How is usual interstitial pneumonia (UIP) defined on a computed tomography (CT) scan?

CT Definition of Usual Interstitial Pneumonia

Usual interstitial pneumonia (UIP) on CT scan is definitively characterized by subpleural and basal predominant distribution with honeycombing, with or without peripheral traction bronchiectasis or bronchiolectasis. 1

Definite UIP Pattern on CT

The 2018 ATS/ERS/JRS/ALAT guidelines establish clear criteria for a definitive UIP pattern that requires ALL of the following features: 1

• Subpleural and basal predominant distribution - this is the hallmark spatial distribution, though it may occasionally be diffuse or asymmetrical 1, 2
• Honeycombing - this is the essential and required finding for definite UIP pattern 1
• Peripheral traction bronchiectasis or bronchiolectasis may be present with the honeycombing 1, 2
• Heterogeneous distribution is characteristic, with areas of normal lung interspersed with fibrotic changes 1

The presence of honeycombing is absolutely critical - without it, the pattern cannot be classified as definite UIP. 1 Honeycombing appears as clustered cystic airspaces, typically 3-10mm in diameter, with well-defined walls in subpleural locations. 1

Probable UIP Pattern on CT

When honeycombing is absent but other features suggest UIP, the pattern is classified as "probable UIP": 1, 2

• Same subpleural and basal predominant distribution as definite UIP 1, 2
• Reticular pattern with peripheral traction bronchiectasis or bronchiolectasis - this replaces honeycombing as the key finding 1, 2
• Mild ground-glass opacities may be present superimposed on the reticular pattern 1, 2
• Heterogeneous distribution is maintained 1

This probable UIP category was introduced because a significant minority of patients with histopathologic UIP do not fulfill HRCT criteria for definite UIP, yet still have the disease. 1

Features That Exclude or Suggest Alternative Diagnosis

Certain CT findings are inconsistent with UIP and should prompt consideration of alternative diagnoses: 1, 2

Distribution patterns inconsistent with UIP:

• Upper or mid-lung predominance 1, 2
• Peribronchovascular predominance 1, 2
• Perilymphatic distribution 1

CT features inconsistent with UIP:

• Predominant ground-glass opacities (when extent exceeds reticular abnormality) 1, 2
• Profuse micronodules or centrilobular nodules 1
• Consolidation as a predominant feature 1
• Marked mosaic attenuation 1
• Cysts (distinct from honeycombing) 1

Ancillary findings suggesting alternative diagnoses:

• Pleural plaques (consider asbestosis) 1
• Dilated esophagus (consider connective tissue disease) 1
• Extensive lymph node enlargement 1
• Pleural effusions or pleural thickening (consider connective tissue disease or drug toxicity) 1

Technical CT Acquisition Requirements

For accurate UIP diagnosis, proper CT technique is essential: 1

• Thin-section reconstruction (<1.5 mm thickness) with contiguous or overlapping slices 1
• High-spatial-frequency algorithm for reconstruction 1
• Supine inspiratory volumetric acquisition is mandatory 1
• Prone inspiratory scans are optional but helpful to distinguish true subpleural abnormalities from dependent atelectasis 1
• Adequate inspiration is critical - inadequate inspiration can mimic ground-glass attenuation and mask subtle abnormalities 1

Clinical Context and Diagnostic Confidence

The diagnostic confidence for IPF based on CT patterns follows a hierarchy: 2

• Definite UIP pattern = high confidence (>90%) for IPF diagnosis when other causes are excluded 2
• Probable UIP pattern = provisional high confidence (70-89%) for IPF diagnosis 2
• Indeterminate pattern = requires multidisciplinary discussion and often surgical lung biopsy 1, 2

Critical pitfall: Even with a definite UIP pattern on CT, the diagnosis of IPF requires exclusion of other known causes of interstitial lung disease, including hypersensitivity pneumonitis, connective tissue disease, drug toxicity, and occupational exposures. 1, 2 A UIP pattern is not synonymous with IPF - it is a morphologic pattern that can occur in multiple diseases. 3

When CT findings are not definitive (probable UIP or indeterminate patterns), multidisciplinary discussion involving experienced pulmonologists, radiologists, and pathologists is mandatory, and surgical lung biopsy should be strongly considered if the patient is an acceptable surgical candidate. 1, 2