Classification of ILD
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"interstitial lung diseases"[MeSH Terms] AND classification
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ATS ERS interstitial lung disease classification 2025
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https://publications.ersnet.org/content/erjor/12/4/00344-202…
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"interstitial pneumonias" AND "classification"
Classification of interstitial lung disease (ILD)
ILD, also termed diffuse parenchymal lung disease, is best classified clinically by whether there is an identifiable cause. Lung involvement may be interstitial, alveolar, or airway-centered, so not every condition called an ILD is limited to the interstitium. Fishman’s Pulmonary Diseases and Disorders describes this cause-based approach as particularly useful in practice. - Fishman’s Pulmonary Diseases and Disorders, Chapter 52, lines 3501-3520.
1. ILD with a known cause or association
| Group | Examples |
|---|---|
| Connective-tissue disease associated ILD (CTD-ILD) | Rheumatoid arthritis, systemic sclerosis, polymyositis/dermatomyositis, Sjögren syndrome, systemic lupus erythematosus, mixed CTD |
| Exposure-related ILD | Hypersensitivity pneumonitis due to birds, molds, farming dusts; occupational pneumoconioses due to silica, asbestos, coal dust, beryllium |
| Drug- or treatment-induced ILD | Amiodarone, methotrexate, nitrofurantoin, bleomycin, immune-checkpoint inhibitors; radiation-induced lung disease |
| Post-infectious or other associated disorders | Some chronic infections, aspiration-related lung injury, graft-versus-host disease |
| Smoking-related ILD | Respiratory bronchiolitis-associated ILD (RB-ILD), desquamative interstitial pneumonia (DIP), pulmonary Langerhans cell histiocytosis (PLCH) |
2. Idiopathic interstitial pneumonias (IIPs)
These are diagnosed after excluding a specific cause.
| Traditional subgroup | Disorders |
|---|---|
| Chronic fibrosing IIPs | Idiopathic pulmonary fibrosis (IPF), with usual interstitial pneumonia (UIP) pattern; idiopathic nonspecific interstitial pneumonia (NSIP) |
| Smoking-related IIPs | RB-ILD; DIP |
| Acute/subacute IIPs | Cryptogenic organizing pneumonia (COP); acute interstitial pneumonia (AIP) |
| Rare IIPs | Idiopathic lymphoid interstitial pneumonia (LIP); idiopathic pleuroparenchymal fibroelastosis (PPFE) |
| Unclassifiable IIP | Features do not permit a confident single diagnosis, often because clinical, HRCT, and pathology findings conflict or are incomplete |
3. Granulomatous ILD
- Sarcoidosis
- Chronic hypersensitivity pneumonitis
- Berylliosis
- Selected infections and vasculitic disorders
4. Other rare ILDs
- Lymphangioleiomyomatosis (LAM)
- Pulmonary Langerhans cell histiocytosis (PLCH)
- Pulmonary alveolar proteinosis (PAP)
- Pulmonary amyloidosis
- Lymphangitic carcinomatosis
- Genetic/familial fibrotic ILD syndromes
5. Unclassifiable ILD
Applied when no definite diagnosis can be made despite multidisciplinary assessment, usually owing to:
- Inadequate clinical or exposure data
- Indeterminate HRCT pattern
- No biopsy or inadequate tissue
- Discordant clinical, radiologic, and pathologic findings
Newer ERS/ATS 2025 pattern-based framework
The newer ERS/ATS framework expands beyond idiopathic interstitial pneumonias and uses three broad categories:
-
Interstitial patterns
- Fibrotic: UIP, fibrotic NSIP, bronchiolocentric interstitial pneumonia
- Non-fibrotic: cellular NSIP and other inflammatory interstitial patterns
-
Alveolar filling patterns
- Organizing pneumonia
- Diffuse alveolar damage
- Alveolar macrophage pneumonia, previously termed DIP
- Eosinophilic pneumonia and related alveolar filling processes
-
Rare or unclassifiable patterns/disorders
Key terminology changes include:
- Acute interstitial pneumonia → idiopathic diffuse alveolar damage
- Desquamative interstitial pneumonia → alveolar macrophage pneumonia
- Recognition of bronchiolocentric interstitial pneumonia as an airway-centered pattern.
This update is summarized in the 2025 ERS/ATS classification review and the ERS overview.
Exam-friendly flowchart
ILD
│
├─ Known cause / associated ILD
│ ├─ CTD-associated
│ ├─ Exposure/occupational
│ ├─ Drug/radiation-induced
│ └─ Smoking-related
│
├─ Idiopathic interstitial pneumonias
│ ├─ Chronic fibrosing: IPF/UIP, NSIP
│ ├─ Smoking-related: RB-ILD, DIP
│ ├─ Acute/subacute: COP, AIP
│ └─ Rare: LIP, PPFE
│
├─ Granulomatous ILD
│ ├─ Sarcoidosis
│ └─ Hypersensitivity pneumonitis
│
├─ Other rare ILD
│ ├─ LAM, PLCH
│ └─ PAP, amyloidosis, etc.
│
└─ Unclassifiable ILD
A recent systematic review highlights that ILD guidance and classifications are evolving, so older exam classifications may use the 2013 IIP terminology while newer respiratory/pathology sources may use the 2025 framework (Ng et al., 2025).
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