Detection of Mycobacterium avium-Mycobacterium intracellulare in blood cultures using concentrated and unconcentrated blood in conjunction with a radiometric detection system.

Studies were performed in order to determine optimal methods for isolating mycobacteria from blood. The combined use of a lysis-centrifugation (Isolator) system and a radiometric-detection system (Bactec) was compared to the use of either unconcentrated blood or centrifuged blood pellet in conjunction with the Bactec mycobacterial system. Results showed that for cultures with greater than 10 colony forming units (CFU) per ml of blood, the use of unconcentrated blood and centrifuged blood pellet was as sensitive as use of the Isolator concentrate. For cultures with lower colony counts (less than 10 CFU/ml), however, Isolator concentrate was consistently better than either centrifuged or unconcentrated blood. Since over half of our positive cultures had low colony counts, the use of Isolator concentrate inoculated into either a Bactec mycobacterial culture vial or onto Middlebrook agar slants is recommended over the use of either centrifuged blood pellet or unconcentrated blood inoculated into a Bactec mycobacterial culture vial.