Factitious hypophosphatemia related to mannitol therapy.

We observed factitious hypophosphatemia in a patient who was receiving large amounts of intravenous mannitol. Mannitol concentrations as low as 25 mmol/L inhibited phosphorus measurement by the Dupont aca endpoint method; a kinetic method was unaffected. The mechanism of the mannitol interference was binding to molybdate in the reaction, decreasing both the rate of color development and the endpoint measurement. We conclude that the molybdate concentration in the DuPont method is suboptimal for measuring phosphorus in specimens containing mannitol.