Table 5. Histopathological criteria for usual interstitial pneumonia (UIP) pattern

UIP pattern (all four features)	Probable UIP pattern	Possible UIP pattern (all three features)	Not UIP pattern (any of the features)
Evidence of marked fibrosis/architectural distortion±honeycombing in a predominantly subpleural/paraseptal distribution	Evidence of marked fibrosis/architectural distortion±honeycombing	Patchy or diffuse involvement of lung parenchyma by fibrosis, with or without interstitial inflammation	Hyaline membranes^#
Presence of patchy involvement of lung parenchyma by fibrosis	Absence of either patchy involvement or fibroblastic foci, but not both	Absence of other criteria for UIP	Organising pneumonia^#,¶
Presence of fibroblast foci	Absence of features against a diagnosis of UIP suggesting an alternate diagnosis	Absence of features against a diagnosis of UIP suggesting an alternate diagnosis	Granulomas^¶
Absence of features against a diagnosis of UIP suggesting an alternate diagnosis	OR Honeycomb changes only⁺		Marked interstitial inflammatory cell infiltrate away from honeycombing
			Predominant airway-centred changes
			Other features suggestive of an alternate diagnosis

^#: can be associated with acute exacerbation of idiopathic pulmonary fibrosis; ^¶: an isolated or occasional granuloma and/or a mild component of organising pneumonia pattern may rarely coexist in lung biopsies with an otherwise UIP pattern; ⁺: this scenario usually represents end-stage fibrotic lung disease where honeycombed segments have been sampled but where a UIP pattern might be present in other areas. Such areas are usually represented by overt honeycombing on high-resolution computed tomography (HRCT) and can be avoided by pre-operative targeting of biopsy sites away from these areas using HRCT. Reproduced from [2] with permission from the publisher.