Multicenter evaluation of the MB-redox medium compared with radiometric BACTEC system, mycobacteria growth indicator tube (MGIT), and Lowenstein-Jensen medium for detection and recovery of acid-fast bacilli

MB-Redox is a new manual culture system designed for the recovery of mycoba cteria from clinical specimens. It consists of a liquid medium (modified Ki rchner medium) containing a redox indicator, a colorless tetrazolium salt, which is reduced to colored formazan by actively growing mycobacteria. Acid fast bacilli (AFB) are easily defected in the medium as pink to purple pin head-sized particles. We report the results of a multicenter study (involvi ng four Italian microbiology laboratories processing 2370 clinical specimen s) aiming to evaluate the recovery rates of AFB and time required for their detection by using the MB-Redox medium. Two different protocols were set u p: in Protocol A (1580 specimens) the performance of MB-Redox was compared with those of the radiometric BACTEC 460 TB system (B460) and Lowenstein-Je nsen medium (L-J), whereas in Protocol B (790 specimens) it was compared wi th those of the Mycobacteria Growth Indicator Tube (MGIT) and L-J. A total of 213 mycobacteria were recovered, including 172 Mycobacterium tuberculosi s complex (MTB) isolates and 41 nontuberculous mycobacteria (NTM) isolates. In Protocol A, recovery rates were 81% for MB-Redox system, 84% for B460 s ystem, and 77% for L-J. In Protocol B the recovery rates by individual syst em were 87, 83, and 76% for MB-Redox, MGIT, and L-J, respectively. Differen ces in both the protocols were not statistically significant. The MB-Redox system plus L-J (Combination 1) recovered 94% of the isolates in Protocol A and 93% in Protocol B, while B460 plus L-J (Combination 2) and MGIT plus L -J (Combination 3) detected 91 and 89% of all mycobacteria isolates respect ively. No statistically significant differences were found among the combin ations. The mean time to detection of mycobacteria teas 16.3 days in Protoc ol A and 19.1 days in Protocol B with the MB-Redox system, 22.4 and 25.9 da ys with L-J, 13.2 days with B460, and 18.2 days with MGIT. The contaminatio n rates were 2.1, 2.0, 1.9, and 3.6 for MB-Redox, B460, MGIT, and L-J respe ctively. The MB-Redox is a reliable, nonradiometric system for growth and d etection of mycobacteria. When used in combination with a solid medium it p roved to be an effective replacement for B460. The MB Redox system is a lab or-intensive method requiring much handling during the visual reading proce dures. (C) Elsevier Science Inc.