The most popular and accepted presumptive test for the
presence of semen is the test for seminal acid phosphatase (SAP).
This enzyme has the ability to catalyze the hydrolysis of organic
phosphates which forms a product that will react with a diazonium
salt chromogen to cause a color change [10]. This is the basic
principle behind all the variations of the SAP test. One popular
substrate/color developer combination is alpha-naphthyl phosphate and Brentamine Fast Blue. Other combinations that have
been successful are beta-naphthol with Fast Garnet B, and alphanaphthol with Fast Red AL [8]. Additional reagents used have been
sodium thymolphthalein monophosphate due to its high selectivity, stability, and low hazardous risk [51], and a combination of pnitroaniline, NaNO3, a-naphthyl phosphoric acid, and aqueous
magnesium chloride [52]. Of course there are false positives such
as some plant materials and even vaginal acid phosphatase (VAP),
so this technique cannot be considered confirmatory [8]. One way
to avoid a potential false positive for VAP is to observe a color
change that only occurs between 5 and 30 s since VAP has never
given a positive result that quickly [12].