Monoclonal Gammopathy with Double M-bands Mimicking Biclonal Gammopathy: A Case Series.

BACKGROUND: Monoclonal gammopathies are characterised by the presence of a serum or urine monoclonal protein (M-pro-tein), typically detected as a single narrow band on serum protein electrophoresis (SPEP). Rarely, double M-bands may appear, raising the suspicion of biclonal gammopathy. Distinguishing true biclonality from paraprotein polymerization is crucial to prevent diagnostic misclassification and inform appro-priate clinical decisions. CASE: We report a case series of four multiple myeloma patients with twin M-bands who presented to AIIMS Patna between March 2023 and March 2024. All four patients exhibited two discrete M-bands on SPEP, initially suggestive of biclonal gammopathy. However, immunofixation electrophoresis confirmed that both bands in each case belonged to the same immunoglobulin isotype and light chain, establishing a monoclonal origin. Treatment of samples with β-mercaptoethanol resulted in the collapse of double bands into a single monoclonal band, confirming paraprotein polymerization. IgA and IgM paraproteins were most frequently implicated in dual-band appearance. CONCLUSION: The presence of double M-band on serum protein electrophoresis does not necessarily indicate biclonal gammopathy. Depending on SPEP results alone may lead to misclassification, and confirmatory techniques such as IFE and reducing agent studies are essential to establish the correct diagnosis.