Diffuse alveolar hemorrhage (DAH) is a syndrome of widespread bleeding from the pulmonary microcirculation (capillaries, arterioles, venules) into the alveolar spaces. It is a medical emergency with mortality up to 50 percent if untreated. Defining features: hemoptysis (absent in 33 percent — be vigilant), bilateral pulmonary infiltrates on chest imaging, anemia (often progressive over hours), hypoxemic respiratory failure, and BAL with progressively bloodier returns from sequential aliquots (gold-standard diagnostic, 30 percent or more red cells in last aliquot or hemosiderin-laden macrophages > 20 percent for chronic bleeding).
Pathologic classification: (1) Pulmonary capillaritis (most common immune cause) — neutrophilic infiltration of alveolar capillaries with fibrinoid necrosis; etiologies: ANCA-associated vasculitis (granulomatosis with polyangiitis = Wegener, microscopic polyangiitis, eosinophilic granulomatosis with polyangiitis = Churg-Strauss), anti-GBM disease (Goodpasture syndrome), SLE, antiphospholipid syndrome, IgA vasculitis (Henoch-Schönlein), cryoglobulinemic vasculitis, polyangiitis with mixed connective tissue disease, drug-induced (propylthiouracil, hydralazine, levamisole-contaminated cocaine); (2) Bland DAH (no inflammation) — anticoagulants (warfarin, DOACs, heparin), antiplatelets, thrombolytics, severe coagulopathy, mitral stenosis with elevated PCWP, mitral regurgitation, idiopathic pulmonary hemosiderosis; (3) Diffuse alveolar damage with DAH — acute respiratory distress syndrome, drug-induced (amiodarone, nitrofurantoin, sirolimus, all-trans-retinoic acid, mTOR inhibitors), bone marrow / hematopoietic stem cell transplant (DAH 1–4 percent of HSCT, very high mortality), invasive infection (CMV, aspergillosis, leptospirosis, dengue), radiation pneumonitis acute.
Diagnosis: comprehensive serology (ANCA-MPO, ANCA-PR3, anti-GBM, ANA, anti-dsDNA, anti-Sm, complement C3/C4, antiphospholipid antibodies, cryoglobulins, ASO, complement), HRCT chest (bilateral ground-glass opacities and consolidations with central predominance, sparing of subpleural areas), bronchoscopy with sequential BAL (essential — confirms DAH and excludes infection), urinalysis (red cell casts in pulmonary-renal syndrome — Goodpasture, vasculitis), kidney biopsy (rapidly progressive glomerulonephritis), surgical lung biopsy if etiology unclear after non-invasive workup.