Plasma alkaline phosphatase assay: interconversion of results by two methods.

Two methods for measuring plasma alkaline phosphatase activity are compared: one makes use of phenyl phosphate, carbonate-bicarbonate buffer, and continuous-flow methodology; the other of p-nitrophenyl phosphate, diethanolamine buffer, and reaction-rate analysis. Results by the methods correlate well (r = 0.98) over a wide range of values (up to 10-fold the upper limit of normal). A factor can therefore be applied to convert results by one method into those that would be obtained by the other. The possibility that the presence of different proportions of isoenzymes in the plasma will affect this factor is considered. We have used the new method, with a conversion factor, as the routine method of alkaline phosphatase measurement in a clinical chemistry laboratory, with no problems.