Evaluation of a tetrazolium salt test to determine absence of live mycobacteria in tuberculin purified protein derivative

Current methodology to determine absence of live mycobacteria in tuberculin purified protein derivative (PPD) takes up to 8 weeks to perform and may a lso involve testing on animals. In this paper we describe an in vitro test utilising the tetrazolium salt, 2,3-bis-(2-methoxy-4-nitro-5-sulphenyl)-(2H )-tetrazolium-5-carboxanilide (XTT) to monitor the absence of live Mycobact erium tuberculosis (Mtb) in PPD. In the presence of live cells XTT is conve rted to a coloured formazan product that can be measured spectrophotometric ally. Live mycobacteria present in spiked PPD were detected by a marked cha nge in optical density above background levels. This; test is easy to perfo rm and is complete in just 48 hr.