THE ESTIMATION OF THE URINARY MONOCLONAL PROTEINS: STILL AN OPEN PROBLEM

The quantification of urinary monoclonal component (UMC), usually as κ or λ light chain, is used to monitor therapeutic response in multiple myeloma patients (Katzmann JA et al, 2011 Clin Chem 57;12:1687). The recommended method is an estimation based on the percentage of UMC identified on the densitometric scan of the electrophoretic separation of total urinary proteins (TUP); however TUP quantification may give different results when different total protein assays are used. To overcome this limitation we tested another approach based on nephelometric determination of urinary albumin (AlbU), which always visible on the immunofixation gel, to obtain another UMC estimations by comparing the two areas of AlbU and UMC. Twenty urinary samples from multiple myeloma patients showing monoclonal bands on immunofixation were selected, with TUP values, quantified by the automated benzethonium chloride method, ranging from 1.4 to 1800 mg/dl. A densitometric scan of the gel by the software ImageJ was performed and the areas of UMC and AlbU were estimated. Two different calculations of UMC were then performed: one referred to the TUP and the other one to the AlbU. On the same samples urinary light chains κ or λ (ULC) were also directly determined by nephelometry. We found a good correlation between the values calculated by using TUP and AlbU (r=0.9368), TUP and ULC (r=0.9560), ULC and AlbU (r=0.9605), but the ULC determined by nephelometry were always higher than the estimated values, even if the degree of overestimation was highly variable, confirming the discrepancy between methods. It is worth to note that in the samples where elevated values of UMC were estimated with TUP (>50 mg/dl), AlbU based calculation restituted far lower values than ULC determination, likely due to the saturation of the protein staining on the electrophoretic gel.