Presence of an Abnormal Serum Free Light Ratio Is an Independent Risk Factor for Progression in Monoclonal Gammopathy of Undetermined Significance (MGUS).
Authors: Rajkumar S. Vincent , Kyle Robert A. , Therneau Terry M. , Melton L. Joseph , Bradwell Arthur R. , Clark Raynell J. , Larson Dirk R. , Plevak Matthew F. , Katzmann Jerry A. 
Published: October 2019

Background: Monoclonal gammopathy of undetermined significance (MGUS) affects 3% of persons age 50 yrs or greater. Reliable predictors of progression of MGUS to myeloma or related malignancy are needed. We hypothesized that the presence of monoclonal unbound (free) kappa or lambda immunoglobulin light chains in MGUS, as detected by an abnormal serum kappa/lambda free light chain (FLC) ratio would indicate clonal evolution in the neoplastic plasma cell and increase risk of progression to malignancy.

Methods: Patients (pts) from Southeastern Minnesota with MGUS seen at the Mayo Clinic from Jan 1, 1960 through Dec 31, 1994 who had baseline serum samples collected and stored at the time MGUS was first recognized were studied. Free kappa and lambda chains were measured using the serum FLC assay (FREELITETM, The Binding Site Ltd).

Results: Of 1384 pts with MGUS during the defined time period, baseline serum samples obtained within 30 days of diagnosis of MGUS were available in 1148 pts. At a median follow-up of 15 yrs, malignant progression has occurred in 87 (7.6%) pts: myeloma (53 pts), IgM lymphoma (17), primary amyloidosis (6), macroglobulinemia (6), chronic lymphocytic leukemia (3) and plasmacytoma (2). An abnormal kappa/lamda FLC ratio (kappa/lambda ratio <0.26 or >1.65) indicating presence of monoclonal FLC was detected in 379 (33%) pts. In a Cox proportional hazards model, the risk of progression in pts with an abnormal kappa/lamda FLC ratio was significantly higher (hazard ratio 3.5, 95% CI 2.3–5.5; p<0.001) compared to pts with a normal ratio. After adjusting for the size of the serum monoclonal (M) protein by multivariate analysis, the hazard ratio was only slightly reduced to 2.8, p<0.001. The risk of progression to myeloma or related malignancy at 10 years was 17% with an abnormal kappa/lambda FLC ratio versus 5% with a normal ratio; corresponding rates at 20 years were 35%, and 13%, respectively. MGUS pts with both an abnormal serum FLC ratio and high serum M protein had a risk of progression at 20 years of 46% (high-risk), versus 26% with one of two risk factors (intermediate-risk) and 7% with no risk factors (low-risk) (Table). The true life-time (28 year) risk of myeloma or related malignancy in the low-risk group was only 5.3% when other competing causes of death were taken into account.

Conclusions: Presence of monoclonal FLC in the serum as detected by an abnormal kappa/lamda FLC ratio is a major independent risk factor for progression of MGUS to myeloma or a related malignancy. The new risk-stratification identifies a low-risk subset with a remarkably small life-time risk of progression, a finding of significant importance for the management of MGUS.

Risk-stratification of MGUS using the size of the serum M spike and free light chain ratio

Risk Group No. of patients Hazard ratio Absolute risk of progression at 20 years Low-risk (Serum M protein <1.5 gm/dL and normal FLC ratio [0.26–1.65]) 606 1 7% Intermediate-risk (Either risk factor present) 373 3.5 26% High-risk (Serum M protein >/=1.5 gm/dL and abnormal FLC ratio [<0.26 or >1.65]) 169 6.8 46%