In addition to acute rejection, transbronchial biopsy is typically used for lung transplant recipients for diagnosis of opportunistic infections (cytomegalovirus, Pneumocystis, etc.), which are discussed in their respective separate chapters. Occasionally, acute rejection and infection may coexist in the same patient and both may be sampled in the same transbronchial biopsy. Some features traditionally associated with infection such as organizing pneumonia are now also thought to occasionally be features of acute rejection, further complicating interpretation of transbronchial biopsies. Correlation with the clinical findings may be very helpful in these situations.

In addition to acute rejection and infection, other histopathologic findings may be seen in transbronchial biopsies from lung transplant recipients. Chronic rejection is a transplant-associated pathology that is potentially diagnosable by transbronchial biopsy. Chronic rejection can be seen months to years posttransplant and is characterized by an obliterative bronchiolitis with inflammation of small airways with fibrosis resulting in complete luminal occlusion. The histopathologic diagnosis of chronic rejection requires sampling of a bronchiole by the bronchoscopist, which does not always occur with every biopsy. Clinicians may also make a diagnosis of bronchiolitis obliterans syndrome on a clinical basis. The finding of intra-alveolar macrophages is suggestive of small airways obstruction when a bronchiole is not sampled on the transbronchial biopsy.

Various other conditions can be potentially sampled on a transbronchial (or endobronchial) biopsy. Accelerated vascular arteriosclerosis is usually seen in long-term transplant patients who develop patchy arteriosclerosis of large and small vessels with narrowing of lumens due to fibrointimal and myointimal proliferation, concentric intimal sclerosis of veins, and arteriosclerotic plaque formation. Primary graft failure, which is usually seen within 3 days of transplant, is an acute lung injury syndrome resulting from ischemia-reperfusion injury and characterized by patchy or diffuse formation of hyaline membranes, interstitial edema, and organization (diffuse alveolar damage, or DAD). Anastomotic complications can be seen with necrosis of bronchial mucosa, submucosa, and cartilage. Thrombus formation, granulation tissue formation with mucosal ulceration, and fibrosis of submucosa can also be seen with anastomotic complications. Rarely hyperacute rejection can occur within minutes to hours posttransplant and is often fatal. Biopsy tissue shows edema, hyaline membrane formation, intra-alveolar hemorrhage, fibrin thrombi, and foci of necrosis of septa and bronchiolar mucosa.