Acid phosphatase testing is one of the most common tests for semen detection in forensic laboratories.
The Acid Phosphatase test detect presence of semen by direct testing or indirect testing.
For Direct testing a drop of extract is placed onto filter/blotting paper which contains Suspected sample and in the case of Indirect testing, disputed sample with stain is pressing with dampened filter/blotting paper on to the surface to move some fluid/stain on paper. Then filter/blotting papers are tested with a chemical reagent, which changes colour from orange to purple which confirms the presence of acid phosphatase. This is due to hydrolysis of the α-napthylphosphate to produce α-napthol, which couples with a Brentamine Fast Black/Blue salt, produce purple azo dye.