Non-Invasive Diagnostic Strategies for Membranous Nephropathy in the NEPTUNE Study

Background: Clinical practice guidelines recommend that a kidney biopsy is no longer required to confirm a diagnosis of membranous nephropathy (MN) in patients with nephrotic syndrome and a positive test for anti-phospholipase A2 receptor antibodies (PLA2R-Ab). However, the optimal diagnostic strategy for using the PLA2R-Ab enzyme-linked immunosorbent assay (ELISA), PLA2R-Ab indirect immunofluorescence (IIF) test, and genetic risk score for diagnosing MN, including the tests’ optimal thresholds for positivity among incident patients with proteinuria, is still unknown. Methods: We used serum samples at or before the first clinically indicated kidney biopsy from participants in the Nephrotic Syndrome Study Network (NEPTUNE) to analyze test performance characteristics using different combinations and cut-offs of the PLA2R-Ab ELISA, IIF, and genetic risk score for diagnosing MN. Secondary analyses included serum samples within 6 months after biopsy but before any immunosuppression use. Results: There were 325 study participants with serum samples available on or before the day of kidney biopsy and an additional 143 study participants with samples within six months after biopsy but before any immunosuppression use. Of these participants, 26% (N=85) had biopsy-confirmed MN. The combination of ELISA ≥2 RU/mL and positive IIF was the optimal approach, with sensitivity of 0.60, specificity of 1.00, negative predictive value of 0.92, and positive predictive value of 1.00. Using IIF to confirm only borderline ELISA titers between 2-20 RU/ml resulted in similar sensitivity but specificity of >0.99. In our multiethnic study sample, we did not find improved diagnostic performance with the addition of genetic risk scores. Conclusions: In the NEPTUNE cohort, combined PLA2R-Ab testing with ELISA and IIF provided optimal test characteristics in making a non-invasive diagnosis of MN before or soon after kidney biopsy, including in patients with sub-nephrotic proteinuria. Further studies in multiethnic populations are needed to assess whether genetic data can augment this approach.