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
