CHARACTERISTICS OF MONOCLONAL IMMUNOGLOBULINS THAT INTERFERE WITH SERUM INORGANIC-PHOSPHATE MEASUREMENT

The measurement of inorganic phosphate using an unmodified acid/molybd ate assay is known to be subject to interference when paraproteinaemia exists. This phenomenon, due to precipitation in the reaction mixture , is not common to all paraproteins. We studied sera from 35 patients to determine whether interference in the assay was related to particul ar electrophysical characteristics of the paraproteins. There were spu riously elevated phosphate concentrations in 48.6% of the sera assayed . This could not be related to a direct effect of light chain type, el ectrical charge or IgG subclass. No IgA paraproteins were found to cau se interference but there were immunoglobulin G (IgG) and immunoglobul in M (IgM) paraproteins in both the 'interfering' and 'non-interfering ' groups. The median paraprotein concentration was similar in both gro ups but, where interference occurred, the degree increased in line wit h the paraprotein concentration. Although it does not seem possible to predict which samples will cause interference, it is important that t he clinical implications of this problem are appreciated.