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SAP Staining In the Diagnosis of MGMID

By Dr. Rebecca May

Sep 01, 2021

Membranous-like glomerulopathy with masked IgG kappa deposits (MGMID) is a rare condition in which subepithelial deposits are present on electron microscopy (compatible with membranous glomerulopathy), but routine immunofluorescence is exclusively positive for C3. When a different processing technique is used and immunofluorescence is performed on the paraffin-embedded tissue, staining is “unmasked” revealing deposits that stain with IgG1, C3, and kappa. These patients are more often younger women and have a vague autoimmune phenomenon, such as a weakly positive ANA. To identify an antigen for membranous-like glomerulopathy with masked IgG kappa deposits, mass spectrometry was performed on laser captured glomeruli. There was a high expression of serum amyloid P (SAP) as compared to non-MGMID cases. SAP co-localized with IgG within deposits by confocal microscopy in MGMID cases. On a larger cohort, SAP was positive in membranous-like glomerulopathy with masked IgG kappa deposits cases and few membranous cases that did not mask, but were IgG1 kappa restricted, suggesting that not all cases are masked on immunofluorescence. Staining for SAP provides a convenient way to test for MGMID in clinical practice.

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