ERS/ATS 2025 Update: A CT-First Approach to Interstitial Pneumonias

A practical radiology review of ERS/ATS 2025 Update: A CT-First Approach to Interstitial Pneumonias, focused on imaging findings, differential diagnosis, reporting points, and high-yield teaching pearls.

Purpose & Scope

• 2025 ERS/ATS official update (replaces 2013)
• Unifies idiopathic + secondary interstitial pneumonias
• Built on multidisciplinary diagnosis (MDD)
• CT pattern recognition = first step

MAJOR CONCEPTUAL SHIFTS

• No longer “idiopathic-only” → one system for all causes
• Pattern ≠ Diagnosis (e.g. NSIP pattern ≠ idiopathic NSIP)
• Interstitial vs Alveolar filling disorders
• Interstitial → fibrotic vs non-fibrotic

Diagnostic confidence must be stated

• Molecular tools = research only, not routine

INTERSTITIAL PATTERNS (CT-FIRST THINKING)

UIP

• Subpleural + basal
• Reticulation, traction bronchiectasis, honeycombing
• IPF vs CTD-UIP vs HP-UIP
• Probable UIP = no honeycombing

NSIP

• Lower lobes
• Subpleural sparing (key!)
• GG ± reticulation ± traction
• CTD-associated >> idiopathic
• Cellular ↔ fibrotic spectrum

BIP (NEW – MAJOR PATTERN )

• Airway-centred disease
• Mosaic attenuation, air-trapping, 3-density sign
• Seen in HP, CTD, aspiration, drugs
• BIP is a morphological pattern; hypersensitivity pneumonitis remains an etiological diagnosis.
• When appropriate, report “BIP pattern; hypersensitivity pneumonitis favoured” and correlate clinically.
• Idiopathic BIP is a provisional diagnosis of exclusion.

DAD

• Diffuse GG ± consolidation
• Organising → fibrotic phase
• AIP Idiopathic DAD

PPFE

• Upper-lobe pleural fibrosis
• Volume loss, platythorax
• Often combined with UIP/NSIP/BIP

LIP

• Thin-walled cysts + GG
• Mid–lower lungs
• Often secondary (CTD, immune deficiency)

ALVEOLAR FILLING DISORDERS

Organising Pneumonia (OP)

• Patchy consolidation
• Reversed halo, perilobular pattern

RB-ILD

• Upper-lobe centrilobular GG nodules
• Smoking-related

AMP (NEW TERM, alveolar macrophage pneumonia)

• DIP Alveolar Macrophage Pneumonia
• Diffuse/patchy GG ± emphysema
• Smoking common

Rare AFPs

AEP / CEP

PAP

• Lipoid pneumonia

COMBINED & UNCLASSIFIABLE

• Mixed patterns common (e.g. UIP + PPFE)
• Unclassifiable ILD = no dominant pattern or low confidence

DIAGNOSTIC CONFIDENCE (MANDATORY)

• Confident ≥90%
• Provisional 51–89%
• Unclassifiable <50%
• Apply to CT, pathology & final MDD

RADIOLOGY REPORTING RULES

• Name the PATTERN
• Add favoured diagnosis + DDx
• State confidence level
• Acknowledge evolution over time

ULTRA–HIGH-YIELD TAKEAWAYS

• Describe the BIP morphology and separately state when hypersensitivity pneumonitis is the favoured cause.
• Pattern-based CT thinking is now central
• Confidence + clarity > definitive labels
• This framework aligns imaging, pathology & therapy

Non-fibrotic BIP

• Diffuse centrilobular ground-glass nodules
• Airway-centred distribution
• No traction bronchiectasis or fibrosis
• Think: inflammatory / early HP-type BIP

Fibrotic BIP

Three-density sign (mosaic attenuation)

• ⬆ Increased density (inflamed lung)
• Normal lung
• ⬇ Decreased density (air-trapping)
• Reticulation + traction bronchiectasis
• Scattered cysts (often lower lobes)
• This morphology may be described as a fibrotic BIP pattern; hypersensitivity pneumonitis can be stated separately as the favoured aetiology when supported.