Direct colorimetric determination of phosphorus in serum and urine

A simple, sensitive and reliable method is presented for the direct photometric measurement of inorganic phosphate in serum or urine. Reagents are stable, inexpensive and minimal in number. The phosphomolybdate complex formed in an acid medium is reduced by p-methylaminophenol sulfate ; subsequent alkalinization with monoethanolamine produces a clear blue color, which is read at 660 nm. Results compare favorably with the Goldenberg-Fernandez manual method, which requires prior serum deproteinization, and the Kessler automated method. Inter-run precision is ± 3% (2 C.V.%). Bilirubin interference is slight and minimal hemolysis can be tolerated. Elevated serum protein levels do not interfere with the method.