Genexpert mtb rif test

The GeneXpert MTB/RIF test (Cepheid, Sunnyvale, CA, USA), which is based on NAA and detection of an MTB-specific region of the rpoB gene, uses real time-PCR (RT-PCR) with molecular beacons. The primers in this assay amplify a portion of the rpoB gene containing the 81 base pair "core" region. The probes can differentiate between the conserved wild-type sequence and mutations in the core region that are associated with rifampicin resistance, and the test also detects mutations associated with rifampin resistance. An amount of 200 µl sediment prepared from skin biopsy specimens after decontamination and homogenization processes was resuspended in phosphate-buffered saline to a 500 µl volume. The sample reagent supplied with the test (1.5 ml) was then added. The mixture was vortexed for 30 s to ensure all bacteria were resuspended. The sample was left to stand for 15 min with intermittent manual shaking. The solution was then transferred to the Xpert cartridge using a Pasteur pipette, and the cartridge was loaded onto the Xpert machine for analysis. Results are interpreted as positive or negative for M. tuberculosis. Positive results are ranked in four categories: Very low, low, medium, or high. Rifampicin resistance results were reported as susceptible or resistant.