Why measure serum FLCs?
Historically, the biochemical methods to diagnose myeloma, and especially LCs, via protein chemistry have been problematic, slow to perform, and lacked both sensitivity and specificity. In contrast, the measurement of serum FLC by nephelometry is rapid (hours); more sensitive (1 to 3 mg/L); and, along with an SPE (to determine the presence or a whole Ig component), will diagnose the majority of patients with myeloma, amyloidosis, and other MIDD and is now the preferred diagnostic test. An abnormal sFLC ratio (normal κ/λ 0.26 to 1.65) is due to an overproduction of a single κ or λ clone (with suppression of the other) and this excess is detectable in the serum before urinary tubular catabolism is exceeded and before the SPE or IFE is abnormal. In patients with chronic kidney disease (CKD), significant accumulation of sFLC occurs (approximately fivefold) due to reduced excretion; so the normal range is adjusted to reflect this ( κ/λ 0.37–3.17) and reduces the over-diagnosis of monoclonal gammopathy in CKD. In patients with myeloma and severe AKI from MCN the sFLC always exceeds 1000 mg/L and the ratio is always abnormal. The measurement of urine FLC does not improve diagnostic yield, and the measurement of sFLC instead of urine IFE is now incorporated into hematologic guidelines. Serial measurements of FLC also provide real-time and quantitative monitoring of the response to chemotherapy and dialysis because of the short half-life (hours) of the sFLC compared to whole Igs (3 weeks) when measured by SPE.
