Reference change values of M-protein, free light chain and immunoglobulins in monoclonal gammopathy

Abstract

Objectives: Clinical decisions in patients with monoclonal gammopathies may be highly imprecise because of variations of parameters used in diagnosis. In this study, we aimed to calculate the variation in M-protein, free light chains (FLCs), and immunoglobulins in respective patients.

Design & methods: We analyzed the data of clinically stable patients with monoclonal gammopathy (MG), which were monitored for 7-years to determine the biological variations and reference change values (RCV) of serum M-protein, monoclonal serum FLCs and immunoglobulin (Ig) concentrations. Patients that were included in the study had no change in diagnosis and showed <5 g/L change in serum M-protein during the monitoring. From the patients included at least 3 consecutive samples were analyzed within 8 months and 7 years of initial diagnosis.

Results: The total coefficient of variations (CV) was calculated for M-protein and involved/uninvolved fractions of FLCs and immunoglobulins. From 38 patients and 456 samples that were included in the study, the total CVs were calculated for serial M-proteins (8.9%), serum involved FLCs (iFLC, 21.4%), involved Ig (i-Ig, 8.7%) and uninvolved Ig (u-Ig, 9.1%). Combining these CVs and the interassay analytical CVs, we calculated the biological CV for the serum M-protein (8.4%), serum iFLC concentration (21.1%), i-Ig (8.6%) and u-Ig (9.0%). A significant correlation was found in multiple myeloma patients between the κ/λ light chain ratio (rFLC) with i-Ig, the difference between i-Ig level and u-Ig level (d-Ig) and ratio Ig (r-Ig) (r = 0.790, 0.703 and 0.711, respectively). These correlations were not found in patients suffering from MG of undetermined significance and smoldering multiple myeloma.

Conclusions: i-Ig determinations may be an alternative to M-protein for MGs. The variations in serum FLC measurements during MG monitoring were greater than those observed in serum M-proteins and therefore need to be more rigorously revised for recommendations.

Keywords: FLC; IgG; Immunoglobulin; M-protein; Monoclonal gammopathy; Multiple myeloma.