Cord formation in BACTEC 7H12 medium for rapid, presumptive identification of Mycobacterium tuberculosis complex.

We evaluated cord formation in BACTEC 7H12 medium as a criterion for rapid identification of Mycobacterium tuberculosis complex. Kinyoun-stained smears, prepared from 270 radiometrically positive BACTEC 7H12 bottles, were examined independently by three observers. Smears from 93.2, 88.6, and 83.0% of the M. tuberculosis complex cultures were read as cord positive, and smears from 97.3, 97.8, and 99.5% of the mycobacteria other than M. tuberculosis cultures were read as cord negative by the three observers, respectively. There was 93.3% agreement between the observers. The presence of cords in BACTEC 7H12 medium can be a reliable criterion for rapid, presumptive identification of M. tuberculosis complex.