Utility of Direct Immunofluorescence Using Proteinase-k for Antigen Retrieval in Formalin-Fixed, Paraffin Embedded Kidney Biopsy Sections

Background: Immunofluorescence on frozen sections (IF-F) is crucial for diagnosing immune complex-related glomerular diseases. Immunofluorescence on para"in block (IF-P) is applicable when IF-F is unfeasible. The e"icacy of IF in formalin fixed, para"in-embedded (FFPE) kidney biopsies with proteinase-K was evaluated. Methods: This prospective study encompassed a total of 100 cases (control-set = 30, study-set = 70). Proteinase-K was employed for enzymatic digestion to facilitate antigen retrieval on FFPE blocks. The sensitivity and specificity of detecting Immunoglobulin G (IgG), Immunoglobulin A (IgA), Immunoglobulin M (IgM), C3, C1q, kappa, and lambda with IF-P were compared to IF-F. Results: In the control group, the total positive for all antibodies, excluding C3c, on IF-P was comparable to that on IF-F, demonstrating a sensitivity of 100%. The sensitivity for C3c was 54.26%. Specificity exceeded 90% for all antibodies, with the exception of C1q and lambda. The intensities on IF-F were brighter than those on IF-P. Of the 70 study-set cases, the IF-P results were consistent with the light microscopy (LM) diagnosis in 66 cases. However, IF-P was negative in one case of classic membranous nephropathy and 3 cases of infection-related glomerulonephritis diagnosed on LM. Conclusion: Immunofluorescence on para"in block is optimally utilized as a rescue strategy or to enhance the sensitivity of immunoglobulin detection in instances where IF-F is ine"ective. Cite this article as: Akurathi H, Ayesha SM, Dondapati S, et al. Utility of direct immunofluorescence using proteinase-k for antigen retrieval in formalin-fixed, para"in-embedded kidney biopsy sections. Turk J Nephrol. 2025;34(2):117-123.