Evaluation of reverse transcription-PCR and a bacteriophage-based assay far rapid phenotypic detection of rifampin resistance in clinical isolates ofMycobacterium tuberculosis

New rapid phenotypic assays for the detection of rifampin resistance in Myc obacterium tuberculosis have recently been described, but most of these req uire liquid cultures, which reduces the utility of many tests in terms of t urnaround times. In the United Kingdom, over 90% of rifampin-resistant isol ates are also resistant to isoniazid, so rifampin resistance can be used as a sensitive marker for multidrug-resistant tuberculosis. In this study, tw o new rapid phenotypic assays were compared to the standard resistance rati o method on 91 clinical isolates of M. tuberculosis. One, the phage amplifi ed biologically (PhaB) assay, has been described previously and is based on the inability of susceptible isolates of M. tuberculosis to support the re plication of bacteriophage D29 in the presence of inhibitory doses of rifam pin. The other employed reverse transcription (RT)-PCR to demonstrate a red uction in inducible dnaK mRNA levels in susceptible isolates treated with r ifampin, After incubation for 18 h with 4 mu g of rifampin per mi, the PhaB assay showed concordance with the resistance ratio method for 46 of 46 (10 0%) susceptible and 31 of 31 (100%) resistant isolates, while RT-PCR showed concordance for 46 of 48 (96%) susceptible and 35 of 36 (97%) resistant is olates. We believe these assays provide a reliable rapid means of susceptib ility testing with a total turnaround time of only 48 h, although the PhaB assay is butter in terms of its lower technical demand and cost and its app licability to tuberculosis susceptibility testing in developing countries.