The diagnostic usefulness of a DNA probe for Mycobacterium tuberculosis complex (Gen-Probe) in Bactec cultures versus other diagnostic methods.

The combination of the radiometric Bactec system and the Gen-Probe for Mycobacterium tuberculosis complex (MTB) in detection and identification of mycobacteria was evaluated. Löwenstein-Jensen and Coletsos media for isolation, and Bactec-NAP test and cellular morphology and grouping for identification of MTB and mycobacteria other than tuberculosis (MOTT) were also evaluated. The study included all specimens submitted to our laboratory for mycobacteria detection over a 6-month-period. The mean recovery times of MTB were 13.7, 23.3, and 21.2 days for Bactec, Löwenstein-Jensen and Coletsos, respectively. Bactec system recovered 87.8% of MTB strains and 98% of MOTT, and the conventional media 82.9% of MTB and 19.2% of MOTT. Ziehl-Neelsen smears and Bactec-NAP were effective in differentiating MTB and MOTT strains (3.3% and 9.6% respectively of the cultures were uninterpretable). Gen-Probe (cutoff point for MTB = 5% of hybridisation) was applied to 100 positive vials of Bactec. All cultures of MTB with a growth index (GI) > 400 displayed hybridisation > 9% (average 28.9%) but for a GI < 400, 17% of cultures showed < 5% of hybridisation (average 18.6%). All cultures for MOTT had < 5% hybridisation. With the combination Bactec/Gen-Probe the average time of the final report could be reduced to 15.5 days.