| Key histologic features |
| Cellular pattern* |
| Mild to moderate interstitial chronic inflammation |
| Type II pneumocyte hyperplasia in areas of inflammation |
| Fibrosing pattern* |
| Dense or loose interstitial fibrosis lacking the temporal heterogeneity pattern and/or patchy features of UIP |
| Lung architecture may appear lost on examination of H&E-stained section, but is relatively preserved with elastin stains |
| Interstitial chronic inflammation - mild or moderate |
| Pertinent negative findings |
| Cellular pattern |
| Dense interstitial fibrosis: absent |
| Organizing pneumonia: absent or not prominent |
| Diffuse severe alveolar septal inflammation: absent |
| 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 |
| Both patterns |
| Acute lung injury pattern, especially hyaline membranes: absent |
| Eosinophils: inconspicuous or absent |
| Granulomas: inconspicuous or absent |
| Viral inclusions and organisms on special stains: not present |
