# Bronchiolitis Obliterans

Bronchiolitis obliterans is an irreversible small-airway fibrotic syndrome most often encountered after lung or allogeneic hematopoietic stem-cell transplantation. Early spirometric decline, infection exclusion, and transplant-team evaluation are central because established airflow obstruction is difficult to reverse.

**Clinical question:** How should physicians recognize, monitor, and manage suspected bronchiolitis obliterans after lung or allogeneic hematopoietic stem-cell transplantation?

Updated: 2026-08-21T01:33:35.155713+00:00

## What matters in practice
- After allogeneic hematopoietic stem-cell transplantation, bronchiolitis obliterans syndrome is closely linked to chronic graft-versus-host disease and generally develops within 2 years. [1]
- New obstructive physiology on pulmonary function testing is the central clinical basis for bronchiolitis obliterans syndrome; declining FEV1 may precede established obstruction. [1][4]
- Established post-transplant bronchiolitis obliterans syndrome is commonly irreversible despite immunosuppressive treatment, making surveillance and prompt investigation of functional decline high-value. [1]
- Do not use azithromycin prophylaxis after allogeneic hematopoietic stem-cell transplantation to prevent airflow decline; the ALLOZITHRO randomized trial evaluated this strategy in this setting. [1]
- Evidence for treatment remains limited; a 2024 ERS/EBMT clinical practice guideline addresses pulmonary chronic graft-versus-host disease in adults, and management should be coordinated with transplant and pulmonary specialists. [22]

## Recognize the relevant bronchiolitis obliterans phenotype

The term has distinct clinical uses that should not be conflated.

Bronchiolitis obliterans is a small-airway injury syndrome characterized pathologically by bronchiolar inflammation and fibrosis that narrows or obliterates terminal airways, producing progressive airflow obstruction. Airway epithelial injury and chronic intrapulmonary inflammation are implicated in environmental and occupational disease. [2][18]

In transplant practice, bronchiolitis obliterans syndrome (BOS) is a clinical syndrome defined by airflow obstruction rather than a requirement for surgical-pathologic confirmation. It occurs after lung transplantation and after allogeneic hematopoietic stem-cell transplantation (HSCT), where it is a pulmonary manifestation of chronic graft-versus-host disease (cGVHD). [1][4][11][18]

Keep post-HSCT BOS distinct from the pediatric acute viral bronchiolitis guideline literature, which addresses a different disease and does not inform BOS treatment. [21]

*Clinical settings in which the bronchiolitis obliterans label has materially different implications. [1][2][4][11][18]*

| Setting | Clinical implication |
| --- | --- |
| Allogeneic HSCT | BOS is linked to cGVHD and presents as new obstructive pulmonary function abnormality, often within 2 years after transplantation. [1][4] |
| Lung transplantation | BOS denotes obstructive chronic lung allograft dysfunction and is staged clinically using spirometric data. [8][11] |
| Environmental or occupational exposure | Epithelial injury and chronic intrapulmonary inflammation are central pathogenic considerations; exposure history can direct etiologic investigation. [2] |

## Find airflow decline before fixed obstruction is advanced

Symptoms alone are an insensitive trigger for evaluation in transplant recipients.

In allogeneic HSCT recipients, obtain and trend spirometry because an FEV1 decrease may occur before overt obstructive lung disease. New cough, dyspnea, increased oxygen requirement, a reduced pulmonary-function result, or abnormal high-resolution CT findings should prompt focused evaluation. [1][9]

Pediatric guidance consistently endorses pulmonary-function-test surveillance after HSCT, although reported recommended intervals range from every 3 months to annually during the first post-transplant year. This heterogeneity supports using the responsible transplant program's surveillance pathway rather than assuming a universally established interval. [7]

Children must be able to perform reproducible pulmonary function testing for BOS criteria to be applicable; this is generally feasible after approximately age 5 years. [10]
- Compare spirometry with the patient's prior post-transplant values; a trajectory of FEV1 decline is clinically meaningful even before a new obstructive pattern is fully established. [1]
- In a symptomatic or declining patient, involve transplant pulmonology and the transplant team early; established disease has limited reversibility. [1][20]
- Use high-resolution CT and infection-directed assessment selectively as part of the evaluation of symptoms or physiologic decline; published biopsy practice has incorporated symptoms, pulmonary function, and HRCT findings. [9]

## Anchor the diagnosis in physiology and exclude competing causes

BOS is a clinical diagnosis; biopsy is not routinely required to establish it.

For post-HSCT disease, the defining clinical signal is new obstructive lung disease on pulmonary function testing in the appropriate transplant and cGVHD context. For lung allograft recipients, BOS remains a spirometry-based clinical entity within obstructive chronic lung allograft dysfunction. [1][4][8][11]

The supplied sources do not provide sufficiently detailed consensus thresholds for FEV1, FEV1/FVC, residual volume, or imaging criteria. Avoid applying uncited numeric cutoffs from memory when using this evidence set; use current transplant-specific consensus criteria and interpret results against the patient's baseline. [20][22]

Do not equate an obstructive spirometric pattern automatically with BOS. Infection, acute allograft or airway processes, medication effects, and alternative structural or parenchymal disease require assessment in the relevant transplant context. This is especially important because symptoms, oxygen requirement, pulmonary-function decline, and HRCT abnormalities have all been used to select patients for lung biopsy in published post-HSCT cohorts. [9]
- Review pre- and post-transplant spirometric trajectories rather than relying on a single test. [1]
- Assess for concurrent cGVHD manifestations in allogeneic HSCT recipients because BOS is closely associated with cGVHD. [1][4]
- Reserve biopsy decisions for multidisciplinary discussion when a competing diagnosis would alter treatment; the available evidence describes clinically directed biopsy, not routine diagnostic biopsy. [9]

## Treat as a transplant complication with limited evidence for reversal

Management priorities differ between prevention, early detection, and established disease.

Established BOS after HSCT is generally considered irreversible despite immunosuppressive therapy. Treatment should therefore prioritize confirmation of the phenotype, evaluation for remediable competing processes, control of associated cGVHD, serial pulmonary-function assessment, and specialist-directed optimization of immunosuppression and supportive care. [1][20][22]

Systemic glucocorticoids were endorsed as first-line systemic treatment in an earlier consensus discussion, but the available search results also emphasize poor response to corticosteroids or immunosuppressants and the absence of established treatment approaches. This tension reflects limited and evolving evidence rather than a reliably effective reversal strategy. [3][24]

For adult pulmonary cGVHD, use the 2024 ERS/EBMT clinical practice guideline as the current guideline anchor and coordinate decisions with the transplant center. The supplied excerpts do not provide its specific recommendations, agent selection, or dosing; these should not be reconstructed from the citations here. [22]
- Do not start azithromycin solely as prophylaxis against post-allogeneic-HSCT airflow decline. The ALLOZITHRO randomized clinical trial examined azithromycin prophylaxis in this population. [1]
- Do not extrapolate lung-transplant prevention data to HSCT BOS: inhaled fluticasone did not prevent BOS or improve survival in a small randomized lung-transplant trial. [14]
- For lung-transplant BOS, optimization of immunosuppression to limit further allograft rejection is a major treatment focus; management is transplant-center specific. [15]

### Therapies with important evidence limits

Ruxolitinib has been studied in a registered trial for BOS after allogeneic hematopoietic cell transplantation, but the supplied record does not report efficacy, safety, or dosing results. Trial registration alone should not be treated as evidence for routine use. [22]

Reports of successful treatment and newer treatment-option reviews are hypothesis-generating or observational in the supplied results. They cannot establish comparative efficacy or a standardized regimen. [16][17]

*Evidence-informed management boundaries for post-transplant BOS. [1][3][14][15][22][24]*

| Decision | What the available evidence supports | What not to infer |
| --- | --- | --- |
| Post-HSCT prophylaxis with azithromycin | A randomized trial evaluated azithromycin prophylaxis for airflow decline–free survival after allogeneic HSCT. [1] | Do not use it as routine BOS prophylaxis on the basis of these results. [1] |
| Systemic immunosuppression for HSCT-associated BOS | Historical consensus endorsed systemic glucocorticoids as first-line systemic treatment; current adult pulmonary cGVHD guidance is available through ERS/EBMT. [3][22] | Do not promise reversal of established obstruction; response is often poor. [1][24] |
| Inhaled fluticasone after lung transplantation | A placebo-controlled trial found no prevention benefit for BOS or survival. [14] | Do not extrapolate a negative prevention trial into evidence about all inhaled regimens or treatment of established disease. [14] |
| Lung-transplant BOS | Optimization of immunosuppression to reduce further allograft rejection is a major therapeutic focus. [15] | Do not apply HSCT cGVHD management pathways without transplant-specific review. [1][15] |

## Communicate the risk of irreversible respiratory decline

Prognostic estimates vary by transplant population and study era.

After allogeneic HSCT, BOS has been reported in 4% to 6% of recipients overall and in up to 14% of those with cGVHD. It usually develops within 2 years after transplantation. [1]

A pediatric HSCT trial description cites approximately 40% to 60% mortality and 13% 5-year survival once BOS is diagnosed; these estimates should be communicated cautiously because they derive from a trial background summary rather than a contemporary comparative cohort. [19]

The key prognostic consequence is that airflow obstruction is often progressive and difficult to reverse. This supports active surveillance, early review of FEV1 decline, and timely transplant-center involvement rather than waiting for severe symptomatic limitation. [1][20]

## Common questions

### Is bronchiolitis obliterans syndrome the same as chronic graft-versus-host disease?

After allogeneic HSCT, BOS is closely linked to cGVHD and is regarded as a pulmonary manifestation of cGVHD, but it is identified clinically through new obstructive pulmonary physiology. [1][4][18]

### Should azithromycin be prescribed to prevent BOS after allogeneic HSCT?

No. Azithromycin prophylaxis should not be used solely to prevent post-allogeneic-HSCT airflow decline or BOS; this strategy was evaluated in the ALLOZITHRO randomized trial. [1]

### When should a transplant recipient with suspected BOS undergo lung biopsy?

Biopsy is not required for the clinical BOS construct. Published post-HSCT practice selected biopsy using symptoms, pulmonary-function decline, oxygen need, and/or HRCT abnormalities; use multidisciplinary review when histology would change management. [9][11]

### How often should pulmonary function be monitored after pediatric HSCT?

Pediatric guidelines endorse surveillance, but recommendations cited in the available literature range from every 3 months to annually in the first post-transplant year. Follow the transplant program's protocol and escalate testing for symptoms or declining FEV1. [7][1]

## References
1. Effect of Azithromycin on Airflow Decline–Free Survival After Allogeneic Hematopoietic Stem Cell Transplant: — jamanetwork.com — https://jamanetwork.com/journals/jama/fullarticle/2647869
2. Environmental and occupational bronchiolitis obliterans — www.thelancet.com — https://www.thelancet.com/journals/ebiom/article/PIIS2352-3964(23)00325-0/fulltext
3. What we know and mostly do not know about bronchiolitis ... — www.nature.com — https://www.nature.com/articles/bmt201138
4. Bronchiolitis obliterans syndrome after allogeneic hematopoietic SCT: phenotypes and prognosis — www.nature.com — https://www.nature.com/articles/bmt2012241?error=cookies_not_supported
5. Bronchiolitis obliterans following pediatric allogeneic hematopoietic stem cell transplantation | Bone Marrow Transplantation — www.nature.com — https://www.nature.com/articles/bmt200819?error=cookies_not_supported
6. To Respond or Not to Respond: That's the Question... — journals.lww.com — https://journals.lww.com/transplantjournal/fulltext/2026/02000/to_respond_or_not_to_respond__that_s_the_question.10.aspx
7. Detection of Bronchiolitis Obliterans Syndrome after Pediatric ... — academic.oup.com — https://academic.oup.com/ajrccm/article/210/3/262/8491330
8. Quantitative Image Analysis at Chronic Lung... — journals.lww.com — https://journals.lww.com/transplantjournal/fulltext/2022/06000/quantitative_image_analysis_at_chronic_lung.26.aspx
9. Biopsy-Verified Bronchiolitis Obliterans and Other ... — www.sciencedirect.com — https://www.sciencedirect.com/science/article/pii/S1083879114013974
10. Bronchiolitis Obliterans - an overview — www.sciencedirect.com — https://www.sciencedirect.com/topics/nursing-and-health-professions/bronchiolitis-obliterans
11. Bronchiolitis obliterans syndrome is not specific for ... — www.sciencedirect.com — https://www.sciencedirect.com/science/article/abs/pii/S1053249814014259
12. Management and clinical outcomes after lung ... — onlinelibrary.wiley.com — https://onlinelibrary.wiley.com/doi/10.1111/tid.13084
13. Lung transplantation in adults and children: Putting ... — onlinelibrary.wiley.com — https://onlinelibrary.wiley.com/doi/full/10.1111/resp.12370
14. Addition of inhaled corticosteroids to systemic... : Transplantation — journals.lww.com — https://journals.lww.com/transplantjournal/abstract/00007890-200206150-00016~addition-of-inhaled-corticosteroids-to-systemic
15. Extracorporeal Photopheresis for Bronchiolitis... — journals.lww.com — https://journals.lww.com/transplantjournal/fulltext/2018/07000/extracorporeal_photopheresis_for_bronchiolitis.14.aspx
16. Successful treatment of severe post hematopoietic stem ... — onlinelibrary.wiley.com — https://onlinelibrary.wiley.com/doi/full/10.1002/ccr3.6267
17. Treatment Options for Bronchiolitis Obliterans Secondary to ... — onlinelibrary.wiley.com — https://onlinelibrary.wiley.com/doi/abs/10.1111/ctr.70274
18. Prevalence of Bronchiolitis Obliterans Syndrome (BOS) Following Allogeneic Hematopoietic Stem Cell Transplant (alloHSCT) in the USA, Europe and Japan - ScienceDirect — www.sciencedirect.com — https://www.sciencedirect.com/science/article/pii/S0006497118636043
19. Study Details | NCT03072849 | Early Detection and Management of Bronchiolitis Obliterans Syndrome Following Pediatric Hematopoietic Stem Cell Transplantation | ClinicalTrials.gov — clinicaltrials.gov — https://clinicaltrials.gov/study/NCT03072849
20. How I treat bronchiolitis obliterans syndrome after ... — ashpublications.org — https://ashpublications.org/blood/article/129/4/448/36179/How-I-treat-bronchiolitis-obliterans-syndrome
21. Clinical Practice Guideline: The Diagnosis, Management ... — pediatrics.aappublications.org — https://pediatrics.aappublications.org/content/134/5/e1474
22. Study Details | NCT03674047 | Ruxolitinib for Bronchiolitis Obliterans Syndrome (BOS) After Allogeneic Hematopoietic Cell Transplantation (HCT) | ClinicalTrials.gov — clinicaltrials.gov — https://clinicaltrials.gov/study/NCT03674047
23. Study Details | NCT01327625 | Efficacy Study of Azithromycin-based Therapy for Bronchiolitis Obliterans | ClinicalTrials.gov — clinicaltrials.gov — https://clinicaltrials.gov/study/NCT01327625
24. Tamibarotene for the Treatment of Bronchiolitis Obliterans ... — journal.chestnet.org — https://journal.chestnet.org/article/S0012-3692(18)32340-7/fulltext

## Editorial note

Prepared from cited clinical literature using Astra's research workflow. Verify recommendations against current guidance and patient-specific factors.