a quantitative analysis of 3 - oxo - 5β - steroid is carried out by acting 3 - oxo - 5β - steroidδ 4 - dehydrogenase on a sample in the presence of a reducing chromophoric agent , and measuring the optical density of the chromophoric substance thereby produced . a typical reducing chromophoric agent is a tetrazolium compound . the invention also provides a reagent for the quantitative analysis of 3 - oxo - 5β - steroid comprising 3 - oxo - 5β - steroidδ 4 - dehydrogenase and a tetrazolium compound . the analysis and reagent provide a simple and reliable method for a liver function test .