Precision in Diagnosis - KM55 (Galactose Deficient IgA1) Staining in Clinically Diagnosed Cases of SLE with IgA Nephropathy.

Abstract Systemic lupus erythematosus (SLE) is a multisystem autoimmune disease. Lupus nephritis (LN) is its most common visceral manifestation. The description of nephritis of other etiologies in patients clinically diagnosed with SLE is an uncommon finding and can confound diagnosis. KM55, an antibody directed against galactose deficient IgA1, is an important tool to resolve differential diagnoses in IgA containing glomerular diseases. We report three cases of IgA Nephropathy (IgAN) in patients clinically diagnosed as SLE with nephritis. On light microscopy, the biopsies showed features of mesangioproliferative or membranoproliferative glomerulonephritis. The immunofluorescence study showed variable mesangial granular uptake of IgA, IgG, IgM, C3, Kappa, Lambda and C1q with IgA being the dominant or codominant immunoglobulin raising the possibility of an IgA Nephropathy. Immunofluorescence for KM55(galactose deficient IgA1) performed in all three cases, showed prominent mesangial granular positivity, thus strongly supporting the diagnosis of IgAN and excluding a LN with atypical immunofluoresence. The renal biopsy diagnosis in the setting of positive ANA is highly significant as an additive criteria in the current EULAR/ACR 2019 recommendations for diagnosis of SLE highlighting the need for precision in diagnosis. The utility of KM55 in this clinical setting with review of current literature on the antibody is presented.