How is acute respiratory distress syndrome (ARDS) diagnosed in adults?

Acute Respiratory Distress Syndrome (ARDS) is a life-threatening condition characterized by widespread inflammation in the lungs, leading to severe hypoxemia and bilateral pulmonary infiltrates. Diagnosis relies on a combination of clinical assessment, imaging, and objective laboratory and physiological criteria—not on a single definitive test.

The diagnosis of ARDS follows the widely accepted Berlin Definition, which requires four key criteria: (1) onset within one week of a known clinical insult or new/worsening respiratory symptoms; (2) bilateral opacities on chest imaging (e.g., chest X-ray or CT scan) that are not fully explained by effusions, lobar/lung collapse, or nodules; (3) respiratory failure not fully attributable to cardiac failure or fluid overload—objective assessment (e.g., echocardiography) is recommended if no risk factors for ARDS are present; and (4) impaired oxygenation quantified by the PaO₂/FiO₂ ratio, measured while the patient is receiving at least 5 cm H₂O of positive end-expiratory pressure (PEEP) or continuous positive airway pressure (CPAP). Severity is stratified as mild (200 mmHg < PaO₂/FiO₂ ≤ 300 mmHg), moderate (100 mmHg < PaO₂/FiO₂ ≤ 200 mmHg), or severe (PaO₂/FiO₂ ≤ 100 mmHg).

Supportive diagnostic evaluations include arterial blood gas (ABG) analysis to assess acid-base status and quantify hypoxemia; complete blood count (CBC), comprehensive metabolic panel (CMP), and inflammatory markers (e.g., CRP, procalcitonin) to identify underlying infection or systemic inflammation; blood cultures and sputum/bal fluid microbiology when infectious etiology is suspected; and echocardiography to exclude hydrostatic pulmonary edema. In select cases—particularly when diagnosis remains uncertain—bronchoalveolar lavage (BAL) may be performed to evaluate for alternative causes such as alveolar hemorrhage, eosinophilic pneumonia, or opportunistic infections.

It is critical to emphasize that ARDS is a clinical diagnosis of exclusion. No biomarker or imaging finding alone confirms ARDS; rather, integration of temporal course, radiographic pattern, physiological data, and careful evaluation for mimics (e.g., cardiogenic pulmonary edema, diffuse alveolar hemorrhage, or acute interstitial pneumonia) is essential for accurate identification and timely management.