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

Abstract

ObjectivesClinical 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 & methodsWe 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. ResultsThe 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. Conclusionsi-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.