ALGORITHM FOR USE OF NUCLEIC-ACID PROBES FOR IDENTIFYING MYCOBACTERIUM-TUBERCULOSIS FROM BACTEC 12B BOTTLES

Nucleic acid probes (Gen-Probe, San Diego, Calif.) can be used to iden tify mycobacteria in BACTEC 12B broth cultures prior to detection of g rowth on solid media. We developed an algorithm that can be used to ma ke an initial choice of a probe (either Mycobacterium tuberculosis com plex [MTB] or M.avium complex [MAC]) for use in testing respiratory sp ecimens. The algorithm was based on both the fluorochrome smear result of the concentrated specimen and the time from inoculation until the BACTEC 12B broth culture is flagged (growth index 10) as presumptively positive. The MTB probe is used first for all 4+ smear specimens, 3smear specimens positive in 5 days, 2+ and 1+ smear specimens positive in 7 days, and smear-negative specimens positive in 11 days. The MAC probe is used for all other specimens. The algorithm is used when othe r information about the culture (e.g., previous positive cultures and colonial morphology of growth on solid media) is unknown. Use of the a lgorithm to probe 102 respiratory BACTEC 12B broth cultures (35 with M TB; 1 with MTB, MAC, and M.gordonae; 47 with MAC; and 19 with other my cobacterial species) from 1 September through 30 November 1992 resulte d in the initial use of the MTB probe for 35 (97%) of the cultures pos itive for MTB and the use of the MAC probe for 35 (73%) of the culture s positive for MAC. Use of the algorithm aided in the efficient use of laboratory resources without delaying the time to identification of M TB isolates.