Pseudohypophosphataemia as a result of bilirubin interference

During the routine use of a discrete analyser it was noted that, when the serum bilirubin concentration was greater than 50 mumol/L, there was interference with serum phosphate determination measured by the formation of unreduced phosphomolybdate using a bichromatic system of measurement. The degree of interference was assessed by comparison with a reduced phosphomolybdate method (molybdenum blue). The interference cannot be removed by changing the secondary wavelength or by the use of a sample blank. It is proportional to the serum bilirubin concentration, but is not significant when this is less than 50 mumol/L. The monochromatic non-reduced phosphomolybdate method compares well with the reduced method.