A colorimetric method for quantitative measurement of the susceptibility of Mycobacterium tuberculosis to antimicrobial agents is described. The method utilizes an oxidation-reduction dye, Alamar blue, as an indicator of growth. By this method, MICs of isoniazid, rifampin, streptomycin, and ethambutol were determined for 50 strains of M. tuberculosis. Colorimetric MIC results were available on the 7th, 10th, or 14th day of incubation for 29 (58%), 14 (28%), and 7 (14%) of the 50 strains, respectively. When MIC susceptibility results were compared with results obtained by the agar proportion method, increased levels of resistance detected by agar proportion were associated with higher MICs obtained by the colorimetric method. Tentative interpretive criteria for colorimetric MIC results which showed good agreement with results obtained by the agar proportion method were established. Interpretive agreement between the two methods was 98% for isoniazid, rifampin, and ethambutol and 94% for streptomycin. Overall, there was agreement between the two methods for 194 of 200 test results (97%). The colorimetric method is a rapid, quantitative, nonradiometric method for determining the antimicrobial susceptibility of M. tuberculosis.

Several studies have reported using methods based on polymerase chain reaction (PCR) to detect Mycobacterium tuberculosis in respiratory tract specimens. However, little is known about the actual clinical utility of PCR-based tests, and it is uncertain if PCR technology can be transferred to the clinical laboratory. To determine its utility, we evaluated a commercially developed PCR test system in a clinical laboratory using consecutive respiratory tract specimens. Microscopic examination of smears stained with acid-fast bacilli (AFB), culture, and a PCR-based test (Amplicor Mycobacterium tuberculosis assay; Roche Molecular Systems) were used to evaluate 535 consecutive sputum and bronchoalveolar lavage specimens from 227 patients. A clinical case definition of tuberculosis was used as the reference-standard to determine the utility of all diagnostic tests. For all specimens from patients with a new or a treatment-failure case of pulmonary tuberculosis, the positivity rate of PCR (58%) was similar to that of culture (56%) (p > 0.90) and substantially greater than microscopic examination of AFB-stained smears (22%) (p < 0.001). PCR and culture detected M. tuberculosis in 46 and 43%, respectively, of the specimens from patients who did not have AFB on microscopic examination of their respiratory tract specimens (p > 0.90). PCR had a false positive rate of 0.8%. In several instances, PCR detected M. tuberculosis when culture did not; and vice versa. The clinical utility of this PCR-based test is similar to that of culture for detecting M. tuberculosis in respiratory tract specimens.(ABSTRACT TRUNCATED AT 250 WORDS)

The value of the smear for acid-fast bacilli in predicting pulmonary tuberculosis is unclear in a setting where there is a high prevalence of Mycobacterium avium complex in respiratory specimens. To evaluate the impact of a high prevalence of M. avium complex on the predictive value of the acid-fast bacilli smear for tuberculosis, we reviewed findings on smears and results of cultures over a 3-year period at a hospital where M. avium complex is the predominant mycobacterial isolate. In this setting, the predictive value of the acid-fast bacilli smear for Mycobacterium tuberculosis was 92% for expectorated sputum specimens, 71% for induced sputum specimens, and 71% for bronchoalveolar lavage specimens. When multiple specimens collected from the same patient were excluded from the data base, the predictive values were 87%, 70%, and 71%, respectively. Smears of sputum samples were positive at the same rate for patients with tuberculosis who had AIDS and for patients with tuberculosis who did not have AIDS.