Quality of sputum in the performance of polymerase chain reaction for diagnosis of pulmonary tuberculosis

SETTING: faster alternative techniques are required to improve the diagnosis and control of pulmonary tuberculosis. OBJECTIVE: To evaluate the sample quality in the performance of PCR for diagnosis of pulmonary tuberculosis. METHOD: during one year, sputum samples were collected from 72 pulmonary tuberculosis patients and 12 non-tuberculosis controls, which were admitted to the Nereu Ramos hospital, Florianópolis city, Brazil. The samples were subjected to Ziehl-Neelsen-stained sputum smear microscopy and Lowestein-Jensen medium culture, which were defined as gold standard tests for mycobacteria, and polymerase chain reaction (PCR). Those samples that presented more than 40% of viable cells and less than 25% of epithelial cells were defined as high quality samples. RESULTS: PCR showed sensitivity of 55.6%, specificity of 41.7%, positive predictive value of 85.1%, negative predictive value of 13.5%, and accuracy of 53.6%. High quality samples showed sensitivity of 72.4%, specificity of 50%, positive predictive value of 91.3%, negative predictive value of 20%, and accuracy of 69.7%. Low quality samples showed sensitivity of 44.2%, specificity of 37.5%, positive predictive value of 79.2%, negative predictive value of 11.1%, and accuracy of 43.1%. CONCLUSION: use of high quality samples improved significantly the PCR performance, especially on their sensitivity and positive predictive values.