α-naphthylphosphate is hydrolyzed by serum acid phosphatase to α-napthol and inorganic phosphate. The rate of hydrolysis is proportional to the enzyme activity present. The α-naphthol produced is coupled with Fast Red TR to produce a colored complex which absorbs light at 405 nm. The reaction can be quantitated photometrically because the coupling reaction is instantaneous. L-Tartrate inhibits prostatic acid phosphatase but does not interfere with the reaction mechanism. Therefore, if testing is performed in the presence and in the absence of L-Tartrate, the difference between the results of the two assays is the level of prostatic acid phosphatase in the serum.