Interpretive criteria for use of AccuProbe for identification of Mycobacterium avium complex directly from 7H9 broth cultures

Rapid identification of Mycobacterium avium complex (MAC) is possible by use of AccuProbe (Gen-Probe, San Diego, Calif.). To evaluate the reliability of the MAC AccuProbe for testing 7H9 cultures inoculated with broth from MGIT cultures positive for acid-fast bacilli or growth on a solid medium, we compared probe results to results obtained by sequencing a portion of the 16S rRNA gene. Isolates were sequenced if the MAC probe result was <100,000 relative light units (RLU) or if it was ≥100,000 RLU and the colony morphology was not classic or there were two colony types. For the 1,389 cultures tested in phase 1, conducted to evaluate cutoff values for the MAC probe in testing of 7H9 cultures inoculated with broth from MGIT cultures, the sensitivity and specificity of the MAC AccuProbe were 97.7% and 88.8%, respectively, according to the manufacturer's interpretive criteria (≥30,000 RLU is positive). If the cutoff for a positive result were 80,000 RLU, the specificity would be 100% and the sensitivity 92.3%. Of the 344 isolates in phase 2, which was conducted to confirm the 80,000-RLU cutoff for a positive result and therefore included only isolates with a MAC probe result of ≤100,000 RLU, 13 of 16 with results of ≥30,000 but <80,000 RLU were identified as mycobacteria other than MAC, including five Mycobacterium tuberculosis complex isolates. These data support the use of 80,000 RLU as the cutoff for a positive result in testing of 7H9 broth cultures with the MAC AccuProbe.