The Cobas Amplicor MTB test for detection of Mycobacterium tuberculosis complex from respiratory and non-respiratory clinical specimens

The Cobas Amplicor MTB test is a polymerase chain reaction (PCR) technique commonly used for direct detection of Mycobacterium tuberculosis in clinical samples. This assay is only validated for respiratory specimens, but many physicians also request PCR analyses for non-respiratory ones. 877 respiratory and 564 non-respiratory specimens were analysed by this test. Using culture results as standard, the sensitivity, specificity, positive (PPV) and negative predictive values (NPV) of the PCR were, respectively, 97.9%, 100%, 100% and 94.4% for smear-positive respiratory specimens, 68.8%, 99.2%, 87.5% and 97.5% for smear-negative respiratory samples, 57.8%, 98.6%, 78.8% and 96.4% for all non-respiratory specimens, and 42.4%, 98.6%, 66.7% and 96.4% for smear-negative non-respiratory specimens. 154 cerebrospinal fluid samples were analysed and the sensitivity, specificity, PPV and NPV were 55.6%, 97.2%, 55.6% and 97.2%, respectively. These results indicate that the Cobas Amplicor MTB test enables detection of tuberculosis in respiratory specimens, but does not perform well enough in non-respiratory specimens. The method fails particularly in cases where a reliable and rapid test is urgently needed, e.g. in tuberculous meningitis.