Detection of serum free light chains: the problem with antigen excess.

Background: In recent years, measurement of serum immunoglobulin free light chains (FLCs) has greatly facilitated diagnosis and monitoring of various plasma cells dyscrasias, including multiple myeloma. Detection of FLCs by nephelometry depends on the formation of immune complexes. However, it is known that if the antigen (free light chain) is present in great excess, non-precipitating immune complexes can be formed and be detected poorly. This may lead to inaccurate test results.

Methods: Serum samples from 91 patients were subjected prospectively to the detection of free κ and λ light chains by automated nephelometry. A standard dilution of 1:100 was paralleled by a 1:2000 dilution. In addition, samples with values below the effective range (1:100) were subjected to a 1:20 dilution in order to calculate the κ/λ ratio.

Results: Here, we report the incidence of antigen excess in a cohort of 91 patients with a high proportion of monoclonal abnormalities. A standard dilution (1:100) of free light κ chains from a patient with monoclonal immunoglobulin A κ gammopathy were missed repeatedly. In a second patient (with myeloproliferative disease and an apparent incidental FLC monoclonal gammopathy of undetermined significance) free λ chains were also substantially underestimated in the standard dilution. Hence, the incidence of erroneously low test results due to antigen excess was 2.20%.

Conclusions: We found a significantly higher incidence of falsely low test results due to antigen excess than reported previously. Antigen excess may result in erroneous interpretations in sera subjected to nephelometric analysis. Therefore, clinical decisions should not be based solely on a single assay, especially if FLC testing includes only one dilution of the serum sample. Instead, several parallel dilutions should be recommended for screening of patients.