Comparison of sensitivity and specificity of PCR and sputum smear compared to culture in diagnosis of tuberculosis

Molecular detection has recently been proposed by nucleic acid amplification, known as polymerase chain reaction (PCR). The aim of this study was to compare the diagnostic method of smear and polymerase chain reaction with culture in terms of sensitivity, specificity, positive and negative predictive value in the diagnosis of pulmonary tuberculosis.

In this cross-sectional study, sputum samples were collected from 58 patients with suspected pulmonary tuberculosis referred to Ghaem Hospital in Mashhad from the beginning of April 2017 to the end of March 2018. The samples were delivered to the laboratory in less than 72 hours. Patients were sampled for three times. Bronchoscopy and Broncho alveolar lavage were performed in patients who were unable to produce sputum. The smear test was reported by Ghaem’s Laboratory after 24 hours. In our study, the culture method was considered as the gold standard and the sensitivity and specificity of the PCR methods and smear were compared with it.

Patients ranged in age from 18 to 89 years. Among 58 suspected pulmonary tuberculosis, the method of cultivation confirmed the presence of the disease in 25 cases (43.1%). However, with smear, the presence of the disease has been proved in 27 patients (46.6%) and with the method of PCR in 24 patients was (41.4%). Sensitivity of smear in the diagnosis of pulmonary tuberculosis was (100%), and its specificity was 93.9%, the positive predictive value of this test was (92.6%) and the negative predictive value was (100.0%). The sensitivity of the PCR method in diagnosis of pulmonary tuberculosis was 88.0% and its specificity was 93.9%. The positive predictive value of this was (91.7%) and the negative predictive value was (91.2%).

In this study, between the two methods of smear and polymerase chain reaction, the acid fast smear method was more sensitive to the diagnosis of pulmonary tuberculosis than the polymerase chain reaction and the specificity of both methods were the same.