What are the different types of interstitial lung disease (ILD)?

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Last updated: March 14, 2026 • View editorial policy

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Classification of Interstitial Lung Disease

Interstitial lung diseases are broadly classified into four major categories: ILDs with known causes, idiopathic interstitial pneumonias (IIPs), granulomatous lung diseases, and unique entities. 1

Major Classification Framework

The comprehensive classification system divides ILDs into the following categories 1, 2:

1. ILDs with Known Causes or Associations

  • Drug-induced ILD 1
  • Connective tissue disease-associated ILD (CTD-ILD), accounting for approximately 25% of all ILD cases 3
  • Environmental exposures including pneumoconioses 2
  • Genetic causes 1

2. Idiopathic Interstitial Pneumonias (IIPs)

The IIPs represent a heterogeneous group of ILDs of unknown etiology 2. These are further subdivided into:

Major Idiopathic Interstitial Pneumonias:

  • Idiopathic Pulmonary Fibrosis (IPF) - the most common and severe form, accounting for 55% of IIPs and approximately one-third of all ILD cases 1, 3
  • Nonspecific Interstitial Pneumonia (NSIP) - representing approximately 25% of cases 1, 2
  • Respiratory Bronchiolitis-Interstitial Lung Disease (RB-ILD) - accounting for 10-15% of cases 1, 2
  • Desquamative Interstitial Pneumonia (DIP) 1, 2
  • Cryptogenic Organizing Pneumonia (COP) - representing approximately 5% of cases 1, 2
  • Acute Interstitial Pneumonia (AIP) - less than 2% of cases 1, 2
  • Lymphoid Interstitial Pneumonia (LIP) - less than 1% of cases 1, 2

Rare Idiopathic Interstitial Pneumonias:

  • Idiopathic lymphoid interstitial pneumonia 4
  • Idiopathic pleuroparenchymal fibroelastosis 4

3. Granulomatous Lung Diseases

  • Sarcoidosis 1, 2
  • Hypersensitivity pneumonitis (HP) - accounting for approximately 15% of ILD cases 2, 3
  • Fungal infections 1
  • Mycobacterial infections 1

4. Unique Entities

  • Pulmonary alveolar proteinosis (PAP) 1
  • Eosinophilic granulomatosis (EG) 1
  • Eosinophilic pneumonia (EP) 1
  • Lymphangioleiomyomatosis (LAM) 1

5. Unclassifiable Idiopathic Interstitial Pneumonias

This category exists when 4:

  • Inadequate clinical, radiologic, or pathologic data are available
  • Major discordance exists between clinical, radiologic, and pathologic findings
  • Previous therapy has substantially altered radiologic or histologic findings
  • The presentation represents a new entity or unusual variant not adequately characterized by current classification

Clinical Significance and Prognostic Implications

IPF carries the worst prognosis among ILDs, with a 5-year survival rate worse than several types of cancer 1. A histologically confirmed usual interstitial pneumonia (UIP) pattern is associated with significantly worse prognosis than other histological patterns of chronic interstitial pneumonia 1.

Progressive fibrosing phenotype can develop across multiple ILD types and influences therapeutic decisions, requiring antifibrotic therapy rather than immunosuppression alone 5.

Important Diagnostic Considerations

The diagnosis of specific ILD types requires a multidisciplinary approach involving pulmonologists, radiologists, and pathologists 1. High-resolution computed tomography (HRCT) has become central to the diagnostic pathway, with approximately 91% sensitivity and 71% specificity for diagnosing ILD subtypes 3.

Common pitfall: Certain IIP pathologies may coexist in the same patient when multiple lung regions are biopsied, particularly UIP and NSIP patterns 2. Additionally, these pathology patterns are not disease-specific and can occur in other disorders such as connective tissue-associated ILD 2.

References

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