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Some observations suggest that measurement of serum FLC may be better than urine protein electrophoresis for monitoring monoclonal light chains in LCMM patients, but no published studies have validated this approach.
Lastly, the FLC assay is useful for monitoring disease activity in patients with NSMM, LCMM, AL, and LCDD in whom there is no measurable M-spike on serum or urine protein electrophoresis.
 Nonstandard abbreviations: FLC, free light chain; NSMM, nonsecretory multiple myeloma; AL, primary systemic amyloidosis; LCDD, light chain deposition disease; LCMM, light chain multiple myeloma; IFE, immunofixation electrophoresis; PCD, plasma cell disorder; MCUS, monoclonal gammopathy of undetermined significance; and SMM, smoldering multiple myeloma.
If the filtered load is excessive, as occurs in LCMM, this reabsorptive capacity is exceeded and free light chains appear in the urine, where they have also been concentrated by physiologic renal tubular water reabsorption.
Additionally, the conventional tests used for detecting BJPs are falsely negative in approximately one-half of patients with LCMM. Because of the high urinary concentration of light chains, 24-h urine collections have traditionally been used for the detection of light chains in patients with suspected multiple myeloma.
From this registry, 28 patients with LCMM were selected for this analysis.
Of the 28 LCMM patients in the study, 9 were positive for [kappa] light chain and 19 for [lambda] light chain.
Results for sequential serum FLCs and 24-h urinary monoclonal protein were compared for a representative [kappa] and a representative [lambda] LCMM patient (Fig.
This correlation suggests that serum measurements may provide a logical and feasible alternative to 24-h urine collections in monitoring patients with LCMM.