224 TRANSACTIONS OF THE ROYAL SOCIETY OF TROPICAL MEDICINE AND HYGIENE (1992) 86, CORRESPONDENCE
Proper requirements for the QBC@ malaria test Baird and colleagues(1992: Transactions, 86, 3-5) attempted to evaluatethe QBC@malaria test for the diagnosis of malaria in the field. They stated that the technique compared poorly to the standard thick film method. However, the authors usedimproper equipment and technique, thereby rendering their conclusions invalid. Baird et al. (lot. cit.) indicated that they used a standard clinical bench top centrifuge, with a relative centrifugal force (RCF) of 2000-3000 g. The QBC@technique requires the use of a centrifuge that can generatean RCF of 14 387 g for 5 min. Under no circumstances,either in the field or in the laboratory, could good results have beenobtained using the slower speedcentrifuge. Baird et al. also stated that they used a 45x objective lens with immersion oil. The QBC@technique requires the useof a final magnification of at least 500x. Sincethe parasites are observed through the capillary tube, it is crucial that the lens be an oil immersion type and have a long working distance. The 45x lens that the authors used was a high-dry objective not meant to be used with oil. The use of a high-dry objective lens with immersion oil would introduce aberrations so severethat the value of the image would be severely degraded (Barer, R., 1953: Lecture Notes on the Useof the MicroscoPe. Oxford: Blackwell Scientific Publications). This error would also account for the need to have a completely dark room. Studies conducted in Tanzania, Sao Tome, and Venezuela with the Paralens@microscope adaptor (Polsuwan et al., 1992: Transactions, 86, 107) all have been performed indoors without the need for a darkened room. This is significant since the light source for the microscopeadapter is not asbright asthe mercury vapour bulb commonly found in a large researchfluorescencemicroscope. In all of these settings the investigators have been finding at least 20% more positive casescompared with thick films. most of thesebeing low level oarasitaemiasin asymptomatic persons. All OFthese investigations were conducted in very harsh climatic conditions without the problems describedby Baird et al. Ienorina either of the above2 reouirements would signifi~antly yeduce the sensitivity of the QBC@technique; ignoring both renders the technique virtually useless. The authors also used QBC@tubes that were designed to obtain a complete blood count, not for the diagnosisof malaria. The QBC@tubes used for deriving a complete blood count have an antibody coating in the tube together with a carrier protein; it is this protein that caused the stain precipitate referred to by Baird et al. This antibody and carrier protein are not present in the QBC@malaria tubes. These tubes were never intended to be marketed for the diagnosisof malaria, hence the constraints printed on the box regarding temperature and humidity. QBC@malaria tubes are dated for 2 years and can be stored and usedat temperaturesashigh as37°Cin 95% relative humidity. Any diagnostic procedure demands that the user follow appropriate procedure. Even the use of thick films demandsproper pH and staining times. Given the above grosserrors in technique and the lack of basicknowledge of light microscopy and theory of centrifugation, it is not surprising that Baird et al. had poor results with the QBC@malaria technique. Joseph B. Perrone Becton Dickinson Tropical Disease Diagnostics I Becton Drive Franklin Lakes IVVAJersey 07417 1 April 1992
Multiresistant Salmonella typhjinfection in Cairo Reports of typhoid fever due to multiresistant SaZmonells &hi infection have recently outlined the problems of diannosisand treatment. Avvaaari & Pal (1991: Transactio& 85, 302) reported from India an outbreak of multiresistant S. typhi infection in 34 patients. These authors, however, stated that infection with S. typhi resistant to chloramphenicol, ampicillin and co-trimoxazole was quite rare. In our opinion this infection is not asrare as theseauthors suggested.During the past 6 months we in Cairo havediagnosed4 patients, referred to us for prolonged undiagnosed fever of 2-3 months duration, as having enteric fever; blood cultures from all 4 patients grew S. typhi resistant to chloramphenicol, ampicillin, and co-trimoxazole. We stress the importance of recognizing the possibility of multiresistant S. typhi infection in patients with prolonged fever not responding to standard chemotherapy. Quinolonesmay be the drug of choice in the future (Cheesbrough, J. S., et al., 1991: Lancet, 338,_ 127). , Z . Farid US Naval Medical Research Unit No. 3 cloAmeticanEmbassy Cairo 3 September 1991 &et
Proper requirements for the QBC@ malaria test Baird and colleagues(1992: Transactions, 86, 3-5) attempted to evaluatethe QBC@malaria test for the diagnosis of malaria in the field. They stated that the technique compared poorly to the standard thick film method. However, the authors usedimproper equipment and technique, thereby rendering their conclusions invalid. Baird et al. (lot. cit.) indicated that they used a standard clinical bench top centrifuge, with a relative centrifugal force (RCF) of 2000-3000 g. The QBC@technique requires the use of a centrifuge that can generatean RCF of 14 387 g for 5 min. Under no circumstances,either in the field or in the laboratory, could good results have beenobtained using the slower speedcentrifuge. Baird et al. also stated that they used a 45x objective lens with immersion oil. The QBC@technique requires the useof a final magnification of at least 500x. Sincethe parasites are observed through the capillary tube, it is crucial that the lens be an oil immersion type and have a long working distance. The 45x lens that the authors used was a high-dry objective not meant to be used with oil. The use of a high-dry objective lens with immersion oil would introduce aberrations so severethat the value of the image would be severely degraded (Barer, R., 1953: Lecture Notes on the Useof the MicroscoPe. Oxford: Blackwell Scientific Publications). This error would also account for the need to have a completely dark room. Studies conducted in Tanzania, Sao Tome, and Venezuela with the Paralens@microscope adaptor (Polsuwan et al., 1992: Transactions, 86, 107) all have been performed indoors without the need for a darkened room. This is significant since the light source for the microscopeadapter is not asbright asthe mercury vapour bulb commonly found in a large researchfluorescencemicroscope. In all of these settings the investigators have been finding at least 20% more positive casescompared with thick films. most of thesebeing low level oarasitaemiasin asymptomatic persons. All OFthese investigations were conducted in very harsh climatic conditions without the problems describedby Baird et al. Ienorina either of the above2 reouirements would signifi~antly yeduce the sensitivity of the QBC@technique; ignoring both renders the technique virtually useless. The authors also used QBC@tubes that were designed to obtain a complete blood count, not for the diagnosisof malaria. The QBC@tubes used for deriving a complete blood count have an antibody coating in the tube together with a carrier protein; it is this protein that caused the stain precipitate referred to by Baird et al. This antibody and carrier protein are not present in the QBC@malaria tubes. These tubes were never intended to be marketed for the diagnosisof malaria, hence the constraints printed on the box regarding temperature and humidity. QBC@malaria tubes are dated for 2 years and can be stored and usedat temperaturesashigh as37°Cin 95% relative humidity. Any diagnostic procedure demands that the user follow appropriate procedure. Even the use of thick films demandsproper pH and staining times. Given the above grosserrors in technique and the lack of basicknowledge of light microscopy and theory of centrifugation, it is not surprising that Baird et al. had poor results with the QBC@malaria technique. Joseph B. Perrone Becton Dickinson Tropical Disease Diagnostics I Becton Drive Franklin Lakes IVVAJersey 07417 1 April 1992
Multiresistant Salmonella typhjinfection in Cairo Reports of typhoid fever due to multiresistant SaZmonells &hi infection have recently outlined the problems of diannosisand treatment. Avvaaari & Pal (1991: Transactio& 85, 302) reported from India an outbreak of multiresistant S. typhi infection in 34 patients. These authors, however, stated that infection with S. typhi resistant to chloramphenicol, ampicillin and co-trimoxazole was quite rare. In our opinion this infection is not asrare as theseauthors suggested.During the past 6 months we in Cairo havediagnosed4 patients, referred to us for prolonged undiagnosed fever of 2-3 months duration, as having enteric fever; blood cultures from all 4 patients grew S. typhi resistant to chloramphenicol, ampicillin, and co-trimoxazole. We stress the importance of recognizing the possibility of multiresistant S. typhi infection in patients with prolonged fever not responding to standard chemotherapy. Quinolonesmay be the drug of choice in the future (Cheesbrough, J. S., et al., 1991: Lancet, 338,_ 127). , Z . Farid US Naval Medical Research Unit No. 3 cloAmeticanEmbassy Cairo 3 September 1991 &et
