Urine analysis in monoclonal gammopathies at diagnosis: settling cut-off values.

Objectives: Laboratory testing has an extensive role in the diagnosis of monoclonal gammopathies. Since the last updates of the International Myeloma Working Group (IMWG) guidelines for the diagnosis of monoclonal gammopathies, debate has arisen as to whether urine analysis remains relevant for the diagnosis of these entities.

Methods: We carried out a retrospective study with data from 132 patients with a newly diagnosed serum M-protein. Patients were divided into two groups depending on the presence of M-protein in urine and different variables were recorded and statistically compared between groups.

Results: The aim of the study was to find a serum M-protein cut-off value under which urine analysis could be avoided in the first laboratory diagnosis. The results show that when the concentration of serum M-protein is ≤3.5 g/L and eGFR is >30 mL/min/1.73 m² (sensitivity (S): 100 %, specificity (Sp): 49 %, negative-predictive value (NPV): 100 %) the probability of finding M-protein in urine is negligible. Patients with alpha heavy chain have a higher probability of having M-protein in urine. Thus, when the heavy chain of the M-protein is gamma or mu, serum cut-off value can be raised up to ≤4.9 g/L (S: 97 %, Sp: 52 %, NPV: 98 %).

Conclusions: Settling these two cut-off values when a new M-protein is discovered could avoid a significant number of urine analyses, optimizing laboratory and healthcare resources.