Revised free light chain reference intervals enhance risk stratification in monoclonal gammopathy of undetermined significance and reduce overdiagnosis

The free light chain (FLC) ratio is a critical part of risk stratification for monoclonal gammopathy of undetermined significance (MGUS). Recently, revised FLC reference intervals developed using the iStopMM cohort, accounting for age and renal function, have reduced the rate of abnormal findings. Here, we examine the implications of the revision in an independent Danish MGUS cohort. Of 6993 MGUS individuals, 2641 had an abnormal FLC ratio by the original intervals, of whom 844 (32%) were reclassified as normal using the revised intervals. Reclassified individuals had no significantly increased risk of progression compared to those with a normal FLC ratio (hazard ratio (HR): 1.07, 95% confidence interval (CI) 0.74–1.57). Those with an abnormal FLC ratio by the revised reference intervals had an increased risk of progression (HR 2.23, 95% CI 1.79–2.78). Using the revised reference intervals, 490 individuals (16%) were reclassified to low-risk from a higher risk group. These individuals had a similar progression risk compared to others in the low-risk group. The findings validate the revised FLC reference intervals, enhancing prognostic accuracy and improving risk stratification to accurately identify MGUS individuals at risk of progression while reducing unnecessary classifications as high-risk.