Evaluation of direct microscopy as a screening test in the diagnosis of pulmonary tuberculosis

A 'false case' in reference to pulmonary tuberculosis is designated as one where, the primary smear shows presence of Acid Fast Bacilli (AFB). However, the subsequent cultural examination fails to grow the pathogen. The present study is directed to determine the incidence of false positivity and attempt its correlation with the number of bacilli in the primary smear, age of the patient and chemotherapy. Of the 820 sputum samples processed, 14.63 percent revealed presence of bacilli both in smear and by cultural examination (true positive), 64.02 percent were true negative (smear and culture negative), 15.97 percent were false negative (smear negative culture positive) while 5.36 percent displayed false positivity. The data analysis has further revealed an inverse relationship between the number of bacilli in the entire smear and false positivity. Aged patients (more than 40 years) who were on prolonged and irregular antitubercular therapy showed a higher incidence of false positivity (50 percent) as compared to others. The present study has indicated that primary scrutiny of the smear should be done with due care and a due consideration should be given to clinical presentation, radiological findings and account of chemotherapy while assessing the prognosis. We further recommend that every sample should be simultaneously processed for cultural examination to avoid the false positivity, if any.