Overestimation of Immunoglobulins in the Presence of Rheumatoid Factor by Kinetic Immunonephelometry and Rapid Immunoturbidimetry

The United Kingdom External Quality Assessment Scheme for Specific Proteins has demonstrated that one method group, comprised of kinetic immunonephelometric assays, gives significantly higher results for immunoglobulins in certain pathological sera containing rheumatoid factor. Using IgM as a model, we have compared kinetic immunonephelometry and a rapid (5 min incubation) immunoturbidimetric assay with radial immunodiffusion (RID) on seven sera. Compared with RID significantly higher results were obtained by kinetic nephelometry on all five sera positive for rheumatoid factor, and by turbidimetry on three of the four of these studied. In a serum with IgM increased due to primary biliary cirrhosis lower results (P0·05) were obtained by nephelometry than by RID. The remaining serum was normal, and no significant intermethod differences were found. The possible causes for these discrepancies are discussed.