Falsely Low Serum Free Light Chain Concentration in Patients with Monoclonal Light Chain Diseases.

Introduction: Measurement of serum free light chains (FLC) is useful in the diagnosis and monitoring of nonsecretory myeloma, AL amyloidosis (AL) and light-chain myeloma (LCMM). Long-term monitoring of monoclonal FLC disease requires reproducible assay performance. We assessed the FLC assay performance in regard to falsely low FLC values.

Methods: We assessed:

1. FLC immunoreactivity and,

2. FLC assay recovery on sample dilution.

FLC concentration was measured in serial serum samples collected from patients with AL and LCMM, using a kit assay (The Binding Site Ltd., Birmingham, UK) with the IMMAGE^TM (Beckman Coulter, Brea, CA) and Hitachi Modular P (Roche Diagnostics) analysers. Monoclonal paraprotein was detected by serum and/or urine protein electrophoresis (Sebia, Issy-les-Moulineaux, France) and immunofixation.

Results:

1. FLC immunoreactivity. Of 23 patients with LCMM, one patient had discordant Bence Jones protein (BJP) and FLC results. The patient was a 70-year-old male with kappa LCMM where changes in the level of KBJP correlated with the KFLC concentration and the serum kappa to lambda (K/L) FLC ratio in samples measured between 2002 and mid 2004. Thereafter the FLC ratio remained within the reference range despite increasing urine KBJP. Subsequent investigations revealed that the patient’s monoclonal KFLC had failed to react with three different lot numbers of kit reagent. A fourth reagent lot, however, gave elevated KFLC concentration and K/L ratio consistent with disease relapse. A partial response to the patient’s monoclonal KFLC was observed for the same samples using the Modular assay (i.e., KFLC concentration was approximately 50% or less of the IMMAGE value).

2. FLC recovery. In four patients with AL or LCMM the monoclonal FLC was underestimated when using the manufacturer’s recommended 1:10 on-line dilution and gave higher recoveries at 1:20 to 1:110 sample dilution. Values for the monoclonal FLC differed by up to 100% and recovery on dilution did not parallel the standard curve response.

Conclusions:

1. We report the first case of non-reaction of a monoclonal kappa FLC in a patient with LCMM. Urinary BJP monitoring remains necessary in patients with LCMM.

2. Dilutional non-linearity of FLC suggests that the immunoreactive response of some monoclonal FLC does not parallel that of polyclonal calibration materials. This may result in falsely low FLC results.

3. While the FLC assay represents a major advance for these diseases, clinicians should be aware of the assay’s limitations.