Clinical relevance of discordant urine and serum electrophoresis immunofixation cases.

Protein electrophoresis (PEP) is a key laboratory technique for identifying and quantifying proteins in body fluids, commonly used for detection of paraproteins in plasma cell disorders like multiple myeloma (MM), amyloidosis, and related conditions. The utility of urine PEP and urine immunofixation electrophoresis (uIFE) has been criticised in recent years due to its poorer sensitivity and precision than the corresponding serum assays. This study, conducted in an Australian cohort, aimed to assess the utility of uIFE when serum immunofixation electrophoresis (sIFE) results are negative and determine the predictors for such cases. We conducted a single-laboratory retrospective cross-sectional study over 18 months (2023-2024) involving 1065 individual patient encounters who had a uIFE test performed. Samples were run on the Sebia HYDRASYS system and were read independently by three experienced readers. We identified 51 discordant cases (5% of cases) where uIFE detected monoclonal proteins absent in paired sIFE. Common diagnoses in these 51 patients included MM (n=1, 22%) and amyloidosis (n=9, 18%). A random selection of 51 age- and gender-matched uIFE patients with positive sIFE results were compared to the discordant cases. Renal function or serum-free light chain status did not influence the presence of discordant results. Discordant uIFE/sIFE results may be seen in clinically relevant syndromes. Our findings highlight the importance of uIFE in detecting low-level monoclonal proteins, particularly in conditions where serum assays fall short, such as in minimal residual disease or light chain amyloidosis.