Background: There are reports in the literature describing artefactually raised phosphate concentrations in serum samples of patients with myeloma. However, IgA paraproteins have been reported only rarely as a potential cause. Methods: Following the detection of a grossly elevated phosphate concentration in a patient with an IgA paraprotein and another with an IgG paraprotein, we estimated phosphate concentrations in a further 73 patients with paraproteins using the Bayer ADVIA 1650 and Ortho Vitros 950 analysers. The latter method has been reported to be unaffected by pseudohyperphosphataemia. Results: Deproteinization of the serum samples containing the IgA and IgG paraproteins showed that they were responsible for the interference. No significant difference in serum phosphate concentrations measured by the two analysers was noted for the larger study of serum IgG or IgM paraproteins. However, a statistically significant but clinically trivial difference in phosphate concentration was noted for serum IgA paraproteins, with the Ortho Vitros 950 giving slightly higher phosphate concentrations. Deproteinization of these samples yielded similar phosphate concentrations. Phosphate estimation in serum samples without paraproteins using both analysers yielded results that were not statistically different. Conclusion: On occasion, serum samples containing IgA paraproteins may give rise to erroneous phosphate concentrations when the Bayer ADVIA is used
