Overestimation of free light chain antigen excess rate.
BACKGROUND
Free light chains (FLC) are useful biomarkers for diagnosis and follow-up of plasma cell disorders. FLC quantification is encumbered by non-linearity and antigen excess (>4-fold difference between results obtained at the 2000- and 100-fold dilution).


METHODS
FLC concentration was measured with Freelite® reagents on the BNII, using 100- and 2000-fold dilutions in 3645 samples. Samples displaying antigen excess were re-measured at the 2000- and/or 400-fold dilution. Carryover was evaluated by tracing samples to cuvettes and by measuring a normal sample in cuvettes that previously contained samples with a high FLC concentration.


RESULTS
Antigen excess occurred in 0.93% of samples for κ and in 0.55% of samples for λ. In 81.5% of the cases, it could not be confirmed by a re-analysis of a 2000-fold and/or a 400-fold diluted sample. Real antigen excess was documented in 0.25% and 0.03% of the samples for κ and λ FLC, respectively. In the low concentration range (2000-fold dilution), imprecision was high. False antigen excess was reduced by batch analysis, introducing cleaning and rinsing procedures and using the 400-fold dilution. No antigen excess was detected in samples with normal FLC concentrations.


CONCLUSIONS
Falsely high results occur by imprecision in the low concentration range and/or by carryover in cuvettes.
Department of Hematology, Universitair ziekenhuis Brussel (UZ Brussel), Laarbeeklaan 101, Brussels 1090, Belgium; Research group: Reproductive Immunology and Implantation (REIM), Vrije Universiteit Brussel (VUB), Laarbeeklaan 103, Brussels 1090, Belgium. Electronic address: Martine.Vercammen@uzbrussel.be.
Martine J
MJ
Vercammen
Department of Hematology, Universitair ziekenhuis Brussel (UZ Brussel), Laarbeeklaan 101, Brussels 1090, Belgium.
Linda
L
Broodtaerts
Department of Hematology, Universitair ziekenhuis Brussel (UZ Brussel), Laarbeeklaan 101, Brussels 1090, Belgium.
Patricia
P
Meirlaen
Laboratory Medicine, Department of Microbiology and Immunology, University Hospitals Leuven and Experimental Laboratory Immunology, KU Leuven, Herestraat 49, Leuven 3000, Belgium.
Xavier
X
Bossuyt
D006801:Humans; D007118:Immunoassay; D007147:Immunoglobulin Light Chains / Q000097:blood*; D010265:Paraproteinemias / Q000097:blood*
D007147:Immunoglobulin Light Chains
Antigen excess; Free light chain; Hook effect; Immunoglobulin; Monoclonal gammopathy