Misdiagnosing renal amyloidosis as minimal change disease.

BACKGROUND Minimal change disease (MCD) accounts for 10-15% of all adult nephrotic syndrome cases and requires normal renal histology by light microscopy and negative immunohistology. Foot process effacement on electron microscopy (EM) is typical. Renal amyloid deposits demonstrate pathognomonic green birefringence when viewed under cross-polarized light after staining tissue with Congo red (CR) and may reveal fibrils on EM. Late diagnosis and delayed treatment of renal amyloidosis negatively impact on renal and patient survival. METHODS A retrospective analysis was performed on 2116 patients referred to the National Amyloidosis Centre between 2001 and 2013, in whom renal amyloidosis was confirmed histologically. Twenty-seven of these patients had renal histology initially interpreted to be MCD. RESULTS Among 26 patients in whom biopsy specimens and/or reports were retrieved, the median age at MCD diagnosis was 62 years and presenting proteinuria averaged 7.8 g/24 h. The median time period between the two diagnoses was 241 days (range: 20-2632 days). MCD was diagnosed without CR in 17/26 (65%) biopsies, but all specimens contained amyloid on retrospective CR staining. MCD was diagnosed without EM in 17/26 (65%) cases and all of 10 such biopsies subsequently demonstrated fibrils. Sixteen patients were subjected to two or more renal biopsies when their proteinuria proved steroid refractory. CONCLUSION This study highlights the need to stain renal biopsies from proteinuric adults with CR, examine them under cross-polarized light and perform EM wherever possible. If the suspicion of renal amyloidosis remains high, despite apparent negative histology, specimens should be reviewed at specialist centres before undertaking a second kidney biopsy.