Interferences of mannitol with phosphate determination.

Methods commonly used for serum phosphate analysis depend upon the combination of inorganic phosphorus with molybdate and subsequent quantitation of the resultant phosphomolybdate by spectrophotometry. This is a case report of mannitol interference with phosphate quantitation by a method not employing dialysis (DuPont ACA) but not with another method which includes a dialysis step (Technicon SMAC). This interference, occurring at an extremely high level of serum mannitol (3.5 g/dl) in a patient with acute renal failure, appears to be due to direct inhibition of the formation of the phosphomolybdate complex by mannitol.