Table 1 2018 ATS/ERS/JRS/ALAT guidelines for the histopathologic diagnosis of idiopathic pulmonary fibrosis and 2020 ATS/JRS/ALAT guidelines for the histopathologic diagnosis of fibrotic hypersensitivity pneumonitis.

UIP

Probable UIP

Indeterminate for UIP

Alternative diagnosis

2018 ATS/ERS/JRS/ALAT Guidelines for UIP

• Dense fibrosis with architectural distortion (i.e., destructive scarring and/or honeycombing)

• Predominant subpleural and/or paraseptal distribution of fibrosis

• Patchy involvement of lung parenchyma by fibrosis

• Fibroblast foci

• Absence of features to suggest an alternate diagnosis

• Some histologic features from column 1 are present but to an extent that precludes a definite diagnosis of UIP/IPF

AND

• Absence of features to suggest an alternative diagnosis

OR

• Honeycombing only

• Fibrosis with or without architectural distortion, with features favoring either a pattern other than UIP or features favoring UIP secondary to another cause

• Some histologic features from column 1, but with other features suggesting an alternative diagnosis

• Features of other histologic patterns of IIPs (e.g., absence of FF or loose fibrosis) in all biopsies

• Histologic findings indicative of other diseases (e.g., hypersensitivity pneumonitis, Langerhans cell histiocytosis, sarcoidosis, LAM)

fHP

Probable fHP

2020 ATS/JRS/ALAT Guidelines for Fibrotic Hypersensitivity Pneumonitis

• Chronic fibrosing interstitial pneumonia (architectural distortion, FF, honeycomb OR fNSIP)

OR

• Airway-centered fibrosis

AND

• Poorly formed nonnecrotizing granulomas

• Absence of features that might suggest an alternate diagnosis (see third column)

• Chronic fibrosing interstitial pneumonia (architectural distortion, FF, honeycomb OR fNSIP)

OR

• Airway-centered fibrosis

AND

• Absence of features that might suggest an alternate diagnosis (see third column)