Performance evaluation of the Cobas TaqMan MTB assay on respiratory specimens according to clinical application

- The performance of the Cobas assay was evaluated according to the likelihood of tuberculosis.
- Significantly different performance was found between the high TB probability group and the low TB probability group.
- The clinical application needs to be considered when interpreting the assay results.

ObjectiveTo evaluate the performance of the Cobas TaqMan MTB assay (Cobas assay) with respect to its clinical application.MethodsThis was a retrospective analysis of 1154 results from 1034 patients for whom mycobacterial cultures and the Cobas assay were performed simultaneously. Based on the patient medical records, two categories of clinical application were defined: (1) the diagnosis of patients with a high probability of pulmonary tuberculosis according to clinical and radiological features (n = 128), and (2) the exclusion of tuberculosis in clinically indeterminate patients (n = 1026). Standard culture was used as the reference method.ResultsThe sensitivity of the Cobas assay for the detection of Mycobacterium tuberculosis was 70.4% (95% confidence interval (CI) 49.7-85.5%) for category 1, but only 25.0% (95% CI 4.5-64.4%) for category 2. The specificity was ≥95.0% for both categories. The positive predictive value was 79.2% (95% CI 57.3-92.1%) for category 1 and 33.3% (95% CI 6.0-75.9%) for category 2, while the negative predictive value was 92.3% (95% CI 85.0-96.4%) for category 1 and 99.4% (95% CI 98.7-99.8%) for category 2.ConclusionsThe results of this study indicate that Cobas assay results must be interpreted carefully according to the clinical purpose of the assay.