As a part of international evidence-based guidelines adopted by a collaborative effort of the American Thoracic Society (ATS), the European Respiratory Society (ERS), the Japanese Respiratory Society (JRS), and the Latin American Thoracic Association (ALAT), specific diagnostic HRCT criteria for usual interstitial pneumonia (UIP) pattern were adopted in 2011. These criteria have been shown to have moderate interobserver reliability among thoracic radiologists ⁴. 

In 2018, the Fleischner Society has published an updated white paper on the approach for the HRCT diagnosis of usual interstitial pneumonia: diagnostic HRCT criteria for usual interstitial pneumonia (UIP) pattern - Fleischner society guideline (2018). 

Classification

• UIP pattern (definite)
• possible UIP pattern
• inconsistent UIP pattern

This helps radiologists to determine the certainty of usual interstitial pneumonia diagnosis based on HRCT chest findings. The importance of this guideline is that definite UIP pattern on chest HRCT precludes the need for tissue diagnosis ^1,2. However, unfortunately up to 20% of inconsistent with UIP group (or actually atypical UIP) can be UIP on biopsy or progress clinically to a diagnosis idiopathic pulmonary fibrosis (IPF).

UIP pattern

All four features present:

1. subpleural, basal predominance
2. reticular abnormality
3. honeycombing +/- traction bronchiectasis
4. absence of features listed as "inconsistent with UIP pattern" (see below)

Possible UIP pattern

All three features present:

1. subpleural, basal predominance
2. reticular abnormality
3. absence of features listed as "inconsistent with UIP pattern" (see below)

Inconsistent with UIP pattern

Any one of the following seven features present:

• upper or mid-lung predominance
• peribronchovascular predominance
• extensive ground-glass abnormality (i.e. more than reticular abnormality)
• profuse micronodules (bilateral, predominantly upper lobes)
• discrete cysts (multiple, bilateral, away from honeycombing)
• diffuse mosaic attenuation / air-trapping (bilateral in ≥3 lobes)
• consolidation in bronchopulmonary segment(s) or lobe(s)

See also

• ATS criteria for the diagnosis of UIP on pathology

• diagnostic categories of UIP pattern based on HRCT chest- Fleischner society guideline 2018