Some pitfalls in the quality control of alkaline phosphatase activity

A study of the pH optimum of alkaline phosphatase activity in calibration and control serum preparations reveals a wide variation in pH optima. It is shown that as small an error of +/- 0.05 pH units in preparation of the buffered substrate results in an error in the order of +/- 5% in the response between the calibrating serum and patient specimens. It is recommended that a calibration serum should closely parallel the pH optimum of alkaline phosphatase in human sera and that a quality control serum containing alkaline phosphatase of alternative source to that present in the calibration serum be included with patient samples as a sensitive means of monitoring pH changes in the buffered substrate.