Peripheral blood based C-PCR assay for diagnosing extra-pulmonary tuberculosis

Extra pulmonary tuberculosis (EPTB) constitutes around 20% of all tuberculosis cases in India. Conventional methods are of limited use in diagnosing this form of the disease. Polymerase chain reaction (PCR) has emerged as a sensitive and specific tool for documenting the presence of Mycobacterium tuberculosis in clinical samples but lacks quantitative ability. The present study evaluates peripheral blood as an alternative clinical specimen for diagnosing EPTB. Peripheral blood samples from 38 EPTB and 89 non tuberculous subjects were analyzed for the presence of tubercle bacilli by MPB 64 gene based PCR method. The assay gave an overall sensitivity of 60.53% with negative predictive value of 76.92% which is superior to present gold standard of mycobacterial culture (10.53 and 72.36%). Additionally, 43.82% of non tuberculous subjects gave positive results with the PCR, thus mitigating the clinical utility of this test. An in-house Competitive PCR (C-PCR) assay was used to determine the mycobacterial load in peripheral blood from culture positive, culture negative EPTB patients and non tuberculous controls which ranged from 7498-12498, 602-4797 and 101-800 genome equivalent (ge)/mL, respectively. The data clearly demonstrated that C-PCR assay can furnish insightful information in diagnosing extra pulmonary disease.