| Critical findings for diagnosis |
|
| Cellular pattern |
Mild to moderate interstitial chronic inflammation |
|
Type II pneumocyte hyperplasia in areas of inflammation |
| Fibrosing pattern |
Dense or loose interstitial fibrosis with uniform appearance |
|
Lung architecture is frequently preserved |
|
Interstitial chronic inflammation: mild or moderate |
| Findings that should consider other diagnosis |
|
| Cellular pattern |
Dense interstitial fibrosis: absent |
|
Organizing pneumonia is not the prominent feature (<20% of biopsy specimen) |
|
Lack of diffuse severe alveolar septal inflammation |
| Fibrosing pattern |
Temporal heterogeneity pattern: fibroblastic foci with dense fibrosis are inconspicuous or absent; this is especially important in cases with patchy involvement and subpleural or paraseptal distribution |
|
Honeycombing: inconspicuous or absent |
|
Enlarged fibrotic airspaces may be present |
| Both patterns |
Acute lung injury pattern, especially hyaline membranes: absent |
|
Eosinophils: inconspicuous or absent |
|
Granulomas: absent |
|
Lack of viral inclusions and organisms on special strains for organisms |
|
Dominant airway disease such as extensive peribronchiolar metaplasia |
