BMC Public Health 2007, 7:356 http://www.biomedcentral.com/1471-2458/7/356 used in parallel with a high clinical suspicion of SNPTB, ranged from 98 to 99%, among HIV seropositive and HIV seronegative subjects. In Health Units, in which the prev- alence ranges from 15% to 20%, usually in General Hos- pitals or Ambulatory Reference Centers (TB Clinics), negative PV of this diagnostic strategy ranged from 96% to 97%. In Reference TB Hospitals where the TB prevalence ranges from 30% to 40%, among HIV seronegative indi- viduals, NPV of PCR colorimetric dot-blot, used in paral- lel with a high clinical suspicion of SNPTB, ranged from 95% to 92%, and among HIV seropositive individuals, this figure was of 94% and 91%, respectively (Figure 1). The median time to reveal growth of MTB was 30 days (interquartile range [IQR] 30 to 45) for culture and the median time for detection of MTB by PCR was 3.32 days (IQR 3.0 to 3.75), respectively (p < 0.01). Within one week, of the 30 positive cultures, none were positive, whilst PCR detected 93% of positive specimens (p < 0.01). Discussion We evaluated the performance of tests in SNPTB suspects. The strengths of this study, carried out in a developing country, included: a) a large number of SNPTB suspects, b) comparison were made according to HIV status, history of previous anti-TB treatment, the different levels of clini- cal suspicion and pretest probability and, c) the prospec- tive design, ensuring a more complete clinical, laboratory, and radiographic information. Individuals were carefully characterized by independent reviews to determine the final diagnosis. In this study, performed at the Hospital Reference Center, we observed a high prevalence of active PTB (48.8%) and TB/HIV co-infection (29.8%), confirming the epidemio- logical data described by the TB Control Program of Porto Alegre City, where 29% of all new TB cases reported yearly are diagnosed in hospitals and where there is a high TB and HIV burden [23]. The prevalence of PTB for the high pretest probability group was 67.4%, lower than that described in the litera- ture where smear-positive and smear-negative PTB were included in analyses, the prevalence of PTB for intermedi- ate pretest probabilities was 24%, similar to that (29%) described by Catanzaro, but higher than that described by Lim (3.4%). PTB prevalence for low pretest probability was 7.5%, higher than that cited elsewhere with low bur- dens of TB and HIV infection in health settings [24-26]. The moderate PCR sensitivity and specificity in SNPTB suspects (sensitivity: 65%; specificity: 83%) observed were similar to the sensitivities (61%–83%) and specifici- ties (84%–92%) described by others [11,13,14]; however the specificities obtained were lower than those described in industrialized countries (>95%) [27]. As mentioned by others, in this report, the sensitivity of PCR was not influ- enced by the HIV status of the patient [12,28]. Decreased sensitivities of new diagnostic tests are expected among smear-negative pulmonary TB cases. In this study, this may be due to: 1) the presence of inhibitors that remain in the specimen after the extraction procedure; 2) a small number of mycobacteria unequally distributed in the test suspension; 3) a mycobacterium level that is below the detection limit of in house PCR (50 CFU) [16]. The proportion of inhibitors was 2.3%, and this result was lower than that reported by other studies using home- made PCR (22.7%) and similar to those using automated NAA (0.85%–5.0%) [17,27,29]. Twenty-three specimens presented less than 50 CFU in culture, which is below the detection limit of the test. Partial loss of mycobacterial homogeneity, leading to unequal distribution in the test suspension, may be due to the division of the suspension into three aliquots for use in laboratory tests. Addition- ally, a potential source of decreased sensitivity may be the use of the IS6110 insertion element as the target for PCR, since MTB can present low copy numbers of the element. Meanwhile, DNA fingerprinting studies, performed in Brazil, did not find the presence of these strains, as men- tioned by Sperhacke et al.[16]. The decrease in specificity was due to eighteen false-posi- tive results from patients that referred to previous anti-TB treatment, thus it is not surprising that DNA could be detected in their respiratory specimens, however the period elapsed between the end of previous anti-TB treat- ment in these patients and the reported positive PCR anal- yses was not collected, and this is a limitation of this study. Using a high pretest probability as a diagnostic test for SNPTB diagnosis, this test had a sensitivity of 72% and a specificity of 86%; these values were similar to those observed with culture and PCR. These low accuracies can be explained by the fact that the evaluation of patients was performed by young or less experienced physicians, as described by LIM et al.[30]. The large disparity in sensitiv- ities found when the clinical suspicion (pretest probabil- ity) is used as a diagnostic test, can be explained due to: a) the classification in risk categories based on based on clin- ical data, b) different years of experience in the SNPTB diagnosis of physicians that evaluated the patients. There- fore, when a high, intermediate and low probability pre- test is used as a diagnostic test, patients with high risk had sensitivities higher than those of patients with intermedi- ate and low risk, because these patients had more symp- toms compatible with SNPTB. Page 7 of 10 (page number not for citation purposes)