Rapid identification of Mycobacterium avium complex in culture using DNA probes

A commercial DNA probe procedure for identification of Mycobacterium avium complex isolates was evaluated for accuracy and applicability for use in the clinical laboratory. The test (Gen-Probe Rapid Diagnostic System for Mycobacterium Avium Complex; Gen-Probe Corp., San Diego, Calif.) uses hybridization in solution of two 125I-labeled cDNA probes. One probe is complementary to rRNA from M. avium, and the other is complementary to rRNA from M. intracellulare. Results are expressed as absolute percent hybridization, with values greater than or equal to 10% considered positive. The procedure accurately identified all 134 M. avium complex isolates and concomitantly identified them to species level. There were no false-positives with 66 other mycobacteria, including 22 M. tuberculosis and 18 M. kansasii isolates, or with 8 Nocardia isolates. The mean percent hybridization (+/- the standard deviation) of M. avium probe-positive isolates (94 isolates) was 48.0 +/- 9.9 (range, 11.5 to 72.7); for M. intracellulare (40 isolates), it was 45.7 +/- 8.8 (range, 22.7 to 60.7). Among the 74 non-M. avium complex isolates, the percent hybridization range was 1.0 to 4.2, except for a single value of 9.7 which was less than 5 when the test was repeated. Four M. avium complex isolates reacted positively with both probes on initial testing, and three were confirmed. On repeat testing of subcultures, each reacted with only one probe, suggesting the presence of a mixed culture. The procedure can be completed in as little as 2 h and could easily be performed in most clinical laboratories.