Predicting culture results for Mycobacterium Tuberculosis complex: Amplified mycobacterium tubercolosis direct test and acid-fast bacilli microscopy

Study objective: To evaluate the usefulness of the Gen-Probe Amplified Mycobacterium Tuberculosis Direct Test (AMTDT; Gen-Probe; San Diego, CA) in predicting the results of cultures in routine laboratory analysis of a patient population with a high incidence of tuberculosis (TB). Patients: Three hundred ten patients suspected of pulmonary mycobacterial infection or receiving antituberculous chemotherapy, accrued between 1996 and 1997. Setting: Tertiary-care facility located in Northern Italy. Design: We retrospectively compared the AMTDT results with the results of cultures. AMTDT results were also compared with those of acid- fast bacilli (AFB) staining of the same specimens. The study included 360 respiratory specimens from 310 patients collected between 1996 and 1997. In 1996, we used the initial version of AMTDT (50 μL of sediment); in 1997, we used the new version of AMTDT (450 μL of sediment). Results: Compared with cultures, AMTDT and AFB staining had sensitivities of 87.2% and 68.4%, and specificities of 70.0% and 89.7%, respectively. When AMTDT and AFB staining were both positive, the sensitivity and specificity were 89.3% and 96.9%, respectively. When AMTDT and AFB staining were in disagreement, the sensitivity and specificity of AMTDT were 81.8% and 18.1%, respectively. Conclusion: We conclude that when AMTDT is used to predict culture outcome, the results should be evaluated in conjunction with AFB staining results before making decisions about TB management.