Assessment of morphology for rapid presumptive identification of Mycobacterium tuberculosis and Mycobacterium kansasii

Mycobacterium tuberculosis often exhibits serpentine cording when grown in liquid medium, whereas Mycobacterium kansasii can be larger and cross-barre d, We assessed the use of these morphologic characteristics as a cost-effec tive method for rapid presumptive identification of isolates from BACTEC bo ttles. Without specific training, using the Kinyoun acid-fast stain, defini tive cording was found in 237 of 373 specimens positive for M. tuberculosis (64%) and cross-barring was recognized within 63 of 76 (83%) of the specim ens positive for M. kansasii, giving sensitivities specificities, positive predictive values, and negative predictive values of 63.5, 96, 92, and 79%, respectively, for M. tuberculosis and 83, 95, 59, and 98%, respectively, f or M kansasii. With training and experience, these results improved to 74.5 , 98, 96, and 84% and 93, 98, 79, and 98%, respectively. The major improvem ents were in distinguishing the pseudocording or loose aggregation of Mycob acterium avium complex from M. tubereculosis and the long bended forms of M ycobacterium gordonae from M. kansasii. Mycobacterium asiaticum and Mycobac terium szulgai, which rarely occur, are genetically related to M kansasii a nd morphologically difficult to distinguish. In defined circumstances, serp entine cording and cross-barring can be used for rapid presumptive identifi cation of M. tuberculosis and M. kansasii, respectively, and as guides for initial probe selection to reduce costs.