False Elevations in Inorganic Phosphate (Pseudohyperphosphatemia) Measured by the Ammonium Molybdate Method in Patients with Paraproteinemia

Basis for interference: Formation of a paraprotein precipitate in the reaction mixture, resulting in an increased turbidity with a resulting increase in absorbance to the mixture.

Recognition:

(1) Patient with IgG or IgM paraproteinemia but not IgA.

(2) Use of the ammonium molybdate method for measuring inorganic phosphate. Repeating the same method on a new specimen also gives a similar result.

(3) If inorganic phosphate is above the upper limit of the normal reference range there is no evidence of hyperphosphatemia.

Confirmation:

(1) Deproteination removes the interference. This may be achieved by ultrafiltration or treatment with trichloroacetic acid (TCA).

(2) Use an alternative method not sensitive to the interference. Some ammonium phosphate methods (Kodak slide method, modified Boehringer Mannheim phosphate method) are free of the interference.

where:

• The modified Boehringer Mannheim method used a reduced concentration of sodium chloride (from 148 to 45 mmol/L) and a different nonionic detergent. Increasing the sodium chloride concentration in the reaction mixture can cause interference to occur.