Microscopic-observation drug susceptibility assay provides rapid and reliable identification of MDR-TB

Liquid culture systems are more rapid and sensitive for both the detection and drug susceptibility testing (DST) of Mycobacterium tuberculosis.

St Peter's TB Specialised Hospital and public health laboratory, Addis Ababa.

To compare the microscopic-observation drug susceptibility (MODS) assay with the BACTEC-MGIT 960 system for isoniazid and rifampicin DST (i.e., multidrug-resistant tuberculosis [MDR-TB] identification) of M. tuberculosis.

The evaluation was based on 58 smear- and culture-positive sputum samples from patients diagnosed in Addis Ababa, Ethiopia. BACTEC-MGIT was used as the reference standard.

For the detection of MDR-TB, MODS has a sensitivity, specificity and accuracy rate of respectively 95%, 100% and 98.3% (kappa 0.981, concordance 98.3%). Concurrent culture detection and DST results are obtained in a median of 9 days with MODS, while indirect DST results with BACTEC-MGIT are obtained in a median of 8 days (this does not include time to primary isolate).

MODS is an accurate, rapid and relatively inexpensive method for the identification of MDR-TB.