Severe cryoglobulinaemic vasculitis associated with monoclonal gammopathy: a diagnostic challenge of masked deposits.

Monoclonal gammopathy of renal significance is characterised by kidney injury caused by nephrotoxic monoclonal immunoglobulins produced by a pathogenic B cell or plasma cell clone. Cryoglobulinaemia in this context may lead to severe systemic and renal manifestations. This case report describes a patient who presented with rapidly progressive digital necrosis and acute kidney injury, ultimately diagnosed with severe type II cryoglobulinaemic glomerulonephritis associated with monoclonal gammopathy. Routine immunofluorescence (IF) on frozen tissue failed to demonstrate immunoglobulin deposits; however, IF performed on formalin-fixed paraffin-embedded tissue following pronase digestion revealed masked monotypic lambda light chain deposits. This case highlights the diagnostic challenge posed by masked deposits and underscores the importance of advanced IF techniques in complex cases of cryoglobulinaemia.