Veterinary Microbiology, 24 (1990) 73-80 Elsevier Science Publishers B.V., Amsterdam
73
Diagnosis of bovine brucellosis by enzyme immunoassay of milk* P. Kerkhofs L,Y. Botton 2, P. Thiange 2, P. Dekeyser 1 and J.N. Limet 3 ~National Institute for Veterinary Research, Groeselenberg, 99, B- 1180 Brussels (Belgium) 2Centre de dkpistage des maladie du bktail, chaussbe de Marche, 604 B-5101 Erpent (Belgium) 31nternational Institute for Cellular and Molecular Pathology, avenue Hippocrate, 74, B- 1200 Brussels (Belgium) (Accepted 12 December 1989)
ABSTRACT Kerkhofs, P., Botton, Y., Thiange, P., Dekeyser, P. and Limet, J.N., 1990. Diagnosis of bovine brucellosis by enzyme immunoassay of milk. Vet. Microbiol., 24: 73-80. Enzyme-immunoassays using lipopolysaccharide as antigen were developed for the detection of bovine IgG 1, IgG2 or IgA Brucella antibodies (Ab) in milk. The results of these tests were compared with those of the milk ring test (MRT) by analyzing 3212 bulk milk samples from farms located in regions where brucellosis is prevalent. Among the 105 herds detected by ELISA and/or MRT, 29 infected herds were detected by ELISA only. The 40 MRT-positive herds were also ELISA positive. Five herds became infected during the study and were detected by ELISA 15 days to 6 months prior to detection by MRT. The ELISA IgG1 titration (IgG 1 ELISA) detected 92.8% of the herds found positive in the three ELISA assays. The concomitant use oflgA ELISA raised the sensitivityto 100%but slightly decreased the specificity. The IgG2 ELISA did not improve the diagnosis. The sensitivity of MRT and lgG1 ELISA was compared by testing successive dilutions in negative milk of 110 individual MRT positive milks samples. On average, IgG 1 ELISA was 22 times more sensitive than MRT.
INTRODUCTION
The milk ring test (MRT), developed by Fleischauwer in 1937, is commonly used for brucellosis screening in dairy herds. The coloured Brucella suspension (Alton et al., 1977) is mainly concentrated in the cream layer when milk contains antibodies (Ab). In the absence of Ab, it is homogeneously distributed in the milk under the white cream layer. However, MRT will be less and less able to detect the antibodies of one positive cow in the bulk milk of a refrigerated tank, due to increasing number of lactating cows *This work was supported by the Institut pour l'Encouragement de la Recherche Scientifique dans l'Industrie et l'Agriculture (IRSIA).
0378-1135/90/$03.50
© 1990 Elsevier Science Publishers B.V.
74
P. KERKHOFS ET AL.
per farm. Refrigerated tanks are now capable of containing the milk of 50, 100, and even 200 cows. Moreover, the efficacy of the MRT seems to be due more to the frequency of testing than to its sensitivity (Fensterbank, 1986 ). Forschner and Bringer (1986) described an ELISA for the detection of bovine Brucella Ab in bulk milk. This test succeeded in detecting milk from one infected cow diluted 100 fold in normal milk. In this article, we described immunoassays for independant titration of IgG 1, IgG2 and IgA Brucella Ab in bulk milk. IgA Ab, the most active in the coloured ring formation, are mainly produced in the mammary gland and are therefore more specific for udder infection. IgG 1 Ab, less active in the ring formation, however, are detectable in the milk earlier after infection (Sutra et al., 1986; Sutra and Dubray, 1987 ). The assays developed were evaluated for specificity with 1457 bulk milks from brucellosis free herds. The sensitivity was estimated by the titration of 3212 bulk milks from regions where brucellosis is prevalent and dilutions in negative milk of 110 MRT positive milk samples. MATERIALS AND METHODS
Milk Ring test (MRT) This test was described by Alton et al. ( 1977 ). Briefly, 50/zl ofhematoxylin coloured antigen (IFFA-Merieux, Lyon) was mixed with 1 ml milk in a disposable polystyrene tube of 6 m m in diameter and incubated for 1 h at 37 ° C. A negative result corresponds to a white cream layer upon a uniformly coloured milk. The degree of a positive reaction depends on the colour intensity of the cream layer and the proportion of the coloured antigen remaining in the milk.
ELISA Buffers. Glycine buffer saline (GBS): 0.17 M NaC1, 0.1 M glycine adjusted to pH 9.2 with NaOH containing 400 mg/1 NAN3. GBS-EDTA-Tw: GBS buffer containing 50 mM EDTA and 0.1% Tween 80, pH 9.2. GBS-EDTA-cholate: GBS buffer containing 50 mM EDTA and 2% sodium cholate, pH 9.2. Citrate-phosphate buffer: 0.051 M NazHPO4, 0.024 M citric acid, pH 5. The washing solution was NaCI-Tw.: 0.15 M NaCI, 0.01% Tween 20. Antigen. The lipopolysaccharide (LPS) of Brucella abortus biovar 3 (field strain ) was obtained by phenol extraction of heat-killed and freeze-dried bacteria according to Westphal and Jann (1965) and purified as described by Moreno et al. (1979). Anti-bovine immunoglobulin Mab. Mab 1C8, 3H3 and 16.35 were used separately. They recognize the heavy chain of IgG1, heavy chain of IgG2 and
DIAGNOSIS OF BOVINE BRUCELLOSIS BY ENZYME IMMUNOASSAY OF MILK
75
weakly those o f l g G l , and IgA respectively. The Mab 1C8 and 3H3 were produced and provided by professor Depelchin and Dr. Letesson of "Centre d'Immunologie Appliqu6e ~ l'61evage, "IRSIA", N a m u r (Letesson et al., 1985 ). Mab 16.35 was a gift from Dr. van Zaane of CDI, Lelystad (Van Zaane, 1987). These Mabs were conjugated to periodic acid activated peroxidase (Nakane and Kawao'i, 1974) and stored at - 20 ° C in 50% glycerol.
Assay. Polystyrene microtitration plates (Greiner labortechnic, Stuttgart) were coated by incubation with 100/A of a 1 m g / m l LPS solution in 5-fold water-diluted GBS. The m i n i m u m incubation time for coating was 3 h at 37°C. The coated plates may be stored for several months either at 4°C or washed, dried and packed in a plastic bags with a dessicant capsule. After vigorous stirring, milk samples were 2-fold diluted in GBS-EDTAcholate. Plates were washed four times with NaC1-Tw by using a 12-channel manual i m m u n o w a s h (Nunc), and 50/tl of each milk dilution were dispensed in a well. After 1 h incubation at room temperature, the plates were washed five times. Fifty #1 of conjugated antibody diluted in GBS-EDTA-Tw containing 2% fetal calf serum were dispensed in the wells and the plates were incubated for 1 h at room temperature in the dark. After five washings with NaC1-Tw, the a m o u n t of b o u n d conjugate was revealed by dispensing 100/~1 of 0.4 m g / m l ortho-phenylene-diamine solution and 2 m M H202 in citratephosphate buffer. The colour development was stopped after a m a x i m u m of 20 min incubation in the dark by the addition of 25/~1 of 2 M H2SO 4 in each well. The light absorbance was measured at 492 n m and 620 nm. The tests were carried out in duplicate.
Origin of the samples Brucellosis free herds. These herds were selected in a part of the country where the prevalence of brucellosis is very low. The bulk milk samples we used were negative to the MRT and all the animals of the corresponding herds were negative to the serum agglutination test (SAT) and rose bengal plate test (RBPT).
Herds from regions where brucellosis is prevalent. 3212 bulk milk samples were tested. SAT and RBPT were performed on serum of all the corresponding animals (Alton et al., 1977 ). Positive sera were retested in the complement fixation test ( C F T ) . Brucella organisms were isolated from milk samples or cotyledons and abortion products (Alton et al., 1977) on at least one animal in 51 of the 69 farms which presented positive sera. The animals of 5 herds had been vaccinated and 13 herds included animals with positive serology but no culture had been performed.
76
P. KERKHOFS ET AL.
M R T positive milk samples. To evaluate the sensitivity of the ELISA, MRT and IgG 1, ELISA were carried out in parallel on serial dilutions of 110 MRT positive individual milk samples. The serial dilutions ( 1/3 to 1/2187 ) were done in milk from uninfected cows shown to be negative for both tests. Milk samples were stored either frozen at - 2 0 ° C or at 4 °C after addition of 0.2% formaldehyde. RESULTS
Characteristics of the three ELISAs Calibration curves of the three assays are presented in Fig. 1. By using a 12channel manual i m m u n o w a s h or three washings of 3 min after immersion in the NaC1-Tw, the coefficient of variation within a plate was less than 10%. The optical densities observed for 1457 milk samples from uninfected farms were compared to those of successive dilutions ( 1/ 1000 to 1/ 128 000) of a strongly positive milk sample for IgA ELISA. For the IgG1 and IgG2 ELISA, a reference serum with a CFT value of 1200 IU was used as reference. The optical densities observed for the M R T negative milk samples obtained from brucellosis free farms were generally low (less than 0.060) and 98.9, 98.8, and
o£
10 3
lO 4
10 5
DILUTION (I/X)
Fig. 1. S t a n d a r d curves of the three assays. M e a n a b s o r b a n c e values a n d s t a n d a r d deviations were calculated from data o b t a i n e d with four different plates r u n on the same day.
DIAGNOSIS OF BOVINE BRUCELLOSISBY ENZYME IMMUNOASSAYOF MILK
77
97.9% of the samples were lower than the value corresponding to the mean plus three standard deviations for IgG 1, IgG2 and IgA respectively. However, cut-off values equal to the mean plus eight to fifteen standard deviations have been used to ensure a 100% specificity
Field application of ELISA; comparison with MRT Among the 3212 milk samples tested, 105 samples were positive to one or more of the three ELISAs and MRT. Only 40 samples were positive to both ELISA and MRT. Herds were classified on the basis of the serological examination of their animals, i.e. the results of the SAT, CFT and RBPT on serum. When available, bacteriological results were taken into account; all culture positive herds were also serologically positive. Table 1 presents the ELISA and MRT results in function of this classification. MRT detected only 58.0% of the herds with a positive serology whereas the three ELISAs combined detected all of them. The 40 MRT positive herds were also ELISA positive. Twenty-nine infected herds were positive in ELISA and negative in MRT. Five of them were recently infected and were discovered earlier by ELISA than by MRT. ELISA results were confirmed by MRT fifteen days later (1/5 ), or by serology (4/5). The serological tests became clearly positive from 15 days to 6 months later. Twelve false positive reactions were observed in the MRT and not in ELISA; and one false positive reaction was observed in both tests. ELISA was, however, found positive in 24 herds without serological or bacteriological eviTABLE 1 Results of the MRT and ELISA in function of the serology of the herds taken as index of the infection Positive serology
Negative serology
Sensitivity (%)
MRT + MRT-
40 29
13 3199
58.0
EL IgGl + EL IgGl -
64 5
9 3203
92.8
EL I g G 2 + EL I g G 2 -
34 35
4 3208
49.3
EL IgA+ EL I g A -
34 35
16 3196
49.3
EL Ig + ELIg-
69 0
24 3188
Specificity (%) 99.6 99.7 99.9 99.5
100 99.3
Serological tests were SAT, CFT and RBPT. EL Ig, combination of the results of the three ELISAs.
78
P. K E R K H O F S
E T AL.
dences of infection (0.7%). The specificity and sensitivity of the ELISA were 99.3 and 100%, respectively, compared to 99.5 and 58.0% for MRT. Values for each assay are given in Table 1. IgGl ELISA was clearly the most efficient assay. It detected the greatest number of culture-positive infected herds and herds with positive serology, not confirmed by the isolation of Brucella. The five MRT negative infected herds discovered during the study were IgG1 positive, but five milk samples from infected herds were only IgA ELISA positive. The IgA ELISA slightly improved the sensitivity of detection but detected the greatest number (16) of farms with negative serology. The 3H3 Mab that essentially detects IgG2 gave very few false positive responses (4), but missed 51% of infected herds.
Comparison of the sensitivity of lgG1 ELISA and MRT Serial dilutions of I 10 positive individual milk samples with negative milk were tested by both M R T and ELISA. Fig. 2 illustrates the ratio of the last dilution positive by ELISA compared to MRT. Only six milk samples exhibit the same titre in both tests. The mean titre ratio was 22 and the highest increase in sensitivity observed by ELISA for 12 samples was 81. No milk was IgG 1 ELISA negative and MRT positive. The highest dilution found positive was 1/2187, for only one sample in M R T and for 11 samples in ELISA. The increase of sensitivity was not significantly influenced by the method of preservation. 50
4O ..d tl zs
73
Diagnosis of bovine brucellosis by enzyme immunoassay of milk* P. Kerkhofs L,Y. Botton 2, P. Thiange 2, P. Dekeyser 1 and J.N. Limet 3 ~National Institute for Veterinary Research, Groeselenberg, 99, B- 1180 Brussels (Belgium) 2Centre de dkpistage des maladie du bktail, chaussbe de Marche, 604 B-5101 Erpent (Belgium) 31nternational Institute for Cellular and Molecular Pathology, avenue Hippocrate, 74, B- 1200 Brussels (Belgium) (Accepted 12 December 1989)
ABSTRACT Kerkhofs, P., Botton, Y., Thiange, P., Dekeyser, P. and Limet, J.N., 1990. Diagnosis of bovine brucellosis by enzyme immunoassay of milk. Vet. Microbiol., 24: 73-80. Enzyme-immunoassays using lipopolysaccharide as antigen were developed for the detection of bovine IgG 1, IgG2 or IgA Brucella antibodies (Ab) in milk. The results of these tests were compared with those of the milk ring test (MRT) by analyzing 3212 bulk milk samples from farms located in regions where brucellosis is prevalent. Among the 105 herds detected by ELISA and/or MRT, 29 infected herds were detected by ELISA only. The 40 MRT-positive herds were also ELISA positive. Five herds became infected during the study and were detected by ELISA 15 days to 6 months prior to detection by MRT. The ELISA IgG1 titration (IgG 1 ELISA) detected 92.8% of the herds found positive in the three ELISA assays. The concomitant use oflgA ELISA raised the sensitivityto 100%but slightly decreased the specificity. The IgG2 ELISA did not improve the diagnosis. The sensitivity of MRT and lgG1 ELISA was compared by testing successive dilutions in negative milk of 110 individual MRT positive milks samples. On average, IgG 1 ELISA was 22 times more sensitive than MRT.
INTRODUCTION
The milk ring test (MRT), developed by Fleischauwer in 1937, is commonly used for brucellosis screening in dairy herds. The coloured Brucella suspension (Alton et al., 1977) is mainly concentrated in the cream layer when milk contains antibodies (Ab). In the absence of Ab, it is homogeneously distributed in the milk under the white cream layer. However, MRT will be less and less able to detect the antibodies of one positive cow in the bulk milk of a refrigerated tank, due to increasing number of lactating cows *This work was supported by the Institut pour l'Encouragement de la Recherche Scientifique dans l'Industrie et l'Agriculture (IRSIA).
0378-1135/90/$03.50
© 1990 Elsevier Science Publishers B.V.
74
P. KERKHOFS ET AL.
per farm. Refrigerated tanks are now capable of containing the milk of 50, 100, and even 200 cows. Moreover, the efficacy of the MRT seems to be due more to the frequency of testing than to its sensitivity (Fensterbank, 1986 ). Forschner and Bringer (1986) described an ELISA for the detection of bovine Brucella Ab in bulk milk. This test succeeded in detecting milk from one infected cow diluted 100 fold in normal milk. In this article, we described immunoassays for independant titration of IgG 1, IgG2 and IgA Brucella Ab in bulk milk. IgA Ab, the most active in the coloured ring formation, are mainly produced in the mammary gland and are therefore more specific for udder infection. IgG 1 Ab, less active in the ring formation, however, are detectable in the milk earlier after infection (Sutra et al., 1986; Sutra and Dubray, 1987 ). The assays developed were evaluated for specificity with 1457 bulk milks from brucellosis free herds. The sensitivity was estimated by the titration of 3212 bulk milks from regions where brucellosis is prevalent and dilutions in negative milk of 110 MRT positive milk samples. MATERIALS AND METHODS
Milk Ring test (MRT) This test was described by Alton et al. ( 1977 ). Briefly, 50/zl ofhematoxylin coloured antigen (IFFA-Merieux, Lyon) was mixed with 1 ml milk in a disposable polystyrene tube of 6 m m in diameter and incubated for 1 h at 37 ° C. A negative result corresponds to a white cream layer upon a uniformly coloured milk. The degree of a positive reaction depends on the colour intensity of the cream layer and the proportion of the coloured antigen remaining in the milk.
ELISA Buffers. Glycine buffer saline (GBS): 0.17 M NaC1, 0.1 M glycine adjusted to pH 9.2 with NaOH containing 400 mg/1 NAN3. GBS-EDTA-Tw: GBS buffer containing 50 mM EDTA and 0.1% Tween 80, pH 9.2. GBS-EDTA-cholate: GBS buffer containing 50 mM EDTA and 2% sodium cholate, pH 9.2. Citrate-phosphate buffer: 0.051 M NazHPO4, 0.024 M citric acid, pH 5. The washing solution was NaCI-Tw.: 0.15 M NaCI, 0.01% Tween 20. Antigen. The lipopolysaccharide (LPS) of Brucella abortus biovar 3 (field strain ) was obtained by phenol extraction of heat-killed and freeze-dried bacteria according to Westphal and Jann (1965) and purified as described by Moreno et al. (1979). Anti-bovine immunoglobulin Mab. Mab 1C8, 3H3 and 16.35 were used separately. They recognize the heavy chain of IgG1, heavy chain of IgG2 and
DIAGNOSIS OF BOVINE BRUCELLOSIS BY ENZYME IMMUNOASSAY OF MILK
75
weakly those o f l g G l , and IgA respectively. The Mab 1C8 and 3H3 were produced and provided by professor Depelchin and Dr. Letesson of "Centre d'Immunologie Appliqu6e ~ l'61evage, "IRSIA", N a m u r (Letesson et al., 1985 ). Mab 16.35 was a gift from Dr. van Zaane of CDI, Lelystad (Van Zaane, 1987). These Mabs were conjugated to periodic acid activated peroxidase (Nakane and Kawao'i, 1974) and stored at - 20 ° C in 50% glycerol.
Assay. Polystyrene microtitration plates (Greiner labortechnic, Stuttgart) were coated by incubation with 100/A of a 1 m g / m l LPS solution in 5-fold water-diluted GBS. The m i n i m u m incubation time for coating was 3 h at 37°C. The coated plates may be stored for several months either at 4°C or washed, dried and packed in a plastic bags with a dessicant capsule. After vigorous stirring, milk samples were 2-fold diluted in GBS-EDTAcholate. Plates were washed four times with NaC1-Tw by using a 12-channel manual i m m u n o w a s h (Nunc), and 50/tl of each milk dilution were dispensed in a well. After 1 h incubation at room temperature, the plates were washed five times. Fifty #1 of conjugated antibody diluted in GBS-EDTA-Tw containing 2% fetal calf serum were dispensed in the wells and the plates were incubated for 1 h at room temperature in the dark. After five washings with NaC1-Tw, the a m o u n t of b o u n d conjugate was revealed by dispensing 100/~1 of 0.4 m g / m l ortho-phenylene-diamine solution and 2 m M H202 in citratephosphate buffer. The colour development was stopped after a m a x i m u m of 20 min incubation in the dark by the addition of 25/~1 of 2 M H2SO 4 in each well. The light absorbance was measured at 492 n m and 620 nm. The tests were carried out in duplicate.
Origin of the samples Brucellosis free herds. These herds were selected in a part of the country where the prevalence of brucellosis is very low. The bulk milk samples we used were negative to the MRT and all the animals of the corresponding herds were negative to the serum agglutination test (SAT) and rose bengal plate test (RBPT).
Herds from regions where brucellosis is prevalent. 3212 bulk milk samples were tested. SAT and RBPT were performed on serum of all the corresponding animals (Alton et al., 1977 ). Positive sera were retested in the complement fixation test ( C F T ) . Brucella organisms were isolated from milk samples or cotyledons and abortion products (Alton et al., 1977) on at least one animal in 51 of the 69 farms which presented positive sera. The animals of 5 herds had been vaccinated and 13 herds included animals with positive serology but no culture had been performed.
76
P. KERKHOFS ET AL.
M R T positive milk samples. To evaluate the sensitivity of the ELISA, MRT and IgG 1, ELISA were carried out in parallel on serial dilutions of 110 MRT positive individual milk samples. The serial dilutions ( 1/3 to 1/2187 ) were done in milk from uninfected cows shown to be negative for both tests. Milk samples were stored either frozen at - 2 0 ° C or at 4 °C after addition of 0.2% formaldehyde. RESULTS
Characteristics of the three ELISAs Calibration curves of the three assays are presented in Fig. 1. By using a 12channel manual i m m u n o w a s h or three washings of 3 min after immersion in the NaC1-Tw, the coefficient of variation within a plate was less than 10%. The optical densities observed for 1457 milk samples from uninfected farms were compared to those of successive dilutions ( 1/ 1000 to 1/ 128 000) of a strongly positive milk sample for IgA ELISA. For the IgG1 and IgG2 ELISA, a reference serum with a CFT value of 1200 IU was used as reference. The optical densities observed for the M R T negative milk samples obtained from brucellosis free farms were generally low (less than 0.060) and 98.9, 98.8, and
o£
10 3
lO 4
10 5
DILUTION (I/X)
Fig. 1. S t a n d a r d curves of the three assays. M e a n a b s o r b a n c e values a n d s t a n d a r d deviations were calculated from data o b t a i n e d with four different plates r u n on the same day.
DIAGNOSIS OF BOVINE BRUCELLOSISBY ENZYME IMMUNOASSAYOF MILK
77
97.9% of the samples were lower than the value corresponding to the mean plus three standard deviations for IgG 1, IgG2 and IgA respectively. However, cut-off values equal to the mean plus eight to fifteen standard deviations have been used to ensure a 100% specificity
Field application of ELISA; comparison with MRT Among the 3212 milk samples tested, 105 samples were positive to one or more of the three ELISAs and MRT. Only 40 samples were positive to both ELISA and MRT. Herds were classified on the basis of the serological examination of their animals, i.e. the results of the SAT, CFT and RBPT on serum. When available, bacteriological results were taken into account; all culture positive herds were also serologically positive. Table 1 presents the ELISA and MRT results in function of this classification. MRT detected only 58.0% of the herds with a positive serology whereas the three ELISAs combined detected all of them. The 40 MRT positive herds were also ELISA positive. Twenty-nine infected herds were positive in ELISA and negative in MRT. Five of them were recently infected and were discovered earlier by ELISA than by MRT. ELISA results were confirmed by MRT fifteen days later (1/5 ), or by serology (4/5). The serological tests became clearly positive from 15 days to 6 months later. Twelve false positive reactions were observed in the MRT and not in ELISA; and one false positive reaction was observed in both tests. ELISA was, however, found positive in 24 herds without serological or bacteriological eviTABLE 1 Results of the MRT and ELISA in function of the serology of the herds taken as index of the infection Positive serology
Negative serology
Sensitivity (%)
MRT + MRT-
40 29
13 3199
58.0
EL IgGl + EL IgGl -
64 5
9 3203
92.8
EL I g G 2 + EL I g G 2 -
34 35
4 3208
49.3
EL IgA+ EL I g A -
34 35
16 3196
49.3
EL Ig + ELIg-
69 0
24 3188
Specificity (%) 99.6 99.7 99.9 99.5
100 99.3
Serological tests were SAT, CFT and RBPT. EL Ig, combination of the results of the three ELISAs.
78
P. K E R K H O F S
E T AL.
dences of infection (0.7%). The specificity and sensitivity of the ELISA were 99.3 and 100%, respectively, compared to 99.5 and 58.0% for MRT. Values for each assay are given in Table 1. IgGl ELISA was clearly the most efficient assay. It detected the greatest number of culture-positive infected herds and herds with positive serology, not confirmed by the isolation of Brucella. The five MRT negative infected herds discovered during the study were IgG1 positive, but five milk samples from infected herds were only IgA ELISA positive. The IgA ELISA slightly improved the sensitivity of detection but detected the greatest number (16) of farms with negative serology. The 3H3 Mab that essentially detects IgG2 gave very few false positive responses (4), but missed 51% of infected herds.
Comparison of the sensitivity of lgG1 ELISA and MRT Serial dilutions of I 10 positive individual milk samples with negative milk were tested by both M R T and ELISA. Fig. 2 illustrates the ratio of the last dilution positive by ELISA compared to MRT. Only six milk samples exhibit the same titre in both tests. The mean titre ratio was 22 and the highest increase in sensitivity observed by ELISA for 12 samples was 81. No milk was IgG 1 ELISA negative and MRT positive. The highest dilution found positive was 1/2187, for only one sample in M R T and for 11 samples in ELISA. The increase of sensitivity was not significantly influenced by the method of preservation. 50
4O ..d tl zs
