LAUSR

An 84-year-old man developed a membranoproliferative glomerulonephritis pattern of injury, and the most likely cause detected during a workup was monoclonal IgG-λ in the urine and serum. Predominant IgG and λ light chain deposition was confirmed only by immunofluorescence using formalin-fixed, paraffin-embedded tissue and not by immunohistochemistry. A smaller and non-linear dynamic range of immunohistochemistry makes it less quantitative than immunofluorescence staining and may explain why immunohistochemistry failed to detect the light chain restriction. This case suggests that immunohistochemistry may not serve as a substitute for immunofluorescence on formalin-fixed, paraffin-embedded tissue in detecting masked monoclonal immunoglobulin deposits, although further research is warranted.