title: Detection of serum free light chains: the problem with antigen excess.
abstract: 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.
authors.affiliation: Department of Pathology, University of Michigan Medical School, Ann Arbor, MI 48109-5602, USA. markusb@umich.edu
authors.forename: Markus
authors.initials: M
authors.lastname: Bosmann
authors.forename: Jürgen
authors.initials: J
authors.lastname: Kössler
authors.forename: Herbert
authors.initials: H
authors.lastname: Stolz
authors.forename: Ulrich
authors.initials: U
authors.lastname: Walter
authors.forename: Stefan
authors.initials: S
authors.lastname: Knop
authors.forename: Udo
authors.initials: U
authors.lastname: Steigerwald
mesh_terms: D000911:Antibodies, Monoclonal / Q000276:immunology; D000937:Antigen-Antibody Reactions; D000941:Antigens / Q000097:blood* / Q000276:immunology; D001774:Blood Chemical Analysis / Q000379:methods*; D015331:Cohort Studies; D005260:Female; D006801:Humans; D007147:Immunoglobulin Light Chains / Q000097:blood* / Q000276:immunology; D008297:Male; D008875:Middle Aged; D009391:Nephelometry and Turbidimetry; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
chemical_list: D000911:Antibodies, Monoclonal; D000941:Antigens; D007147:Immunoglobulin Light Chains