. 2020 Dec 21;22(5):811–828. doi: 10.3348/kjr.2020.1132

Table 2. Histopathological Criteria for UIP and NSIP.

UIP^*	Probable UIP^*	Indeterminate UIP^*	Alternative Diagnosis^*
Dense fibrosis with architectural distortion (i.e., destructive scarring and/or honeycombing)	Some histologic features from UIP (column 1) are present but to an extent that preludes a definite diagnosis of UIP/IPF	Fibrosis with or without architectural distortion with features favoring either a pattern other than UIP or features favoring UIP secondary to another cause	Features of other histologic patterns of IIPs (e.g., absence of fibroblast foci or loose fibrosis) in all biopsies

Predominantly subpleural and/or paraseptal distribution of fibrosis	AND
	Absence of features to suggest an alternate diagnosis		Histologic findings indicative of other diseases (e.g., hypersensitivity pneumonitis, Langerhans cell histiocytosis, sarcoidosis, LAM)
	Absence of features to suggest an alternate diagnosis
		Some histologic features from UIP (column 1), but with other features suggesting an alternative diagnosis
Patchy involvement of lung parenchyma by fibrosis	OR
Patchy involvement of lung parenchyma by fibrosis	Honeycombing only

Fibroblast foci

Absence of features to suggest an alternate diagnosis
Fibrosing NSIP^†		Cellular NSIP^†
Key Features	Pertinent Negative Factors	Key Features	Pertinent Negative Factors
Dense or loose interstitial fibrosis with uniform appearance	Temporal heterogeneity	Mild to moderate interstitial chronic inflammation	Dense interstitial fibrosis
		Mild to moderate interstitial chronic inflammation
	Fibroblastic foci with dense fibrosis		Prominence of organizing pneumonia
	Fibroblastic foci with dense fibrosis	Type II pneumocyte hyperplasia in areas of inflammation	Prominence of organizing pneumonia
Frequent preservation of lung architecture		Type II pneumocyte hyperplasia in areas of inflammation
Frequent preservation of lung architecture	Findings above are especially important in case with patchy involvement and subpleural or paraseptal distribution		Diffuse severe alveolar septal inflammation
			Diffuse severe alveolar septal inflammation
Mild or moderate interstitial chronic inflammation

^*Raghu et al. Am J Respir Crit Care Med 2018;198:e44-e68 [8], ^†Travis et al. Am J Respir Crit Care Med 2008;177:1338-1347 [37]. IIP = idiopathic interstitial pneumonia, IPF = idiopathic interstitial fibrosis, LAM = lymphangioleiomyomatosis, NSIP = nonspecific interstitial pneumonia, UIP = usual interstitial pneumonia