Supportive Data

Studies at Mayo Clinic have shown that in some patients with urine monoclonal light chains and negative serum immunofixation, the free light chain (FLC) assay can identify monoclonal FLC in the serum. These studies support the increased sensitivity of the turbidimetric FLC assay. In a series of patients with primary systemic amyloid treated by stem cell transplantation, the quantitation and monitoring of FLC predicted organ response (eg, disease course).

Reference values for this test were updated in February 2026 by Mayo Clinic. Multiinstitutional studies confirmed substantial assay platform-specific variability, reinforcing calls from professional organizations to move toward locally validated or assay-appropriate ranges. Most recently, large population-based datasets from the iStopMM study(1) enabled robust age and kidney function–stratified reference intervals derived from tens of thousands of participants, culminating in updated interpretive workflows. Together, these developments made it clear that updating reference intervals was a necessary step to ensure accurate interpretation and reduce clinically meaningful misclassification. Mayo Clinic pursued a new reference interval (RI) study using current stabilized Optilite Freelite Kappa Free and Lambda Free reagents (The Binding Site Group, Ltd). Data from 489 patients (51% female) between the ages of 30 to 88 years were used. The new Mayo Clinic RIs were established as a central 99% range. The data compares closely with the iSTOPMM(1) recently published 99% central RI for healthy subjects and includes the RI proposed by Bertamini and colleagues(2) for individuals of African descent (Free Light Chain ratio of 0.686 to 2.10).