Histopathology

• Dense fibrosis causing architecture remodelling with frequent honeycombing
• Patchy lung involvement by fibrosis
• Subpleural or paraseptal distribution or both
• Fibroblast foci at the edge of dense scars

• Honeycomb fibrosis only

Or

• Dense fibrosis causing architecture remodelling with frequent honeycombing
• Patchy lung involvement by fibrosis
• Subpleural or paraseptal distribution or both
• Fibroblast foci at the edge of dense scars may or may not be present

• Patients have less compelling histological changes than those classified by the final column
  (eg, occasional foci of centrilobular injury of scarring, rare granulomas or giant celles, only a minor degree of lymphoid hyperplasia or diffuse homogeneous fibrosis favouring fibrotic non specific interstitial pneumonia) These features and the differential diagnoses they call to mind, become part of the multidisciplinary diagnosis of IPF, or not

• Non-UIP pattern
  Patients with features of other fibrotic disorders-eg, fibrotic hypersensitivity pneumonitis, fibrotic non-specific interstitial pneumonia, fibrosing organising pneumonia, pleuroparenchymal fibroelastosis, pulmonary Langerhans cell histiocytosis, or smoking-related interstitial fibrosis
• UIP pattern with ancillary features strongly suggesting an alternative diagnosis
  Eg, prominent diffuse alveolar damage or organising pneumonia (consider acute exacerbation of UIP), granulomas (consider hypersensitivity pneumonitis, sarcoid, infection), marked interstital inflammatory cell infiltrate away from areas of UIP (consider hypersensitivity pneumonia)