BMC Public Health 2007, 7:356 http://www.biomedcentral.com/1471-2458/7/356 specificity of 86% (CI 95%:78%–92%). Using the PCR dot-blot in parallel with high pretest probability as a diagnostic test, sensitivity, specificity, positive and negative predictive values were: 90%, 71%, 75%, and 88%, respectively. Among non-previously TB treated and HIV subjects, this approach had sensitivity, specificity, positive and negative predictive values of 91%, 79%, 81%, 90%, and 90%, 65%, 72%, 88%, respectively. Conclusion: PCR dot-blot associated with a high clinical suspicion may provide an important contribution to the diagnosis of SNPTB mainly in patients that have not been previously treated attended at a TB/HIV reference hospital. Background Tuberculosis (TB) is one of the most widespread mortal- ity-causing infectious diseases in humans. Timely detec- tion of the disease allows the institution of an effective and life-saving treatment, thereby reducing transmission to close contacts. Conventional diagnosis of Pulmonary Tuberculosis (PTB) is time-consuming, and the acid fast bacilli (AFB) smear has a low sensitivity (40%–60%) [1]. HIV infection has been associated with an increased inci- dence of smear negative pulmonary tuberculosis (SNPTB) and a higher mortality rate in TB patients. [2-4]. In Brazil, almost 30% of PTB cases among adults are SNPTB [2-4]. Diagnosis of SNPTB is a difficult task and, in developing countries, the majority of these cases have been treated only on the basis of clinical and chest radiographic find- ings. Without a standardized clinical work up, the misdi- agnosis rates have been estimated to be as high as 35% [5]. Therefore, in settings with a high rate of TB and HIV, the clinical evaluation of new tools for smear negative PTB diagnoses is extremely valuable [1,6] In industrialized countries, tests for Mycobacterium tuber- culosis (MTB), using rapid nucleic acid amplification (NAA), have been considered a major breakthrough in the diagnosis of PTB [7]. In developing countries, the in house polymerase chain reaction (PCR) for amplification of MTB DNA, using the IS6110 insertion element as a target, offers a potentially sensitive, specific and low-cost test that could provide a rapid diagnosis of PTB [8-11]. In these settings, the published evaluations of NAA tech- niques for smear negative PTB diagnosis have been based mainly on laboratory criteria for diagnosis of disease with or without clinical records used to evaluate discrepant results [11-17]. In the present study, we investigated the performance of a home-made colorimetric PCR (PCR dot-blot) to diagnose TB using expectorated sputum from patients suspected of having SNPTB, in isolation and in parallel with pretest probability (based on Clinical Suspicion) in a hospital setting with a high burden of TB and HIV. The PCR tech- nique performance was compared with conventional rou- tine diagnostic methods for smear negative patients. Methods Setting and patient selection Consecutive adults suspected of having SNPTB, referred to the TB and HIV Reference Center, Parthenon Reference Hospital (PRH) in Porto Alegre City, capital of Rio Grande do Sul, State of Brazil, were studied prospectively, from May 2003 to May 2004. SNPTB suspects were referred from community health care units to have their respira- tory specimens cultured for mycobacteria, according to Brazilian National Guidelines [18]. Eligible patients were those: (1) who reported more than 3 weeks of cough; (2) who had two consecutive samples of spontaneous sputum that were acid fast bacilli smear- negative. Patients illegible were those receiving anti-TB treatment. Patients with a history of previous TB were not excluded. Patients were excluded from the study if any of the following conditions were met: (1) culture was con- taminated; (2) when expectorated sputum was not obtained (3) laboratory or clinical data did not fulfill the SNPTB definition; (4) written informed consent was not obtained from the study participant. All clinical samples were sent to the Laboratory of the State of RS, State Foun- dation for Research in Health, Porto Alegre/RS/Brazil, (FEEPS/Lacen/RS) for laboratory analysis. This study was approved by the Institutional Review Boards of FEPPS/RS (n. 01/2002). Suspects of SNPTB, after signing their written informed consent, underwent a validated questionnaire with ques- tions regarding demographic variables and clinical history (e.g.: smoking, alcohol abuse, HIV infection/AIDS)[19]. Chest radiographs and physical examination was per- formed by a respiratory specialist using a standardized form. Respiratory specialists were blinded for the results of cultures and PCR dot-blot, and laboratory technicians were blinded for the chest radiographs results and clinical predictors. HIV testing by ELISA was performed, using Western blot as a confirmatory test. Estimate of pretest probability To estimate pretest probability (clinical suspicion), all eli- gible individuals were classified into three relative risk cat- egories during the first appointment: low (≤ 25%); Page 2 of 10 (page number not for citation purposes)