Early Diagnosis of Extrapulmonary Tuberculosis by a New Procedure Combining Broth Culture and PCR

The diagnosis of extrapulmonary tuberculosis is difficult because of the paucibacillary nature of these infections. We developed a culture-enhanced PCR assay combining a preliminary step of broth culture in BacT/Alert MP bottles with the subsequent detection of Mycobacterium tuberculosis using the GenoType Mycobacteria Direct test. First, the procedure was applied to 10-fold-diluted suspensions of M. tuberculosis prepared in vitro. These experiments showed that a 15-day incubation time was required to detect bacilli in the suspension, with the lowest inoculum size yielding a single colony on Lowenstein-Jensen slants. The efficacy of culture-enhanced PCR at day 15 was subsequently evaluated with 225 nonrespiratory specimens from 189 patients with suspected tuberculosis. All these specimens were smear negative, and 31 (13.8%) from 27 patients were culture positive. The result of culture-enhanced PCR at day 15 was consistent with final culture results in all specimens tested. Compared to culture results, the sensitivity, specificity, positive predictive value, and negative predictive value were 100%. Four patients with a negative culture and a negative PCR result were diagnosed as having tuberculosis on the basis of histological findings or therapeutic response. When using a positive diagnosis of tuberculosis as a gold standard, the sensitivity, specificity, positive predictive value, and negative predictive value were 88.6%, 100%, 100%, and 97.9%, respectively. These results indicate that culture-enhanced PCR is a highly sensitive and specific method for the early detection of M. tuberculosis in extrapulmonary specimens.