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ABSTRACT
The control and standardization of live Mycobacterium bovis BCG vaccine is specified by the World Health Organization. The estimation of vaccine potency is based on number of viable particles in freeze-dried preparation. The current method recommended by WHO for determination of number of viable units (NVU) in BCG vaccine is performed by cultivation on solid medium (Löwenstein-Jensen medium). This method is time-consuming—the estimation of viable counts takes 35 days, and may give variable results due to different factors. Preliminary studies on performing a tetrazolium salt test has been developed and evaluated as a potential additional or replacement method for determination of BCG viability. The tetrazolium salt 2,3-bis-(2-methoxy-4-nitro-5-sulphenyl)-(2H)-tetrazolium 5-carboxanilide (XTT) used as alternative substrate in the assay gave more rapid and reproducible result than the conventional viable count. The XTT assay was effective for viability estimation of suspensions prepared from several BCG vaccine batches covering a range of viable units. The method is easily performed and takes between 24 and 48 hours to estimate and analyze viable cells content compared with 35 day's for the conventional method.