Diagnosis
Bronchiolitis obliterans is often diagnosed based on the symptoms of obstructive lung disease following lung injury. The definitive diagnosis is through biopsy, but due to the variable distribution of lesions, leading to falsely negative tests, and invasive nature of this procedure it is often not performed. Several tests are often needed to diagnose bronchiolitis obliterans, including spirometry, diffusing capacity of the lung tests (DLCO), lung volume tests, chest X-rays,  high-resolution CT (HRCT), and lung biopsy. Pulmonary function testing
Spirometry tests usually show an obstructive pattern and is the most common presentation. A slightly reduced to normal forced vital capacity (FVC), and a reduced FEV1 to FVC ratio and forced expiratory volume (FEV) with little to no correction with the use of bronchodialators are common findings. Lung volume tests may show hyperinflation (excessive air in lungs caused by air trapping). Diffusing capacity of the lung (DLCO) tests are usually normal; people with early-stage OB are more likely to have normal DLCO.FEV1 (forced expiratory volume in 1 second) should be above 80% of predicted values to be considered normal. Bronchiolitis obliterans reduces this to between 16% and 21%. Medical imaging
Early in the disease chest radiography is typically normal but may show hyperinflation. As the disease progresses a reticular pattern with thickening of airway walls may be present. HRCT can also show air trapping when the person being scanned breathes out completely; it can also show thickening in the airway and haziness in the lungs.
These symptoms represent an obstructive pattern that is non-reversible with bronchodilator therapy, and need to be related to various lung insults. These insults include inhalation damage, post transplant auto-immune injury, post-infectious disease, drug reactions, and several auto-immune diseases. Cause
Bronchiolitis obliterans has many possible causes, including collagen vascular disease, transplant rejection in organ transplant patients, viral infection (adenovirus, respiratory syncytial virus, influenza, HIV, cytomegalovirus), Stevens–Johnson syndrome, Pneumocystis pneumonia, drug reaction, aspiration and complications of prematurity (bronchopulmonary dysplasia), and exposure to toxic fumes. Toxins implicated in the condition include diacetyl, sulfur dioxide, nitrogen dioxide, ammonia, chlorine, thionyl chloride, methyl isocyanate, hydrogen fluoride, hydrogen bromide, hydrogen chloride, hydrogen sulfide, phosgene, polyamide-amine dyes, mustard gas and ozone. It can also be present in patients with IBD, systemic lupus erythematosus, juvenile idiopathic arthritis, rheumatoid arthritis, GERD, IgA nephropathy, and ataxia telangiectasia. Activated charcoal has been known to cause it when aspirated. The ingestion of large doses of papaverine in the vegetable Sauropus androgynus has caused it. Additionally, the disorder may be idiopathic (without known cause). Lung transplant
Bronchiolitis obliterans is a common complication in lung transplants because transplanted lungs are at greater risk of alloimmunization as compared to healthy lungs. The disease is often termed bronchiolitis obliterans syndrome (BOS) in the setting of post lung transplantation and hematopoietic stem cell transplant (HSCT). Patients who develop BOS post lung transplant vary in disease latency and severity. Patients often initially have normal lung function on pulmonary function testing and have normal chest radiographs.