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Table 1 Summary of consensus diagnostic criteria used for case classification

From: Comparative utility of LC3, p62 and TDP-43 immunohistochemistry in differentiation of inclusion body myositis from polymyositis and related inflammatory myopathies

Diagnostic criterion PM PM-COX pIBM IBM
Lymphocytic endomysial inflammationa Present Present Present Present
Inflammatory infiltrate compositionb T-cell rich, B-cell poor T-cell rich, B-cell poor T-cell rich, B-cell poor T-cell rich, B-cell poor
Degenerating / regenerating fibersa Present (random distribution) Present (random distribution) Present (random distribution) Present (random distribution)
Fiber invasionb Present Present Present Present
Diffuse MHC-1 positivityb Present Present Present Present
Endomysial fibrosisa None or mild None or mild Moderate to severe Moderate to severe
Fiber size variationa None or mild None or mild Moderate to severec Moderate to severec
Percentage of COX-negative fibersa <1% ≥1% Any (generally >1%) Any (generally >1%)
Ragged red fibersb Absent Present Either (generally present) Either (generally present)
Classic rimmed vacuolesa Absent Absent Absent Present
Rimmed cracks or basophilic granular debrisb Absent Absent Presentd Either (generally present)
  1. a Main criteria (required for diagnosis / classification).
  2. b Supporting criteria.
  3. c Specimens with “moderate to severe fiber size variation” included the entire range of muscle fiber sizes (from atrophic to hypertropic; <5 to >100 μm) rather than just two populations of normal-sized and atrophic fibers, as can be seen in PM.
  4. d For pIBM classification, presence of rimmed cracks (incomplete rimmed vacuoles; Figure 5E, white arrowhead) or basophilic granular debris (material reminiscent of basophilic RV rim) was required in cases with moderate fibrosis but optional in cases with severe fibrosis.