Rapid drug susceptibility testing of Mycobacterium tuberculosis using conventional solid media

Radiometric methods for M. tuberculosis drug susceptibility testing yield much faster results than standard techniques; however, these methods require sophisticated equipment and are expensive. We investigated a rapid drug susceptibility testing method for isoniazid, rifampin, ethambutol, streptomycin and pyrazinamide in specimens from 197 patients with pulmonary tuberculosis using a simplified agar-dilution method. Middlebrook 7H11 agar solid medium and microcolony detection were used to test sputum from 64 smear-positive, and from 70 culture-positive but smear-negative patients. Culture-positive material from bronchoscopy, surgical biopsy, pleural fluid or gastric fluid of 63 patients was tested. In 64 smear-positive patients, the median time for final susceptibility results was 11 days (95% confidence interval (95% CI) 10-12 days) compared to 62 days (95% CI 56-66 days) with the standard method. In 133 smear-negative patients, results were available after a median of 35 days (95% CI 32-40 days) in contrast to 72 days (95% CI 62-83 days) with the standard method, regardless of whether or not sputum or other materials were used for primary culture. The rapid method detected all cases of single-drug resistance (n = 20) and multidrug resistance (n = 14) within 13 days (95% CI 9-17 days) in smear-positive patients (n = 8), or within 38 days (95% CI 35-48 days) in smear-negative patients (n = 26). Only one discrepancy was encountered in 985 resistance tests. Moreover, contamination was not observed. Our rapid susceptibility testing method for M. tuberculosis on Middlebrook 7H11 agar is fast, practical and inexpensive. It provides an alternative when more sophisticated techniques are not available or affordable.