Identification of Mycobacterium gordonae from culture by the Gen-Probe Rapid Diagnostic System: evaluation of 218 isolates and potential sources of false-negative results.

The Mycobacterium gordonae Rapid Diagnostic System (Gen-Probe, Inc., San Diego, Calif.) was evaluated for sensitivity and specificity as well as for its application in the mycobacteriology laboratory. An 125I-labeled cDNA probe complementary to rRNA was employed. Hybridization of greater than or equal to 10% was considered positive. A total of 218 mycobacterial isolates, including 159 isolates of M. gordonae, were tested. Under optimum conditions, the specificity and sensitivity of the probe were 100 and 98.7%, respectively. A number of discrepancies were observed between the probe and conventional biochemical results in one laboratory. Further studies, designed to resolve these discrepancies, revealed a number of potential technical pitfalls. Hybridization incubation temperatures that varied from the manufacturer's recommended optimum, culture suspensions below the density of a no. 1 McFarland nephelometer standard, and extended storage times of culture suspension all adversely affected the final hybridization values. Additionally, it was determined that in one laboratory incorrect functioning of the sonicator caused false-negative hybridization values. The manufacturer's recommendations should be strictly followed, and the performance of the sonicator should be checked on a scheduled basis. Results show that the probe will allow fast and accurate identification of M. gordonae, thus eliminating time-consuming biochemical testing of this organism.