FLOW CYTOMETRIC TESTING OF SUSCEPTIBILITIES OF MYCOBACTERIUM-TUBERCULOSIS ISOLATES TO ETHAMBUTOL, ISONIAZID, AND RIFAMPIN IN 24 HOURS

Susceptibility testing of Mycobacterium tuberculosis is seriously limi ted by the time required to obtain results. We show that susceptibilit y testing of clinical isolates of M. tuberculosis can be accomplished rapidly with acceptable accuracy by using flow cytometry, The suscepti bilities of 35 clinical isolates of M. tuberculosis to various concent rations of isoniazid, rifampin, and ethambutol were tested by the agar proportion method and by flow cytometry, Agreement between the result s from the two methods was 95, 92, and 83% for isoniazid, ethambutol, and rifampin, respectively. Only 11 discrepancies were detected among 155 total tests. The results of bow cytometric susceptibility tests we re available within 24 h of inoculation of drug-containing medium, whi le the proportion method required 3 weeks to complete. The flow cytome tric method is also simple to perform.