Table 2.

Fibrillary GN and the mimickers

DiseasesLMIFCongo Red StainEMa
Fibrillary GN (21)MesGN, MPGN, MGN, DPGN, and DSGNSmudgy appearance of polyclonal IgG and C3Mostly negative, rarely positive (4%) (4)Fibril size 12–24 nm
Variably argyrophilicFibrils are randomly oriented, straight
Amyloidosis (64,65)Mesangial expansion with acellular matrix, PAS negative, nonargyrophilicMonotypic staining of either κ or λ light chain in AL amyloidosisPositiveFibril size 7–12 nm
Deposits can be present in interstitium/vessel wallsFibrils are randomly oriented, solid, and nonbranching
Immunotactoid glomerulopathy (66)MPGN pattern (most common)Mesangial and capillary wall IgG positivityNegativeMicrotubule size 17–52 nm
70% of patients show monotypic stainingMicrotubules have parallel arrangements, centrally hollowed
Fibronectin glomerulopathy (67,68)MPGN patternUsually negative but may show nonspecific staining for Ig and C3NegativeFibril size 12–16 nm
Strongly PAS positive, nonargyrophilicExtensive mesangial and subendothelial deposits with focal fibrillary substructures
Diabetic fibrillosis (69)Mesangial matrix expansion and nodularityLinear glomerular capillary wall and tubular basement membrane staining with IgG and albuminNegativeFibril size 10–20 nm
Strong PAS positive, argyrophilic (FGN deposits are nonargyrophilic)Focal segmental mesangial IgM and C3 stainingFibrils are seen in short bundles in parallel arrays, nonbranching
  • LM, light microscopy; IF, immunofluorescence; EM, electron microscopy; MesGN, mesangioproliferative GN; MPGN, mesangioproliferative GN; MGN, membranous GN; DPGN, diffuse proliferative GN; DSGN, diffuse sclerosing GN; PAS, Periodic acid–Schiff staining; AL, amyloid light chain; FGN, fibrillary GN.

  • a The fibril sizes indicated for each disease are the typical reported sizes (fibril sizes can be smaller or larger than what is noted).