Background: There is a need for rapid diagnosis of pulmonary tuberculosis. We have previously used a PCR to detect circulating Mycobacterium tuberculosis DNA in blood samples from patients (mostly HIV-infected) with pulmonary tuberculosis. We have now prospectively investigated the role of this blood-based PCR assay for diagnosis of this disease in a clinical setting.
Methods: Our PCR assay is specific for the IS6110 insertion element of the M tuberculosis complex of organisms. We used it to test peripheral blood from 88 consecutive patients admitted to a chest ward with suspected pulmonary tuberculosis. Personnel who carried out the assay did not know the results of any clinical investigations and ultimate diagnosis, and clinicians did not know the PCR results. Results of the PCR assay were compared with the final clinical diagnosis. A subgroup of 15 patients had blood samples assayed serially to track the PCR signal over time.
Findings: 41 patients had a final clinical diagnosis of tuberculosis, and the cases were typical of those seen at our hospital: HIV infection was common, and most cases were not sputum-smear positive for acid-fast bacilli. The PCR assay correctly identified 39 of 41 patients with proven pulmonary tuberculosis, 26 (63%) of whom were sputum-smear negative. There were five patients in whom a positive PCR result did not accord with the final clinical diagnosis, and two of the 44 negative PCR results were classified as false negatives. The overall sensitivity and specificity of the PCR assay for a diagnosis of tuberculosis was 95% and 89%, respectively. In 15 patients with pulmonary tuberculosis and a positive blood assay,the PCR result remained positive after 1 month of therapy, but had reverted to negative in 13 of the 15 by 4 months of therapy.
Interpretation: We conclude that peripheral-blood-based PCR detection for the diagnosis of tuberculosis is a technically feasible approach that has a potentially important role in the diagnosis of pulmonary tuberculosis.