Scandinavian Journal of Infectious Diseases, 2014; 46: 376–383
ORIGINAL ARTICLE
Are procalcitonin or other infection markers useful in the detection of group A streptococcal acute tonsillitis?
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ANN MARLENE GRAM CHRISTENSEN1, MARIANNE KRAGH THOMSEN2, THERESE OVESEN1 & TEJS EHLERS KLUG1 From the 1Department of Otorhinolaryngology, Head and Neck Surgery, Aarhus University Hospital, and 2Department of Clinical Microbiology, Aarhus University Hospital, Aarhus, Denmark
Abstract Background: The Centor criteria and the streptococcal rapid antigen detection test (RADT) are commonly used to differentiate sore throat patients with group A streptococci (GAS) from patients with other pathogens. We aimed to investigate if procalcitonin (PCT), C-reactive protein (CRP), white blood cell count (WBC), and absolute neutrophil count (ANC) could increase the diagnostic accuracy when added to the Centor score and RADT, or be used instead of the RADT, in the differential diagnosis. Methods: A 6-month prospective study was carried out in a Danish general practice with 8 physicians. One hundred acute tonsillitis patients aged 15 to 40 y were included. Results: The prevalence of GAS was 26%. The sensitivity (90%) and specificity (97%) of the RADT were high. Mean values of CRP, WBC, and ANC were significantly higher in patients with GAS compared to non-GAS patients (p ⬍ 0.001). However, the sensitivities (66–90%) and specificities (45–75%) were low. No difference in PCT levels was found (p ⫽ 0.334). CRP was the most reliable infection marker (sensitivity 90% and specificity 45%) for GAS aetiology. Conclusions: The sensitivity, specificity, and area under the curve of the RADT were higher than those of the 4 measured infection markers in the differentiation between GAS and non-GAS acute tonsillitis patients. The infection markers did not increase the diagnostic accuracy when added to the Centor score and RADT. When RADT is not available, measurement of CRP or ANC may increase the diagnostic accuracy in the detection of GAS-positive patients. Keywords: Acute tonsillitis, beta-haemolytic group A Streptococcus, streptococcal rapid antigen detection test, infection markers,
procalcitonin, C-reactive protein
Introduction Acute tonsillitis is a very frequent reason for consultation in general practice [1]. Lancefield group A beta-haemolytic Streptococcus (GAS) is considered the predominant bacterial cause (10–26% of all acute tonsillitis cases) [2–4], and in most countries is the only pathogen for which antibiotic therapy is currently recommended [5,6]. The Centor criteria (oral temperature ⱖ 38.3°C, tonsillar exudate, absence of cough, and swollen cervical lymph nodes) can be used to estimate the probability of a GAS infection in acute tonsillitis [7]. The 4-point Centor score and the streptococcal rapid antigen detection test (RADT) are widely used to differentiate sore throat patients with GAS infection from patients infected with other pathogens.
Culture of throat swabs remains the gold standard to diagnose tonsillar bacterial infection [8,9]. A major advantage of culture is that less frequent pathogens such as the Lancefield group C/G betahaemolytic streptococci, Fusobacterium necrophorum, Corynebacterium diphtheriae, and Arcanobacterium haemolyticum are detectable, and the antibiotic resistance patterns can be obtained. The diagnostic delay (48 h) is acceptable in relation to the prevention of rheumatic fever [10]. However, the delay may compromise symptom relief [5] and increase the spread of infection, and secondary contact with patients for whom the culture is positive is time-consuming. Although the biochemical sensitivity (91–97%) and specificity (97–99%) of the RADT are high [11] and the test result is available within a few minutes,
Correspondence: A. M. Gram Christensen, Department of Otorhinolaryngology, Head and Neck Surgery, Aarhus University Hospital, Noerrebrogade 44, 8000 Aarhus C, Denmark. Tel: ⫹ 45 28 74 00 01. E-mail: [email protected] (Received 29 September 2013 ; accepted 1 January 2014 ) ISSN 0036-5548 print/ISSN 1651-1980 online © 2014 Informa Healthcare DOI: 10.3109/00365548.2014.885656
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Infection markers and GAS acute tonsillitis several factors affect its clinical reliability. The clinical specificity is decreased because of the poor capability of the test to differentiate between patients with GAS acute tonsillitis and GAS carriers with a tonsillar infection of other origin [12]. The prevalence of GAS in healthy, asymptomatic individuals is 2–11% depending on the season and age [8,13,14]. The clinical sensitivity of the RADT is influenced by the quality of the tonsillar swab [15], physician experience [16], and the GAS inoculum [17]. Therefore, a biochemical parameter independent of these factors that can increase the accuracy in the differentiation between GAS and non-GAS acute tonsillitis cases may be valuable. Infection markers are appreciated supplements in the clinical diagnosis of infectious diseases [18,19]. A high C-reactive protein (CRP) level is commonly regarded as an indicator of bacterial rather than viral infection [20], and the CRP level can also be used to assess the severity of bacterial infection [21]. CRP synthesis is initiated 4–8 h after an inflammatory stimulus and peaks after 38 h [22]. During an inflammatory response, leukocytes are rapidly released from the bone marrow and the total white blood cell count (WBC) rises within minutes to hours. In bacterial infections, a selective increase in absolute neutrophil count (ANC) is often seen. Blood circulating neutrophils have a life-span of approximately 10 h, whereas the remaining neutrophils die from apoptosis within 24–48 h after entering the inflammatory tissue [23]. Procalcitonin (PCT) is a relatively new marker of infection in the clinical setting. It has the advantages of being more specifically induced by bacteria and having a more rapid synthesis (3–6 h) and peak concentration (6–8 h) than CRP [24,25]. The aims of this study were to investigate if: (1) the addition of PCT, CRP, WBC, or ANC to the RADT and the Centor score may increase the diagnostic accuracy in the distinction between patients with GAS and non-GAS acute tonsillitis, and (2) the addition of PCT, CRP, WBC, or ANC to the Centor score may be used instead of the RADT in the differential diagnosis between patients with GAS and non-GAS acute tonsillitis.
Materials and methods Study population This prospective study was performed at Skoedstrup general practice (with 8 physicians employed) during the period between 27 August 2012 and 5 March 2013. All patients with a sore throat were asked to participate in the study. A screening log was recorded for all the patients with sore throat during the period
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of recruitment. The inclusion criteria were: (1) age between 15 and 40 y, (2) subjective and objective signs of acute tonsillitis meeting 1 or more of the Centor criteria (temperature ⱖ 38.3°C, tonsillar exudate, swollen cervical lymph nodes, and absence of cough), (3) no antibiotic treatment within the past month, (4) no other infections within the past month, (5) no recurrent acute tonsillitis within the last 3 months, (6) no suspicion of a peri-tonsillar abscess, and (7) no previous tonsillectomy. The diagnosis of acute tonsillitis was based on typical symptoms (sore throat and pain on swallowing) and clinical findings of a tonsillar exudate or hyperaemia. The study was approved by the Danish Ethics Committee (ID: 1–10-72–321-12) and the Danish Data Protection Agency (ID: 2007-58-0010). Data were registered at ClinicalTrials.gov (ID: NCT 01657968). Microbiological specimens Simultaneously, 2 swabs were rubbed on both tonsillar surfaces and the posterior pharyngeal mucosa. A white cotton swab was used for the RADT (Alere Test Pack ⫹ Plus including on-board controls) and a charcoal impregnated cotton swab was transported to the Department of Clinical Microbiology in Stuart’s transport medium (SSI Diagnostica, Hillerød, Denmark). Five percent blood agar, chocolate agar, and a specific Fusobacterium agar plate (all from SSI Diagnostica, Hillerød, Denmark) were used for microbiological culture. The swab was semi-quantitatively applied to the various plates using a dilution streak technique.The first quadrant of the plate was streaked using the swab and each successive quadrant was streaked using a new bacteriological loop in order to dilute the number of bacteria in each quadrant. We assume that a semi-quantitative growth of bacteria is correlated to the amount of bacteria present on the swab. The blood agar plates and the chocolate agar plates were incubated in a CO2-enriched atmosphere, whereas the Fusobacterium agar plates were incubated anaerobically. All agar plates were incubated at 35°C for a maximum of 72 h. The plates were inspected for growth of GAS, group C/G betahaemolytic streptococci, F. necrophorum, A. haemolyticum, and C. diphtheriae. Identification was performed by standard methods including matrixassisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS; Bruker Daltonic, Germany). Light to moderate growth of viridans streptococci, Neisseria species, Lactobacillus species, coagulase-negative staphylococci, Prevotella species, and Fusobacterium non-necrophorum alone
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or in mixture was reported as ‘mixed oral flora’. Nongroup A Streptococcus (non-GAS) refers to the growth of all bacteria (including mixed oral flora) other than GAS. Biochemical parameters
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The measurement of PCT was performed using an Elecsys BRHAMS PCT test (Cobas e601 module on a Cobas 6000 instrument), which is an electrochemical luminescence immunoassay test for the quantitative detection of PCT in vitro. Analysis of CRP, WBC, and ANC levels was carried out daily at the Department of Clinical Biochemistry. Statistics For the statistical analysis of normally distributed data, the Student’s t-test and analysis of variance (ANOVA) were used. The normality of the data was assessed using histograms and quantile–quantile (QQ) plots. The Kruskal–Wallis test was used for the non-normally distributed data. The optimal cut-off values of the biochemical parameters in differentiating GAS and non-GAS acute tonsillitis were calculated using the Youden index (sensitivity ⫹ specificity ⫺ 1) [26]. The highest Youden index reflected the best cut-off value. For the statistical analysis of binary data, the Fisher’s exact test and the Chi-square test were used. Statistical significance was assumed at p ⬍ 0.05.
Results During the 6-month study period, 197 patients (69 men) sought a consultation due to sore throat. In total, 100 patients (31 men) with acute tonsillitis were included. The reasons for exclusion are illustrated in Figure 1. The mean age of the included patients was 28 y (range 15–40 y). The median duration of symptoms was 3 days (range 1–21 days). Clinical data and biochemical test results are presented in Table I. GAS was recovered from 26 patients, F. necrophorum from 12 patients, and group C/G beta-haemolytic streptococci from 10 patients. Both F. necrophorum and beta-haemolytic streptococci were found in 4 patients. The remaining 48 patients grew mixed oral flora. Acute tonsillitis patients who met only 1 of the Centor criteria (6%) had a significantly lower probability of GAS infection compared to patients with a Centor score of 2 (31%) or 3 (52%) (p ⬍ 0.001; Chi-square test). A tonsillar exudate (p ⫽ 0.001; Fisher’s exact test) and absence of cough (p ⫽ 0.003) were significantly more common among patients
Figure 1. Flow chart illustrating the causes for exclusion of patients.
growing GAS (GAS group) compared to patients without GAS recovery (non-GAS group). Although anamnestic fever was reported equally often in both groups, a significantly higher mean oral temperature was measured at the time of consultation in the GAS group compared to the non-GAS group (p ⫽ 0.002; Fisher’s exact test). Only 1 patient had an oral temperature ⱖ 38.3°C. The sensitivities, specificities, and area under the curve (AUC) of the Centor score, the RADT, the biochemical parameters, and combinations of these tools for the differentiation between GAS and non-GAS acute tonsillitis are shown in Table II. Figure 2 illustrates the receiver operating characteristics (ROC) curves for the 4 individual infection markers. Ninety-three percent of all patients growing GAS had a Centor score of 2 or higher. When taking into account the Centor score alone, a Centor score
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Table I. Clinical data and test results for patients with group A streptococcal (GAS) and non-GAS acute tonsillitis.
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Variables Patient data Patients, n Men, n (%) Age, y, mean (95% CI) Smokers, n (%) Symptom duration, days, median (range) Clinical tests Per-oral temperature, °C, mean (95% CI) Positive streptococcal rapid antigen detection teste, n (%) Biochemical parameters Procalcitonin, μg/l, median (range) C-reactive protein, mg/l, mean (95% CI) White blood cell count, 109/l, mean (95% CI) Absolute neutrophil count, 109/l, mean (95% CI) Total Centor score, n (%) Score 1 Score 2 Score 3 Score 4 Distribution of Centor criteria, n (%) Oral temperature ⱖ 38.3°C Tonsillar exudate Absence of cough Swollen cervical lymph nodes
GAS
p-Value
71 (37%) (15–39) (13%) (1–21)
0.062b ⬍ 0.001c 0.540b 0.018d
37.1 (36.9–37.3) 26 (90%)
36.7 (36.6–36.9) 2 (3%)
0.002c ⬍ 0.001b
0.03 44 11.1 8.6
0.03 15 8.2 5.1
(0.02–0.51) (10–19) (7.5–8.9) (4.4–5.8)
0.334d ⬍ 0.001c ⬍ 0.001c ⬍ 0.001c
5 33 5 3
29 (17%) (18–40) (17%) (1–14)
non-GASa
(0.02–0.32) (38–60) (9.8–12.3) (7.4–9.7)
26 27 9 3
2 12 15 0
(7%) (41%) (52%) (0%)
30 27 14 0
(42%) (38%) (20%) (0%)
⬍ 0.001b 0.823b 0.003b –
0 24 25 22
(0%) (83%) (86%) (76%)
1 33 38 54
(1%) (46%) (54%) (76%)
1.000b 0.001b 0.003b 1.000b
95% CI, 95% confidence interval. refers to growth of all bacteria (including mixed oral flora) other than GAS. bStudent’s t-test. cFisher’s exact test. dKruskal–Wallis test. eMicrobiological culture results were used as standard of reference. aNon-GAS
of 2 as cut-off between patients with and without GAS infection implies a low specificity (42%) and a Centor score of 3 as cut-off implies a low sensitivity (52%). The sensitivity (90%), specificity (97%), positive predictive value (93%), and negative predictive value (96%) of the RADT were high when applied to patients with a Centor score of 1–4. If the RADT was only applied to patients with a Centor score of 2–4 or even 3–4, the specificity remained high (97% and 99%, respectively). However, in doing so, the sensitivity dropped considerably (Centor score 2–4: 86% and Centor score 3–4: 52%). CRP (p ⬍ 0.001; Student’s t-test), WBC (p ⬍ 0.001), and ANC (p ⬍ 0.001) levels were significantly higher among patients in the GAS group compared to non-GAS patients. However, using the best cut-off levels, the sensitivities, specificities, and area under the curve for CRP (sensitivity 90%, specificity 45%), WBC (sensitivity 69%, specificity 73%), and ANC (sensitivity 66%, specificity 87%) were lower compared to the RADT. No difference in PCT levels (p ⫽ 0.334) was found between patients growing GAS and non-GAS patients (Table I and II).
In order to increase the sensitivity of diagnosing GAS-positive patients, a combination of a positive RADT or measurement of an infection marker above a certain (best) cut-off level was used (correctly diagnosing patients with false-negative RADT). When applied to patients with a Centor score of 1–4, the sensitivity of the RADT was increased from 90% to 97% when combined with CRP. Accordingly, the specificity fell from 97% to 42% (Table II). Similarly, by combining the RADT and measurement of an infection marker above a certain (best) cut-off level, the specificity could be increased (correctly diagnosing patients with false-positive RADT). When applied to patients with a Centor score of 1–4, the specificity of the RADT could be increased from 97% to 100% when combining the test with CRP. Accordingly, the sensitivity fell from 90% to 83% (Table II). Discussion Although the mean values of CRP, WBC, and ANC were significantly higher among patients growing GAS compared to non-GAS patients, the sensitivities and specificities were higher using the RADT
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Table II. Sensitivities, specificities, and area under the curve (AUC) of the Centor score, the streptococcal rapid antigen detection test (RADT), and the infection markers in the differential diagnosis between group A streptococcal (GAS) and non-GASa acute tonsillitis. Centor score 1–4
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Sensitivity %b Specificity %b (absolute (absolute numbers) numbers) Centor score RADT Streptococcal rapid antigen teste Infection markers Procalcitonin, μg/l C-reactive protein, mg/l White blood cell count, 109/l Absolute neutrophil count, 109/l RADT and infection markers RADT and procalcitonin RADT and C-reactive protein RADT and white blood cell count RADT and absolute neutrophil count RADT or infection markers RADT or procalcitonin RADT or C-reactive protein RADT or white blood cell count RADT or absolute neutrophil count
Centor score 2–4
AUC (95% CI)c
Sensitivity %b Specificity %b (absolute (absolute numbers) numbers)
AUC (95% CI)d
100 (29/29)
29 (21/71)
0.73 (0.63–0.84)
93 (27/29)
42 (41/71)
0.61 (0.47–0.75)
90 (26/29)
97 (69/71)
0.93 (0.87–1.00)
86 (25/29)
97 (69/71)
0.94 (0.87–1.01)
72 90 69 66
58 45 73 87
0.59 0.77 0.76 0.82
45 83 66 62
70 70 83 90
0.51 0.76 0.69 0.79
(21/29) (26/29) (20/29) (19/29)
(41/71) (32/71) (52/71) (62/71)
(0.47–0.71) (0.66–0.87) (0.65–0.87) (0.73–0.91)
(13/29) (24/29) (19/29) (18/29)
(50/71) (50/71) (59/71) (64/71)
(0.37–0.65) (0.65–0.88) (0.56–0.82) (0.68–0.90)
45 (13/29) 83 (24/29) 69 (20/29)
97 (69/71) 100 (71/71) 100 (71/71)
0.76 (0.59–0.93) Too little data Too little data
41 (12/29) 79 (23/29) 66 (19/29)
97 (69/71) 100 (71/71) 100 (71/71)
0.58 (0.41–0.75) Too little data Too little data
66 (19/29)
100 (71/71)
Too little data
62 (18/29)
100 (71/71)
Too little data
93 97 90 90
(27/29) (28/29) (26/29) (26/29)
58 42 73 85
(41/71) (30/71) (52/71) (60/71)
0.66 0.65 0.51 0.55
(0.52–0.79) (0.51–0.79) (0.34–0.67) (0.34–0.76)
90 93 86 86
(26/29) (27/29) (25/29) (25/29)
70 69 80 87
(50/71) (49/71) (57/71) (62/71)
0.57 0.66 0.55 0.56
(0.41–0.74) (0.50–0.81) (0.37–0.73) (0.32–0.80)
95% CI, 95% confidence interval. aNon-GAS refers to growth of all bacteria (including mixed oral flora) other than GAS. bThe best cut-off values were calculated using the Youden index for all sensitivities and specificities: procalcitonin, 0.03 μg/l; C-reactive protein, 6 mg/l; white blood cell count, 9.9 ⫻ 109/l; and absolute neutrophil count, 8.0 ⫻ 109/l. cAUC (95% CI) are given for Centor score 1–4. dAUC (95% CI) are given for Centor score 2–4. eMicrobiological culture results were used as standard of reference.
than any of the infection makers. However, when the RADT is not available, measurement of CRP (with a cut-off value of 6 mg/l) or ANC (cut-off 8.0 ⫻ 109/l) is helpful (increasing the specificity) in the detection of GAS acute tonsillitis, especially when applied to patients with a Centor score of 2–4 (Table II). Using culture as the gold standard to differentiate GAS tonsillitis from non-GAS tonsillitis, we found a sensitivity of 90% and a specificity of 97% for the RADT, which is in accordance with previous findings [11]. The performance of the RADT and measurement of an infection marker (with appropriate cut-off levels) was an interesting combination; it may reduce the incidence of GAS carriers wrongly diagnosed with GAS infection because of infection due to other pathogens (false-positive patients). In our study, only 2 false-positive patients growing ‘mixed oral flora’ were found. These patients had low levels of CRP, WBC, and ANC, whereas PCT was slightly elevated. Kaplan and Wannamaker found that 72% of acute tonsillitis patients with heavy growth of GAS had CRP levels above the reference level, compared to 49% of patients with scarce growth of GAS [27].
Similarly, we found that 79% of the acute tonsillitis patients with moderate to heavy growth of GAS (⫹ 3/⫹ 4) had CRP levels above our reference level (ⱕ 8 mg/l), whereas no patients with scarce growth of GAS had increased CRP levels. Conversely, the use of a positive result of the RADT or an infection marker (with the best cut-off levels) is potentially useful when dealing with the problem with false-negative results of the RADT because of insufficient tonsillar swabbing [17]. We found 3 patients with false-negative RADT results. In 2 of these patients, scarce growth of GAS was found. CRP, WBC, and ANC levels were low in all 3 patients. Hence, although we regard these patients as false-negative, we admit that the 2 patients with scarce growth of GAS may represent GAS carriers with non-bacterial infections. The AUCs for all combinations of the RADT or/and an infection marker were considerably lower than the AUC for the RADT alone. The appropriate cut-off values for CRP and WBC have previously been reported to be 35 mg/l [28] and 12 ⫻ 109/l [9], respectively. In our study, the
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Figure 2. Receiver operating characteristics (ROC) curves and area under the curves (AUC) for procalcitonin, C-reactive protein, white blood cell count, and absolute neutrophil count.
best cut-off values were substantially lower: 6 mg/l for CRP and 9.9 ⫻ 109/l for WBC. Hjortdahl and Melbye reported that CRP (sensitivity 73% and specificity 67%) and WBC (sensitivity 71% and specificity 77%) measurements were more reliable as compared to clinical criteria (sensitivity 64% and specificity 71%) in the distinction between acute tonsillitis caused by GAS and that caused by other pathogens. However, they noted that the RADT had a higher specificity (93%) and equal sensitivity (71%) compared to CRP or WBC [29]. Gulich et al. found that the addition of CRP to the clinical assessment increased the sensitivity (from 61% to 78%) and specificity (from 73% to 82%) [28]. We found no difference in mean PCT levels between GAS and non-GAS acute tonsillitis patients. The sensitivity (72%) and specificity (58%) were accordingly low. In a previous study of PCT levels in acute tonsillitis patients, significantly higher mean PCT levels were found in children with bacterial growth compared to children without a bacterial aetiology. Using optimal cut-off levels, the specificity was higher for PCT (87%) than CRP (73%), but the sensitivity was lower (PCT 73%, CRP 80%) [26]. An explanation for the higher PCT reliability may have been the fact that Elsammak et al. distinguished between bacterial and non-bacterial acute tonsillitis rather than between GAS and non-GAS.
In the current study, mean PCT levels were only slightly elevated above the threshold of 0.02 μg/l in both groups (GAS and non-GAS). A possible explanation may be that the patients had relatively localized infections. Hence, PCT may only be a valuable infection marker in more extensive infections [30]. Another explanation may be the fact that PCT levels peak (6–8 h) and culminate (12–48 h) quickly after the release of bacterial endotoxin. For our patients, the median duration of symptoms before consultation was 3 days. The prevalence of tonsillar GAS infection in adults has been reported to be 10–26% [7,31–33]. We isolated GAS in 26% of acute tonsillitis patients aged 15 to 40 y. As in previous studies, we found a correlation between increasing Centor score and the likelihood of positive GAS culture [7,32–34]. The presence of a tonsillar exudate and the absence of cough were significantly more frequent among patients with acute tonsillitis caused by GAS compared to non-GAS patients. Interestingly, our data favour measurement of temperature at consultation over anamnestic fever report, as only the objective temperature was correlated to GAS infection. However, only 1 patient had an oral temperature above the cut-off level used in the Centor criteria (38.3°C). Although Centor et al. measured oral temperature at the time of consultation in the original study of acute
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tonsillitis patients [7], much confusion exists as to whether history of fever [4,12,32–37] or objective temperature measurement [1,31,38] is used in the Centor criteria. Our study favours the use of the RADT in all patients with a Centor score of 1–4, as the sensitivity and specificity of GAS diagnosis were markedly higher using this test compared to relying on clinical criteria alone or in combination with biochemical parameters. Clinicians should bear in mind that the RADT provides no information concerning bacterial pathogens other than GAS, and although not currently recommended, some patients infected with such pathogens may also benefit from antibiotic treatment. Of note, we used the Centor score and not the modified Centor score, which has been used by some other researchers [4,12,32–37]. A considerable proportion (77%) of our patients would have scored 1 point higher on the modified Centor score due to the fact that 78% of the patients reported anamnestic fever and only 1 (1%) patient had a temperature of 38.3°C or higher at the time of consultation. A limitation of our study is the fact that the study was performed at a single general practice and that relatively few patients, aged 15 to 40 y, were included. Hence, bacterial and biochemical findings may be different in age groups outside this range and in areas with different socio-economic structures. We used surface swabs for bacterial culture as standard of reference. Tonsillar tissue specimens and a PCRbased method may have increased the number of potential pathogen recoveries. However, using a thorough swabbing technique (rubbing both tonsils and the posterior oropharyngeal wall) in all patients, we were able to detect GAS in 26% of cases, which is relatively high compared to previous studies [2,3]. In conclusion, PCT was not useful in differentiating GAS from non-GAS acute tonsillitis, either alone or in combination with the Centor score or the RADT. CRP, WBC, and ANC should not be performed in patients with acute tonsillitis, as they do not contribute significantly to an increase in sensitivity or specificity of the RADT. However, measurement of CRP or ANC may be helpful (increasing the specificity) in differentiating GAS from non-GAS acute tonsillitis when the RADT is not available. Acknowledgements The authors are thankful to Skoedstrup general practice, 8541 Skoedstrup, Denmark for providing facilities for the recruitment of patients. Declaration of interest: The authors have no potential conflicts of interest in this study.
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Infection markers and GAS acute tonsillitis [17] Edmonson MB, Farwell KR. Relationship between the clinical likelihood of group A streptococcal pharyngitis and the sensitivity of a rapid antigen-detection test in a pediatric practice. Pediatrics 2005;115:280–5. [18] Dahler-Eriksen BS, Lassen JF, Lund ED, Lauritzen T, Brandslund I. C-reactive protein in general practice—how commonly is it used and why? Scand J Prim Health Care 1997;15:35–8. [19] Tandeter H, Namir S, Herman J. Pride and prejudice: the case of streptococcal pharyngitis and the WBC. J Clin Epidemiol 1993;46:1071–3. [20] Putto A, Meurman O, Ruuskanen O. C-reactive protein in the differentiation of adenoviral, Epstein–Barr viral and streptococcal tonsillitis in children. Eur J Pediatr 1986; 145:204–6. [21] Christensen AM, Kirkegaard MG, Randrup TS, Klug TE. Biochemical tests cannot differentiate between tonsillar and middle ear-derived infections. Dan Med J 2013;60:A4623. [22] Kushner I, Feldmann G. Control of the acute phase response. Demonstration of C-reactive protein synthesis and secretion by hepatocytes during acute inflammation in the rabbit. J Exp Med 1978;148:466–77. [23] Porth CM. Essentials of pathophysiology: concepts of altered health states. 3rd ed. Lippincott Williams & Wilkins; 2010. [24] Maruna P, Nedelníková K, Gürlich R. Physiology and genetics of procalcitonin. Physiol Res 2000;49(Suppl 1):S57–61. [25] Simon L, Gauvin F, Amre DK, Saint-Louis P, Lacroix J. Serum procalcitonin and C-reactive protein levels as markers of bacterial infection: a systematic review and meta-analysis. Clin Infect Dis 2004;39:206–17. [26] Elsammak M, Hanna H, Ghazal A, Edeen FB, Kandil M. Diagnostic value of serum procalcitonin and C-reactive protein in Egyptian children with streptococcal tonsillopharyngitis. Pediatr Infect Dis J 2006;25:174–6. [27] Kaplan EL, Wannamaker LW. C-reactive protein in streptococcal pharyngitis. Pediatrics 1977;60:28–32. [28] Gulich MS, Matschiner A, Glück R, Zeitler HP. Improving diagnostic accuracy of bacterial pharyngitis by near patient
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measurement of C-reactive protein (CRP). Br J Gen Pract 1999;49:119–21. Hjortdahl P, Melbye H. Does near-to-patient testing contribute to the diagnosis of streptococcal pharyngitis in adults? Scand J Prim Health Care 1994;12:70–6. Wacker C, Prkno A, Brunkhorst FM, Schlattmann P. Procalcitonin as a diagnostic marker for sepsis: a systematic review and meta-analysis. Lancet Infect Dis 2013;13:426–35. McIsaac WJ, Kellner JD, Aufricht P, Vanjaka A, Low DE. Empirical validation of guidelines for the management of pharyngitis in children and adults. JAMA 2004;291: 1587–95. Fine AM, Nizet V, Mandl KD. Large-scale validation of the Centor and McIsaac scores to predict group A streptococcal pharyngitis. Arch Intern Med 2012;172:847–52. Llor C, Calviño O, Hernández S, Crispi S, Pérez-Bauer M, Fernández Y, et al. Repetition of the rapid antigen test in initially negative supposed streptococcal pharyngitis is not necessary in adults. Int J Clin Pract 2009;63:1340–4. Dimatteo LA, Lowenstein SR, Brimhall B, Reiquam W, Gonzales R. The relationship between the clinical features of pharyngitis and the sensitivity of a rapid antigen test: evidence of spectrum bias. Ann Emerg Med 2001;38:648–52. Hall MC, Kieke B, Gonzales R, Belongia EA. Spectrum bias of a rapid antigen detection test for group A beta-hemolytic streptococcal pharyngitis in a pediatric population. Pediatrics 2004;114:182–6. Seppälä H, Lahtonen R, Ziegler T, Meurman O, Hakkarainen K, Miettinen A, et al. Clinical scoring system in the evaluation of adult pharyngitis. Arch Otolaryngol Head Neck Surg 1993;119:288–91. Zwart S, Sachs AP, Ruijs GJ, Gubbels JW, Hoes AW, de Melker RA. Penicillin for acute sore throat: randomised double blind trial of seven days versus three days treatment or placebo in adults. BMJ 2000;320:150–4. Worrall G, Hutchinson J, Sherman G, Griffiths J. Diagnosing streptococcal sore throat in adults: randomized controlled trial of in-office aids. Can Fam Physician 2007;53:666–71.
ORIGINAL ARTICLE
Are procalcitonin or other infection markers useful in the detection of group A streptococcal acute tonsillitis?
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ANN MARLENE GRAM CHRISTENSEN1, MARIANNE KRAGH THOMSEN2, THERESE OVESEN1 & TEJS EHLERS KLUG1 From the 1Department of Otorhinolaryngology, Head and Neck Surgery, Aarhus University Hospital, and 2Department of Clinical Microbiology, Aarhus University Hospital, Aarhus, Denmark
Abstract Background: The Centor criteria and the streptococcal rapid antigen detection test (RADT) are commonly used to differentiate sore throat patients with group A streptococci (GAS) from patients with other pathogens. We aimed to investigate if procalcitonin (PCT), C-reactive protein (CRP), white blood cell count (WBC), and absolute neutrophil count (ANC) could increase the diagnostic accuracy when added to the Centor score and RADT, or be used instead of the RADT, in the differential diagnosis. Methods: A 6-month prospective study was carried out in a Danish general practice with 8 physicians. One hundred acute tonsillitis patients aged 15 to 40 y were included. Results: The prevalence of GAS was 26%. The sensitivity (90%) and specificity (97%) of the RADT were high. Mean values of CRP, WBC, and ANC were significantly higher in patients with GAS compared to non-GAS patients (p ⬍ 0.001). However, the sensitivities (66–90%) and specificities (45–75%) were low. No difference in PCT levels was found (p ⫽ 0.334). CRP was the most reliable infection marker (sensitivity 90% and specificity 45%) for GAS aetiology. Conclusions: The sensitivity, specificity, and area under the curve of the RADT were higher than those of the 4 measured infection markers in the differentiation between GAS and non-GAS acute tonsillitis patients. The infection markers did not increase the diagnostic accuracy when added to the Centor score and RADT. When RADT is not available, measurement of CRP or ANC may increase the diagnostic accuracy in the detection of GAS-positive patients. Keywords: Acute tonsillitis, beta-haemolytic group A Streptococcus, streptococcal rapid antigen detection test, infection markers,
procalcitonin, C-reactive protein
Introduction Acute tonsillitis is a very frequent reason for consultation in general practice [1]. Lancefield group A beta-haemolytic Streptococcus (GAS) is considered the predominant bacterial cause (10–26% of all acute tonsillitis cases) [2–4], and in most countries is the only pathogen for which antibiotic therapy is currently recommended [5,6]. The Centor criteria (oral temperature ⱖ 38.3°C, tonsillar exudate, absence of cough, and swollen cervical lymph nodes) can be used to estimate the probability of a GAS infection in acute tonsillitis [7]. The 4-point Centor score and the streptococcal rapid antigen detection test (RADT) are widely used to differentiate sore throat patients with GAS infection from patients infected with other pathogens.
Culture of throat swabs remains the gold standard to diagnose tonsillar bacterial infection [8,9]. A major advantage of culture is that less frequent pathogens such as the Lancefield group C/G betahaemolytic streptococci, Fusobacterium necrophorum, Corynebacterium diphtheriae, and Arcanobacterium haemolyticum are detectable, and the antibiotic resistance patterns can be obtained. The diagnostic delay (48 h) is acceptable in relation to the prevention of rheumatic fever [10]. However, the delay may compromise symptom relief [5] and increase the spread of infection, and secondary contact with patients for whom the culture is positive is time-consuming. Although the biochemical sensitivity (91–97%) and specificity (97–99%) of the RADT are high [11] and the test result is available within a few minutes,
Correspondence: A. M. Gram Christensen, Department of Otorhinolaryngology, Head and Neck Surgery, Aarhus University Hospital, Noerrebrogade 44, 8000 Aarhus C, Denmark. Tel: ⫹ 45 28 74 00 01. E-mail: [email protected] (Received 29 September 2013 ; accepted 1 January 2014 ) ISSN 0036-5548 print/ISSN 1651-1980 online © 2014 Informa Healthcare DOI: 10.3109/00365548.2014.885656
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Infection markers and GAS acute tonsillitis several factors affect its clinical reliability. The clinical specificity is decreased because of the poor capability of the test to differentiate between patients with GAS acute tonsillitis and GAS carriers with a tonsillar infection of other origin [12]. The prevalence of GAS in healthy, asymptomatic individuals is 2–11% depending on the season and age [8,13,14]. The clinical sensitivity of the RADT is influenced by the quality of the tonsillar swab [15], physician experience [16], and the GAS inoculum [17]. Therefore, a biochemical parameter independent of these factors that can increase the accuracy in the differentiation between GAS and non-GAS acute tonsillitis cases may be valuable. Infection markers are appreciated supplements in the clinical diagnosis of infectious diseases [18,19]. A high C-reactive protein (CRP) level is commonly regarded as an indicator of bacterial rather than viral infection [20], and the CRP level can also be used to assess the severity of bacterial infection [21]. CRP synthesis is initiated 4–8 h after an inflammatory stimulus and peaks after 38 h [22]. During an inflammatory response, leukocytes are rapidly released from the bone marrow and the total white blood cell count (WBC) rises within minutes to hours. In bacterial infections, a selective increase in absolute neutrophil count (ANC) is often seen. Blood circulating neutrophils have a life-span of approximately 10 h, whereas the remaining neutrophils die from apoptosis within 24–48 h after entering the inflammatory tissue [23]. Procalcitonin (PCT) is a relatively new marker of infection in the clinical setting. It has the advantages of being more specifically induced by bacteria and having a more rapid synthesis (3–6 h) and peak concentration (6–8 h) than CRP [24,25]. The aims of this study were to investigate if: (1) the addition of PCT, CRP, WBC, or ANC to the RADT and the Centor score may increase the diagnostic accuracy in the distinction between patients with GAS and non-GAS acute tonsillitis, and (2) the addition of PCT, CRP, WBC, or ANC to the Centor score may be used instead of the RADT in the differential diagnosis between patients with GAS and non-GAS acute tonsillitis.
Materials and methods Study population This prospective study was performed at Skoedstrup general practice (with 8 physicians employed) during the period between 27 August 2012 and 5 March 2013. All patients with a sore throat were asked to participate in the study. A screening log was recorded for all the patients with sore throat during the period
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of recruitment. The inclusion criteria were: (1) age between 15 and 40 y, (2) subjective and objective signs of acute tonsillitis meeting 1 or more of the Centor criteria (temperature ⱖ 38.3°C, tonsillar exudate, swollen cervical lymph nodes, and absence of cough), (3) no antibiotic treatment within the past month, (4) no other infections within the past month, (5) no recurrent acute tonsillitis within the last 3 months, (6) no suspicion of a peri-tonsillar abscess, and (7) no previous tonsillectomy. The diagnosis of acute tonsillitis was based on typical symptoms (sore throat and pain on swallowing) and clinical findings of a tonsillar exudate or hyperaemia. The study was approved by the Danish Ethics Committee (ID: 1–10-72–321-12) and the Danish Data Protection Agency (ID: 2007-58-0010). Data were registered at ClinicalTrials.gov (ID: NCT 01657968). Microbiological specimens Simultaneously, 2 swabs were rubbed on both tonsillar surfaces and the posterior pharyngeal mucosa. A white cotton swab was used for the RADT (Alere Test Pack ⫹ Plus including on-board controls) and a charcoal impregnated cotton swab was transported to the Department of Clinical Microbiology in Stuart’s transport medium (SSI Diagnostica, Hillerød, Denmark). Five percent blood agar, chocolate agar, and a specific Fusobacterium agar plate (all from SSI Diagnostica, Hillerød, Denmark) were used for microbiological culture. The swab was semi-quantitatively applied to the various plates using a dilution streak technique.The first quadrant of the plate was streaked using the swab and each successive quadrant was streaked using a new bacteriological loop in order to dilute the number of bacteria in each quadrant. We assume that a semi-quantitative growth of bacteria is correlated to the amount of bacteria present on the swab. The blood agar plates and the chocolate agar plates were incubated in a CO2-enriched atmosphere, whereas the Fusobacterium agar plates were incubated anaerobically. All agar plates were incubated at 35°C for a maximum of 72 h. The plates were inspected for growth of GAS, group C/G betahaemolytic streptococci, F. necrophorum, A. haemolyticum, and C. diphtheriae. Identification was performed by standard methods including matrixassisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS; Bruker Daltonic, Germany). Light to moderate growth of viridans streptococci, Neisseria species, Lactobacillus species, coagulase-negative staphylococci, Prevotella species, and Fusobacterium non-necrophorum alone
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or in mixture was reported as ‘mixed oral flora’. Nongroup A Streptococcus (non-GAS) refers to the growth of all bacteria (including mixed oral flora) other than GAS. Biochemical parameters
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The measurement of PCT was performed using an Elecsys BRHAMS PCT test (Cobas e601 module on a Cobas 6000 instrument), which is an electrochemical luminescence immunoassay test for the quantitative detection of PCT in vitro. Analysis of CRP, WBC, and ANC levels was carried out daily at the Department of Clinical Biochemistry. Statistics For the statistical analysis of normally distributed data, the Student’s t-test and analysis of variance (ANOVA) were used. The normality of the data was assessed using histograms and quantile–quantile (QQ) plots. The Kruskal–Wallis test was used for the non-normally distributed data. The optimal cut-off values of the biochemical parameters in differentiating GAS and non-GAS acute tonsillitis were calculated using the Youden index (sensitivity ⫹ specificity ⫺ 1) [26]. The highest Youden index reflected the best cut-off value. For the statistical analysis of binary data, the Fisher’s exact test and the Chi-square test were used. Statistical significance was assumed at p ⬍ 0.05.
Results During the 6-month study period, 197 patients (69 men) sought a consultation due to sore throat. In total, 100 patients (31 men) with acute tonsillitis were included. The reasons for exclusion are illustrated in Figure 1. The mean age of the included patients was 28 y (range 15–40 y). The median duration of symptoms was 3 days (range 1–21 days). Clinical data and biochemical test results are presented in Table I. GAS was recovered from 26 patients, F. necrophorum from 12 patients, and group C/G beta-haemolytic streptococci from 10 patients. Both F. necrophorum and beta-haemolytic streptococci were found in 4 patients. The remaining 48 patients grew mixed oral flora. Acute tonsillitis patients who met only 1 of the Centor criteria (6%) had a significantly lower probability of GAS infection compared to patients with a Centor score of 2 (31%) or 3 (52%) (p ⬍ 0.001; Chi-square test). A tonsillar exudate (p ⫽ 0.001; Fisher’s exact test) and absence of cough (p ⫽ 0.003) were significantly more common among patients
Figure 1. Flow chart illustrating the causes for exclusion of patients.
growing GAS (GAS group) compared to patients without GAS recovery (non-GAS group). Although anamnestic fever was reported equally often in both groups, a significantly higher mean oral temperature was measured at the time of consultation in the GAS group compared to the non-GAS group (p ⫽ 0.002; Fisher’s exact test). Only 1 patient had an oral temperature ⱖ 38.3°C. The sensitivities, specificities, and area under the curve (AUC) of the Centor score, the RADT, the biochemical parameters, and combinations of these tools for the differentiation between GAS and non-GAS acute tonsillitis are shown in Table II. Figure 2 illustrates the receiver operating characteristics (ROC) curves for the 4 individual infection markers. Ninety-three percent of all patients growing GAS had a Centor score of 2 or higher. When taking into account the Centor score alone, a Centor score
Infection markers and GAS acute tonsillitis
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Table I. Clinical data and test results for patients with group A streptococcal (GAS) and non-GAS acute tonsillitis.
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Variables Patient data Patients, n Men, n (%) Age, y, mean (95% CI) Smokers, n (%) Symptom duration, days, median (range) Clinical tests Per-oral temperature, °C, mean (95% CI) Positive streptococcal rapid antigen detection teste, n (%) Biochemical parameters Procalcitonin, μg/l, median (range) C-reactive protein, mg/l, mean (95% CI) White blood cell count, 109/l, mean (95% CI) Absolute neutrophil count, 109/l, mean (95% CI) Total Centor score, n (%) Score 1 Score 2 Score 3 Score 4 Distribution of Centor criteria, n (%) Oral temperature ⱖ 38.3°C Tonsillar exudate Absence of cough Swollen cervical lymph nodes
GAS
p-Value
71 (37%) (15–39) (13%) (1–21)
0.062b ⬍ 0.001c 0.540b 0.018d
37.1 (36.9–37.3) 26 (90%)
36.7 (36.6–36.9) 2 (3%)
0.002c ⬍ 0.001b
0.03 44 11.1 8.6
0.03 15 8.2 5.1
(0.02–0.51) (10–19) (7.5–8.9) (4.4–5.8)
0.334d ⬍ 0.001c ⬍ 0.001c ⬍ 0.001c
5 33 5 3
29 (17%) (18–40) (17%) (1–14)
non-GASa
(0.02–0.32) (38–60) (9.8–12.3) (7.4–9.7)
26 27 9 3
2 12 15 0
(7%) (41%) (52%) (0%)
30 27 14 0
(42%) (38%) (20%) (0%)
⬍ 0.001b 0.823b 0.003b –
0 24 25 22
(0%) (83%) (86%) (76%)
1 33 38 54
(1%) (46%) (54%) (76%)
1.000b 0.001b 0.003b 1.000b
95% CI, 95% confidence interval. refers to growth of all bacteria (including mixed oral flora) other than GAS. bStudent’s t-test. cFisher’s exact test. dKruskal–Wallis test. eMicrobiological culture results were used as standard of reference. aNon-GAS
of 2 as cut-off between patients with and without GAS infection implies a low specificity (42%) and a Centor score of 3 as cut-off implies a low sensitivity (52%). The sensitivity (90%), specificity (97%), positive predictive value (93%), and negative predictive value (96%) of the RADT were high when applied to patients with a Centor score of 1–4. If the RADT was only applied to patients with a Centor score of 2–4 or even 3–4, the specificity remained high (97% and 99%, respectively). However, in doing so, the sensitivity dropped considerably (Centor score 2–4: 86% and Centor score 3–4: 52%). CRP (p ⬍ 0.001; Student’s t-test), WBC (p ⬍ 0.001), and ANC (p ⬍ 0.001) levels were significantly higher among patients in the GAS group compared to non-GAS patients. However, using the best cut-off levels, the sensitivities, specificities, and area under the curve for CRP (sensitivity 90%, specificity 45%), WBC (sensitivity 69%, specificity 73%), and ANC (sensitivity 66%, specificity 87%) were lower compared to the RADT. No difference in PCT levels (p ⫽ 0.334) was found between patients growing GAS and non-GAS patients (Table I and II).
In order to increase the sensitivity of diagnosing GAS-positive patients, a combination of a positive RADT or measurement of an infection marker above a certain (best) cut-off level was used (correctly diagnosing patients with false-negative RADT). When applied to patients with a Centor score of 1–4, the sensitivity of the RADT was increased from 90% to 97% when combined with CRP. Accordingly, the specificity fell from 97% to 42% (Table II). Similarly, by combining the RADT and measurement of an infection marker above a certain (best) cut-off level, the specificity could be increased (correctly diagnosing patients with false-positive RADT). When applied to patients with a Centor score of 1–4, the specificity of the RADT could be increased from 97% to 100% when combining the test with CRP. Accordingly, the sensitivity fell from 90% to 83% (Table II). Discussion Although the mean values of CRP, WBC, and ANC were significantly higher among patients growing GAS compared to non-GAS patients, the sensitivities and specificities were higher using the RADT
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Table II. Sensitivities, specificities, and area under the curve (AUC) of the Centor score, the streptococcal rapid antigen detection test (RADT), and the infection markers in the differential diagnosis between group A streptococcal (GAS) and non-GASa acute tonsillitis. Centor score 1–4
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Sensitivity %b Specificity %b (absolute (absolute numbers) numbers) Centor score RADT Streptococcal rapid antigen teste Infection markers Procalcitonin, μg/l C-reactive protein, mg/l White blood cell count, 109/l Absolute neutrophil count, 109/l RADT and infection markers RADT and procalcitonin RADT and C-reactive protein RADT and white blood cell count RADT and absolute neutrophil count RADT or infection markers RADT or procalcitonin RADT or C-reactive protein RADT or white blood cell count RADT or absolute neutrophil count
Centor score 2–4
AUC (95% CI)c
Sensitivity %b Specificity %b (absolute (absolute numbers) numbers)
AUC (95% CI)d
100 (29/29)
29 (21/71)
0.73 (0.63–0.84)
93 (27/29)
42 (41/71)
0.61 (0.47–0.75)
90 (26/29)
97 (69/71)
0.93 (0.87–1.00)
86 (25/29)
97 (69/71)
0.94 (0.87–1.01)
72 90 69 66
58 45 73 87
0.59 0.77 0.76 0.82
45 83 66 62
70 70 83 90
0.51 0.76 0.69 0.79
(21/29) (26/29) (20/29) (19/29)
(41/71) (32/71) (52/71) (62/71)
(0.47–0.71) (0.66–0.87) (0.65–0.87) (0.73–0.91)
(13/29) (24/29) (19/29) (18/29)
(50/71) (50/71) (59/71) (64/71)
(0.37–0.65) (0.65–0.88) (0.56–0.82) (0.68–0.90)
45 (13/29) 83 (24/29) 69 (20/29)
97 (69/71) 100 (71/71) 100 (71/71)
0.76 (0.59–0.93) Too little data Too little data
41 (12/29) 79 (23/29) 66 (19/29)
97 (69/71) 100 (71/71) 100 (71/71)
0.58 (0.41–0.75) Too little data Too little data
66 (19/29)
100 (71/71)
Too little data
62 (18/29)
100 (71/71)
Too little data
93 97 90 90
(27/29) (28/29) (26/29) (26/29)
58 42 73 85
(41/71) (30/71) (52/71) (60/71)
0.66 0.65 0.51 0.55
(0.52–0.79) (0.51–0.79) (0.34–0.67) (0.34–0.76)
90 93 86 86
(26/29) (27/29) (25/29) (25/29)
70 69 80 87
(50/71) (49/71) (57/71) (62/71)
0.57 0.66 0.55 0.56
(0.41–0.74) (0.50–0.81) (0.37–0.73) (0.32–0.80)
95% CI, 95% confidence interval. aNon-GAS refers to growth of all bacteria (including mixed oral flora) other than GAS. bThe best cut-off values were calculated using the Youden index for all sensitivities and specificities: procalcitonin, 0.03 μg/l; C-reactive protein, 6 mg/l; white blood cell count, 9.9 ⫻ 109/l; and absolute neutrophil count, 8.0 ⫻ 109/l. cAUC (95% CI) are given for Centor score 1–4. dAUC (95% CI) are given for Centor score 2–4. eMicrobiological culture results were used as standard of reference.
than any of the infection makers. However, when the RADT is not available, measurement of CRP (with a cut-off value of 6 mg/l) or ANC (cut-off 8.0 ⫻ 109/l) is helpful (increasing the specificity) in the detection of GAS acute tonsillitis, especially when applied to patients with a Centor score of 2–4 (Table II). Using culture as the gold standard to differentiate GAS tonsillitis from non-GAS tonsillitis, we found a sensitivity of 90% and a specificity of 97% for the RADT, which is in accordance with previous findings [11]. The performance of the RADT and measurement of an infection marker (with appropriate cut-off levels) was an interesting combination; it may reduce the incidence of GAS carriers wrongly diagnosed with GAS infection because of infection due to other pathogens (false-positive patients). In our study, only 2 false-positive patients growing ‘mixed oral flora’ were found. These patients had low levels of CRP, WBC, and ANC, whereas PCT was slightly elevated. Kaplan and Wannamaker found that 72% of acute tonsillitis patients with heavy growth of GAS had CRP levels above the reference level, compared to 49% of patients with scarce growth of GAS [27].
Similarly, we found that 79% of the acute tonsillitis patients with moderate to heavy growth of GAS (⫹ 3/⫹ 4) had CRP levels above our reference level (ⱕ 8 mg/l), whereas no patients with scarce growth of GAS had increased CRP levels. Conversely, the use of a positive result of the RADT or an infection marker (with the best cut-off levels) is potentially useful when dealing with the problem with false-negative results of the RADT because of insufficient tonsillar swabbing [17]. We found 3 patients with false-negative RADT results. In 2 of these patients, scarce growth of GAS was found. CRP, WBC, and ANC levels were low in all 3 patients. Hence, although we regard these patients as false-negative, we admit that the 2 patients with scarce growth of GAS may represent GAS carriers with non-bacterial infections. The AUCs for all combinations of the RADT or/and an infection marker were considerably lower than the AUC for the RADT alone. The appropriate cut-off values for CRP and WBC have previously been reported to be 35 mg/l [28] and 12 ⫻ 109/l [9], respectively. In our study, the
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Figure 2. Receiver operating characteristics (ROC) curves and area under the curves (AUC) for procalcitonin, C-reactive protein, white blood cell count, and absolute neutrophil count.
best cut-off values were substantially lower: 6 mg/l for CRP and 9.9 ⫻ 109/l for WBC. Hjortdahl and Melbye reported that CRP (sensitivity 73% and specificity 67%) and WBC (sensitivity 71% and specificity 77%) measurements were more reliable as compared to clinical criteria (sensitivity 64% and specificity 71%) in the distinction between acute tonsillitis caused by GAS and that caused by other pathogens. However, they noted that the RADT had a higher specificity (93%) and equal sensitivity (71%) compared to CRP or WBC [29]. Gulich et al. found that the addition of CRP to the clinical assessment increased the sensitivity (from 61% to 78%) and specificity (from 73% to 82%) [28]. We found no difference in mean PCT levels between GAS and non-GAS acute tonsillitis patients. The sensitivity (72%) and specificity (58%) were accordingly low. In a previous study of PCT levels in acute tonsillitis patients, significantly higher mean PCT levels were found in children with bacterial growth compared to children without a bacterial aetiology. Using optimal cut-off levels, the specificity was higher for PCT (87%) than CRP (73%), but the sensitivity was lower (PCT 73%, CRP 80%) [26]. An explanation for the higher PCT reliability may have been the fact that Elsammak et al. distinguished between bacterial and non-bacterial acute tonsillitis rather than between GAS and non-GAS.
In the current study, mean PCT levels were only slightly elevated above the threshold of 0.02 μg/l in both groups (GAS and non-GAS). A possible explanation may be that the patients had relatively localized infections. Hence, PCT may only be a valuable infection marker in more extensive infections [30]. Another explanation may be the fact that PCT levels peak (6–8 h) and culminate (12–48 h) quickly after the release of bacterial endotoxin. For our patients, the median duration of symptoms before consultation was 3 days. The prevalence of tonsillar GAS infection in adults has been reported to be 10–26% [7,31–33]. We isolated GAS in 26% of acute tonsillitis patients aged 15 to 40 y. As in previous studies, we found a correlation between increasing Centor score and the likelihood of positive GAS culture [7,32–34]. The presence of a tonsillar exudate and the absence of cough were significantly more frequent among patients with acute tonsillitis caused by GAS compared to non-GAS patients. Interestingly, our data favour measurement of temperature at consultation over anamnestic fever report, as only the objective temperature was correlated to GAS infection. However, only 1 patient had an oral temperature above the cut-off level used in the Centor criteria (38.3°C). Although Centor et al. measured oral temperature at the time of consultation in the original study of acute
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tonsillitis patients [7], much confusion exists as to whether history of fever [4,12,32–37] or objective temperature measurement [1,31,38] is used in the Centor criteria. Our study favours the use of the RADT in all patients with a Centor score of 1–4, as the sensitivity and specificity of GAS diagnosis were markedly higher using this test compared to relying on clinical criteria alone or in combination with biochemical parameters. Clinicians should bear in mind that the RADT provides no information concerning bacterial pathogens other than GAS, and although not currently recommended, some patients infected with such pathogens may also benefit from antibiotic treatment. Of note, we used the Centor score and not the modified Centor score, which has been used by some other researchers [4,12,32–37]. A considerable proportion (77%) of our patients would have scored 1 point higher on the modified Centor score due to the fact that 78% of the patients reported anamnestic fever and only 1 (1%) patient had a temperature of 38.3°C or higher at the time of consultation. A limitation of our study is the fact that the study was performed at a single general practice and that relatively few patients, aged 15 to 40 y, were included. Hence, bacterial and biochemical findings may be different in age groups outside this range and in areas with different socio-economic structures. We used surface swabs for bacterial culture as standard of reference. Tonsillar tissue specimens and a PCRbased method may have increased the number of potential pathogen recoveries. However, using a thorough swabbing technique (rubbing both tonsils and the posterior oropharyngeal wall) in all patients, we were able to detect GAS in 26% of cases, which is relatively high compared to previous studies [2,3]. In conclusion, PCT was not useful in differentiating GAS from non-GAS acute tonsillitis, either alone or in combination with the Centor score or the RADT. CRP, WBC, and ANC should not be performed in patients with acute tonsillitis, as they do not contribute significantly to an increase in sensitivity or specificity of the RADT. However, measurement of CRP or ANC may be helpful (increasing the specificity) in differentiating GAS from non-GAS acute tonsillitis when the RADT is not available. Acknowledgements The authors are thankful to Skoedstrup general practice, 8541 Skoedstrup, Denmark for providing facilities for the recruitment of patients. Declaration of interest: The authors have no potential conflicts of interest in this study.
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