Paraprotein interference causing pseudohyperphosphataemia: evaluation of an improved methodology

The measurement of serum inorganic phosphorus by direct acid/molybdate assay is known to be subject to interference by some serum paraproteins. Serum samples from 45 paraproteinaemic patients were used for the measurement of inorganic phosphorus using a manufacturer's kit reagent and a detergent-modified reagent from the same manufacturer. In addition, the same samples were analysed with both reagents following precipitation of serum protein with trichloroacetic acid. Using the original reagent pseudohyperphosphataemia was found in 27% (12/45) of the samples. The modified reagent was free from interference with no significant difference between results from serum and protein-free supernatants.