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ILD Classification - 2025 ERS/ATS Update
(Ryerson et al., European Respiratory Journal, 2025)
Overview of the New Approach
The 2025 update expands beyond the previous "idiopathic only" framework (2002, 2013) to a unified classification covering both secondary and primary/idiopathic interstitial pneumonias. The primary stratification is by morphological pattern (histological and radiological), followed by multidisciplinary determination of aetiology.
Classification Table (Table 1 of the Paper)
A. INTERSTITIAL PATTERNS
| Morphological Pattern | Secondary Causes | Primary / Idiopathic Diagnosis |
|---|---|---|
| UIP (Usual Interstitial Pneumonia) | Secondary UIP (e.g. CTD, HP, medications) | IPF (idiopathic UIP) |
| NSIP (Nonspecific Interstitial Pneumonia) | Secondary NSIP (e.g. CTD >>> HP, medications) | Idiopathic NSIP |
| BIP (Bronchiolocentric Interstitial Pneumonia) (new major pattern) | Secondary BIP (e.g. HP >>> CTD, aspiration, inhalational exposures, medications) | Idiopathic BIP (provisional diagnosis) |
| DAD (Diffuse Alveolar Damage) | Secondary DAD (multiple causes) | Idiopathic DAD (= Acute Interstitial Pneumonia) |
| PPFE (Pleuroparenchymal Fibroelastosis) | Secondary PPFE (e.g. IPF, CTD, HP, medications, radiation, transplant, post-TB, occupational) | Idiopathic PPFE |
| LIP (Lymphoid Interstitial Pneumonia) | Secondary LIP (e.g. CTD, immune deficiency) | Idiopathic LIP |
B. ALVEOLAR FILLING PATTERNS
| Morphological Pattern | Secondary Causes | Primary / Idiopathic Diagnosis |
|---|---|---|
| OP (Organising Pneumonia) | Secondary OP (e.g. CTD, post-infectious, drugs) | COP (Cryptogenic OP) |
| RB-ILD (Respiratory Bronchiolitis-ILD) | Secondary RB-ILD (e.g. smoking, inhalational) | Idiopathic RB-ILD |
| AMP (Alveolar Macrophage Pneumonia) (renamed from DIP) | Secondary AMP | Idiopathic AMP |
| Rare AFPs | Acute & chronic eosinophilic pneumonia (AEP/CEP); PAP (idiopathic/secondary); exogenous & endogenous lipoid pneumonia |
C. OTHER CATEGORIES
| Category | Notes |
|---|---|
| Combined patterns | Secondary and primary interstitial pneumonia with overlapping/mixed patterns |
| Unclassifiable ILD | Cases that cannot be assigned to a specific pattern after full multidisciplinary evaluation |
Four Major Changes from 2013
1. Expansion Beyond Idiopathic
- The old 2002/2013 "IIP-only" scope is replaced by a single unified system covering all interstitial pneumonias (both secondary and primary/idiopathic).
- Secondary aetiologies are listed before primary/idiopathic to emphasise excluding an underlying cause first.
2. New Subcategories and Updated Terms
| Old Term (2013) | New Term (2025) | Rationale |
|---|---|---|
| Not recognised as a major pattern | BIP (Bronchiolocentric Interstitial Pneumonia) | Added as a new major interstitial pattern; subsumes HP pattern terminology |
| AIP (Acute Interstitial Pneumonia) | Idiopathic DAD | Aligns pattern (DAD) with multidisciplinary diagnosis |
| DIP (Desquamative Interstitial Pneumonia) | AMP (Alveolar Macrophage Pneumonia) | Better reflects diffuse macrophage accumulation; DIP was a misnomer |
| "HP" used as a pattern name | HP reserved only for multidisciplinary diagnosis | BIP is now the pattern; HP is the clinical diagnosis |
3. Subclassification into Interstitial vs. Alveolar Filling Disorders
- Interstitial disorders are further divided into:
- Fibrotic (UIP, fibrotic NSIP, fibrotic BIP, PPFE)
- Non-fibrotic (non-fibrotic BIP, LIP, cellular NSIP)
- Alveolar filling disorders (OP, RB-ILD, AMP, rare AFPs) are separated as a distinct category.
4. Diagnostic Confidence
- Diagnostic confidence is now formally incorporated into patient evaluation and management decisions - recognising that many diagnoses are made with varying levels of certainty.
Key Imaging-Based Classification Framework (Figure 1)
When ILD is suspected on chest CT, patterns are distinguished by:
Interstitial disorders - key imaging features:
- Craniocaudal distribution: lower (UIP, NSIP, LIP), diffuse (BIP, DAD), upper (PPFE)
- Axial distribution: peripheral (UIP, NSIP, PPFE), subpleural sparing (NSIP), peribronchovascular (BIP, LIP)
- Specific features: honeycombing (UIP), air trapping/three-density sign (BIP), cysts (LIP), ground-glass (DAD), septal thickening (DAD)
Alveolar filling disorders - key imaging features:
- Multifocal consolidation (OP, EP), diffuse ground-glass (AMP, PAP), fat density (LP)
All findings feed into multidisciplinary discussion integrating clinical, molecular, and pathological data to reach a final diagnosis (Table 1).
Source: Ryerson CJ et al. "Update of the international multidisciplinary classification of the interstitial pneumonias: an ERS/ATS statement." Eur Respir J, 2025. https://doi.org/10.1183/13993003.00158-2025
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