BIP
(bronchiolocentric interstitial pneumonia)

a pattern of predominantly airway-centered interstitial pneumonia, which can occur in hypersensitivity pneumonitis, or systemic autoimmune and rheumatic disease, as well as due to several other etiologies; the current definition of this term was proposed in an ERS/ATS 2025 update to the classification of ILD

What does it look like?

HRCT Findings

  • Non-fibrotic BIP: profuse centrilobular ground glass nodules, mosaic attenuation, and/or lobular air trapping

  • Fibrotic BIP: peribronchovascular ground glass, traction bronchiectasis, mosaic attenuation, lobular air trapping, and three density sign; features may overlap with non-fibrotic BIP

Histopathological Findings

  • Non-fibrotic BIP: Bronchiolocentric, chronic, lymphocyte-predominant inflammation ± non-necrotizing granulomas

  • Fibrotic BIP: Bronchiolocentric fibrosis, often with peribronchiolar metaplasia ± bronchiolocentric inflammation

What diseases can produce it?

  • Hypersensitivity pneumonitis

  • Aspiration/GERD

  • SARD, especially rheumatoid arthritis-associated ILD

  • Inhalational and medication exposures

  • Idiopathic BIP (provisional diagnosis)

How confidently can we recognize it?

Histologically, the pattern is recognizable. However, certain features may also suggest specific etiologies. For example, patchy, granulomatous inflammation in a peribronchiolar distribution is highly suggestive of hypersensitivity pneumonitis when present. Similarly, foci of organizing pneumonia, foamy macrophages, multinucleated giant cells, suppurative granulomas, and (especially) foreign material raise concern for aspiration.

However, how to use CT Chest findings to predict histology is less clear for BIP than for some other morphologic patterns. In fact, the ERS/ATS 2025 update to classification explicitly identifies this as a future direction: “What is the radiological-pathological correlation of BIP?” For now:

  • Inflammatory BIP on CT Chest can be regarded as highly suggestive of inflammatory hypersensitivity pneumonitis (HP); however, when centrilobular nodules are not especially profuse, considering other etiologies is worthwhile.

  • Fibrotic BIP on CT Chest is less well characterized; however, with a known exposure and an absence of features suggesting another etiology, it is considered highly suggestive of mixed or fibrotic hypersensitivity pneumonitis (fHP).

How does it inform clinical reasoning?

  • [Widely different ranges]

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[concerns about BIP, especially iBIP]

[HP; HP without identified exposure; iBIP (review concept of provisional diagnosis)]

[should iBIP result in lung biopsy]

References

  • Ryerson CJ, Adegunsoye A, Piciucchi S, Hariri LP, Khor YH, Wijsenbeek MS, Wells AU, Sharma A, Cooper WA, Antoniou K, Borie R. Update of the international multidisciplinary classification of the interstitial pneumonias: an ERS/ATS statement. European Respiratory Journal. 2025 Dec;66(6):2500158.

  • Ryerson CJ, Adegunsoye A, Hariri LP, Piciucchi S, Travis WD, Nicholson AG. Bronchiolocentric interstitial pneumonia is a more accurate interstitial lung disease classification than hypersensitivity pneumonitis: pro. American Journal of Respiratory and Critical Care Medicine. 2026 Mar;212(3):561-3.

  • Myers JL, Remy-Jardin M, Lynch DA, Raghu G. Rebuttal to “Bronchiolocentric interstitial pneumonia is a more accurate interstitial lung disease classification than hypersensitivity pneumonitis: con”. American Journal of Respiratory and Critical Care Medicine. 2026 Mar;212(3):564-7.

  • Cottin V. Bronchiolocentric interstitial pneumonia: words matter. American Journal of Respiratory and Critical Care Medicine. 2026 Mar;212(3):548-50.