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Usual Interstitial Pneumonia (UIP) is a specific histopathologic and radiologic pattern of chronic fibrosing interstitial pneumonia, most classically associated with Idiopathic Pulmonary Fibrosis (IPF). However, it may also occur in connective tissue diseases, asbestosis, and chronic hypersensitivity pneumonitis.

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Etiology

Clinical associations:

Primary Condition Association with UIP Pattern
‣ Classic UIP without known cause (most common)
Rheumatoid Arthritis–associated ILD Second most common cause of UIP
‣ Mimics UIP radiologically
Chronic Hypersensitivity Pneumonitis Can show fibrotic UIP pattern

Radiology

https://youtu.be/TUA0eW24Kyo

https://doi.org/10.1148/radiol.2021204367

HRCT features

Fleischner Society 2020 diagnostic criteria:

Feature Description
Distribution Basal and subpleural predominant
Reticulation Coarse reticular opacities
Honeycombing Clustered cystic airspaces, typically 3–10 mm in diameter, in subpleural areas
Traction bronchiectasis Dilated airways due to fibrotic pull
Absence of features suggesting alternative diagnosis No upper-lobe predominance, mosaicism, or nodules

HRCT UIP pattern classification:

UIP pattern (all four features) Possible UIP pattern (all three features) Inconsistent with UIP pattern
Subpleural, basal predominance Subpleural, basal predominance Upper or mid-lung predominance
Reticular opacities Reticular opacities Peribronchovascular predominance
Honeycombing with or without traction bronchiectasis Absence of features listed as “inconsistent” with UIP pattern Extensive ground-glass opacities (extent > reticular abnormality)
Absence of features listed as “inconsistent” with UIP pattern Discrete cysts multiple, bilateral, away from areas of honeycombing Profuse micronodules (bilateral, predominantly upper lobes)
Diffuse mosaic attenuation or air trapping (bilateral in three or more lobes)
Segmental or lobular consolidation

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Representative images

![(a, b) Axial CT image in a 63-year-old man with usual interstitial pneumonia (UIP)/idiopathic pulmonary fibrosis (IPF) shows bilateral reticular opacities, honeycombing (black arrowheads), and traction bronchiectasis (arrow). In addition, patchy ground-glass opacities are present (white arrowheads) (a). Acute exacerbation in the same patient shows marked progression of ground-glass opacities (arrowheads) (b)

Mueller-Mang, C., Ringl, H., Herold, C. (2017). Interstitial Lung Diseases. In: Nikolaou, K., Bamberg, F., Laghi, A., Rubin, G.D. (eds) Multislice CT. Medical Radiology(). Springer, Cham. https://doi.org/10.1007/174_2017_151](attachment:4d10a35c-e5e1-4947-bc7b-29835f5de332:63847_4_En_151_Fig6_HTML.webp)

(a, b) Axial CT image in a 63-year-old man with usual interstitial pneumonia (UIP)/idiopathic pulmonary fibrosis (IPF) shows bilateral reticular opacities, honeycombing (black arrowheads), and traction bronchiectasis (arrow). In addition, patchy ground-glass opacities are present (white arrowheads) (a). Acute exacerbation in the same patient shows marked progression of ground-glass opacities (arrowheads) (b)

Mueller-Mang, C., Ringl, H., Herold, C. (2017). Interstitial Lung Diseases. In: Nikolaou, K., Bamberg, F., Laghi, A., Rubin, G.D. (eds) Multislice CT. Medical Radiology(). Springer, Cham. https://doi.org/10.1007/174_2017_151

Differentials

Feature UIP NSIP HP
Honeycombing ✔️ (hallmark) ✖️ ✖️ / late stage only
Distribution Basal, subpleural Basal, symmetrical Mid-to-upper lung, patchy
GGO dominance Minimal More prominent Often with mosaicism, nodules
Air trapping ✖️ ✖️ ✔️ (expiratory HRCT key)