BMC Public Health 2007, 7:356 http://www.biomedcentral.com/1471-2458/7/356 The clinical evaluation used in parallel with the PCR test may be an alternative to the use of the PCR test for rapid diagnosis of PTB, especially in a hospital setting with a high burden of TB/HIV co-infection. The combination of clinical judgment and amplification results strongly enhances a rapid and correct diagnosis of PTB [26]. In this study, when we used PCR in parallel with high pre- test probability, the diagnostic appeared to offer a higher negative predictive value in SNPTB subjects that had not been previously treated and in HIV seronegative cases, as described by others [24,25,30]. In non-previously treated and HIV seronegative cases, the performance results (SE: 93%; NPV: 92%) were similar to those recently described by Piersimoni et al, using automated tests in 214 PTB sus- pects [26]. Due to the small number of active TB in the group with the low pretest probability, additional evaluation is war- ranted in order to analyze the appropriateness of the par- allel use of the PCR technique in this group of patients. However, the most difficult group for clinical assessment is the intermediate risk group, where PCR, used in parallel with the intermediary pretest (Clinical Suspicion), appears not to be useful, as already suggested by others. The prevalence of PTB may be overestimated in the inter- mediate risk group, thus the utility of PCR assay in these patients needs further evaluation, using more accurate clinical selection criteria [25,30]. The PCR dot-blot was selected due to its low cost (around U$12), simple extraction method and the colorimetric end point, all factors that might be expected to facilitate the transfer of NAA tests to laboratories in low income countries [28,31]. Additionally, as clinical risk assessment is more likely to reflect physician decision-making, to our knowledge, this is the first prospective study that relates pretest probability with the performance of a PCR in con- secutive patients suspected of having SNPTB, in South America. In our study, we pursued, in a large number of smear neg- ative pulmonary TB suspects, a comprehensive clinical and laboratory approach for TB diagnosis using a home- made PCR, as suggested by Flores et al[32]. Due to the het- erogeneity in the test's accuracy, it was emphasized the necessity to incorporate the clinical information for the better evaluation of NAAs in TB diagnosis among smear negative cases [32,33]. Another possible study limitation was the use of a home- made PCR that may warrant validation in comparison with more reliable techniques, such as an automated standardized test, as described by Greco et al.[33]. Unfor- tunately, an automated test was not available, therefore, the IS6110 element of insertion was used as a target for PCR; as recent meta-analyses demonstrated its higher accuracy in the diagnosis of SNPTB [11]. Differing data in literature may be explained due to some factors. Firstly, few studies have evaluated the utility of the home-made PCR technique among SNPTB suspects in developing nations with a high TB and HIV burden. Secondly, those studies that measured the clinical risk assessment were performed in settings with different TB prevalence [24,25]. Thirdly, clinical judgment and experience can influence the pretest probabilities, interfering with the sample size in each clinical risk group. Finally, the preva- lence of co-morbidities (i.e.: HIV infection) of mycobacte- ria other than tuberculosis (MOTT) disease and the patient's response to interview may differ according to their prevalence in the community. Conclusion The PCR dot-blot used in parallel with the high probabil- ity pretest has a high negative predictive value suggesting that in a hospital setting in developing countries, with a high prevalence of TB and HIV, the PCR technique may be useful for the evaluation of SNPTB suspects. For example, when the pretest probability is high, a negative PCR result indicates an increased likelihood of the absence of active TB in SNPTB suspects, infected or not by HIV. We conclude that our results are in agreement with those of the literature, showing that molecular methods may provide an important contribution to the diagnosis of SNPTB in patients with high clinical suspicion [24,26]. Since home-made PCR is less costly than automated NAA, this test could be introduced more widely after a proper evaluation of its cost-effectiveness with clinical and radio- graphic characteristics to refine estimates of likelihood of TB disease in different settings, as proposed by others [28,34]. Abbreviations AFB – Acid Fast Bacilli MOTT – Mycobacterium Other Than Tuberculosis MTB – Mycobacterium tuberculosis NAA – Nucleic Acid Amplification PCR – Polymerase Chain Reaction PTB – Pulmonary Tuberculosis SNPTB – Smear Negative Pulmonary Tuberculosis PRH – Parthenon Reference Hospital Page 8 of 10 (page number not for citation purposes)