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 and reviewed in 2018.

In 2018, the Fleischner Society has also published a 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). 

Overview 

• the guideline is intended to help clinicians make an accurate diagnosis of IPF and to empower them to implement recommended courses of action, particularly decisions in which diagnostic intervention to pursue
• the guideline applies for the patterns and distribution assessed on HRCT chest
• these criteria have been shown to have moderate interobserver reliability among thoracic radiologists ⁴

Classification

• UIP pattern (definite)
• probable UIP pattern
• indeterminate for UIP pattern
• alternative diagnosis 

This helps radiologists to determine the certainty of usual interstitial pneumonia (UIP) 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 ^1,2,5. 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

Classic HRCT pattern including:

• honeycombing: it is the distinguishing feature of UIP and must be present
  • +/- traction bronchiectasis
• reticular abnormalities
• distribution: subpleural with a basal predominance
• absence of features listed as "inconsistent with UIP pattern" (see below)suggestive of an alternative diagnosis 

Probable UIP pattern

HRCT studies with absent honeycombing, but showing:

• reticular abnormalities 
  • +/- groundglass opacification superimposed on a fine reticular pattern (cf. pure groundglass opacities, which is not a typical UIP feature)
• peripheral traction bronchiectasis or traction bronchioloectasis 
• distribution: subpleural with a basal predominance
• absence of features listed as "inconsistent with UIP pattern" (see below)suggestive of an alternative diagnosis

Indeterminate for UIP pattern

HRCT features of pulmonary fibrosis that do not meet the "UIP" or "probable UIP" patterns above and does not explicitly suggest an "alternative diagnosis".

• subtle reticular abnormalities 
  • +/- groundglass opacification superimposed on a fine reticular pattern (cf. pure groundglass opacities, which is not a typical UIP feature)
• distribution: do not suggest any specific aetiology 

This category includes those patients with very limited subpleural reticulation or groundglass opacities without other obvious features of pulmonary fibrosis and for whom there is some suspicion for an early UIP or probable UIP. 

Alternative diagnosis 

Although there is clinical suspicion for IPF, HRCT findings are suggestive of another aetiology (e.g. hypersensitivity pneumonitis, NSIP, sarcoidosis).​

Findings in a peribronchovascular, perilymphatic or upper or mid-lung predominance suggest an alternative diagnosis. Additional findings include pulmonary cysts, marked mosaic attenuation (air trapping), predominant ground-glass opacities, profuse micronodules, centrilobular nodules, pleural effusions, pleural thickening,  pleural plaques (consider asbestosis), dilated esophagus (consider CTD), distal clavicular erosions (consider RA), extensive lymph node enlargement. 

Previous versions

In 2018 the guidelines were reviewed, and the main changes compared to the 2011 publication are:

• "possible UIP" no longer exists and has been replaced by "probable UIP"
  • studies have shown that most of these patients were likely to have histopathologic UIP pattern despite the radiologic absence of honeycombing 
• similar to the diagnostic HRCT criteria for usual interstitial pneumonia (UIP) pattern - Fleischner society guideline (2018), this reviewed guidelines moved from three to four HRCT categories with a reasonable correspondence among the two 2018 papers 
• now, the patient's management should be based on their HRCT classification in two groups ⁵:
  • HRCT pattern of probable UIP, indeterminate for UIP, and alternative diagnosis
  • HRCT pattern of UIP

See also

• ATS criteria for the diagnosis of UIP on pathology
• diagnostic categories of UIP pattern based on HRCT chest- Fleischner society guideline 2018