The method is based on the hydrolysis of α-naphtyl phosphate at pH 5.0 by acid phosphatase (ACP) to produce α-naphtol and inorganic phosphate. The pentanediol acts as a phosphate
acceptor increasing the reaction sensitivity.
The α-naphtol reacts with Fast Red TR, to produce a coloured complex directly proportional to the activity of the ACP in the sample.