Utility of Direct Immunofluorescence Using Proteinase-k forAntigen 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 paraffin block (IF-P) is applicable when IF-F is unfeasible. The efficacy of IF in formalinfixed, paraffin-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 paraffin block is optimally utilized as a rescue strategy or to enhance the sensitivity of immunoglobulin detection in instances where IF-F is ineffective.