Estimation of serum M-protein concentration from polyclonal immunoglobulins: an alternative to serum protein electrophoresis and standard immunochemical procedures.

BACKGROUND

Serum monoclonal proteins (M-proteins) are usually quantified by electrophoresis or immunochemical measurement. A third alternative involves immunochemical measurement of the monoclonal isotype by subtraction of the polyclonal part of the isotype calculated from the other polyclonal isotypes and light chain. We experimentally calculated heavy chain/light chain equivalence factors for three immunoglobulin (Ig) isotypes and compared serum M-protein concentrations obtained using the three approaches.

METHODS

Equivalence factors were calculated for 1427 samples from 322 patients with a clinical diagnosis of monoclonal gammopathy of undetermined significance and validated in serum samples with and without a monoclonal component. Serum light chain recovery by nephelometry was compared to estimates using factors for serum immunoglobulin concentrations. Some 3735 samples were measured by electrophoresis and nephelometry for comparison of M-protein concentrations.

RESULTS

The experimentally calculated factors were 3.3059 (IgGkappa), 3.5204 (IgGlambda), 3.4567 (IgAkappa), 3.5308 (IgAlambda), 4.9104 (IgMkappa) and 4.8020 (IgMlambda). Light chains quantified by nephelometry in 984 serum samples without M-protein gave recovery of 100.97%+/-6.89% compared to immunoglobulin concentration estimates. Light chain recovery of <80% was observed more often for IgMkappa (61.0%) and IgMlambda (42.5%). Method differences for M-protein were >25% for 37.4%-52.3% of the serum samples. Electrophoresis results were higher than nephelometry results for 5% of serum samples and 23.7% of the estimates, especially for M-protein <20 g/L. Method differences for concentrations >40 g/L were <25% for most IgG and IgA isotypes, but not IgM.

CONCLUSIONS

No method can accurately measure serum M-proteins over the entire concentration range. Estimation using polyclonal immunoglobulins and light chains represents a simple alternative to electrophoresis that is applicable to any serum M-protein concentration, regardless of electrophoretic migration and particularly for M-protein serum <20 g/L.