Types of Interstitial Lung Abnormalities (ILA)
According to the 2025 American Thoracic Society guidelines, ILAs are classified into three distinct subtypes based on their radiologic appearance and distribution: nonsubpleural ILA, subpleural nonfibrotic ILA, and subpleural fibrotic ILA 1.
The Three ILA Subtypes
1. Nonsubpleural ILA
This subtype presents with patchy ground-glass and peribronchovascular opacities, typically predominating in the upper lung zones with minimal reticular opacities. The abnormalities are not primarily located at the lung periphery 1.
2. Subpleural Nonfibrotic ILA
This pattern shows bilateral subpleural reticulations without traction bronchiectasis or honeycombing. The key distinguishing feature is the absence of architectural distortion despite the peripheral location of abnormalities 1.
3. Subpleural Fibrotic ILA
This is the most clinically significant subtype, characterized by bilateral subpleural reticulations with traction bronchiolectasis and/or honeycombing. The presence of architectural distortion defines fibrosis in this context 1.
Clinical Significance of Subtyping
The subpleural fibrotic subtype carries the greatest risk of progression to interstitial lung disease and has the poorest prognosis 2. This classification is not merely academic—it directly impacts:
- Mortality risk: Subpleural fibrotic ILA is associated with hazard ratios of 1.3 to 2.7 for mortality in large cohorts 3
- Progression likelihood: Risk of progression to ILD varies from 20-80% depending on subtype 4
- Treatment complications: Patients with subpleural fibrotic ILA have significantly worse outcomes with lung cancer radiotherapy, including higher rates of grade ≥5 radiation pneumonitis (HR: 3.07) 5
Key Diagnostic Criteria
All ILA subtypes must meet these fundamental criteria 1:
- Bilateral nondependent parenchymal abnormalities on CT (though unilateral findings may be relevant in high-risk populations with family history or genetic variants)
- Involving ≤5% of a lung zone by visual estimate
- Not meeting criteria for interstitial lung disease (ILD)
- Findings must persist on prone imaging if initially seen only in dependent locations
Important Caveats
The distinction between ILA and ILD hinges on extent and features. Once fibrotic abnormalities involve >5% of total lung volume, or when definite honeycombing with traction bronchiectasis is present beyond this threshold, the diagnosis shifts from ILA to ILD 1.
Radiologists must systematically document the specific ILA subtype in their reports, as this information is essential for risk stratification and determining appropriate surveillance intervals 3. The subpleural fibrotic pattern warrants closer monitoring (6-12 month intervals) compared to nonfibrotic patterns (12-36 month intervals) 4.