What are the diagnostic criteria for Acute Respiratory Distress Syndrome (ARDS)?

How to Diagnose ARDS

ARDS is diagnosed using the Berlin Definition criteria: acute onset within 1 week of a known clinical insult, bilateral opacities on chest imaging not fully explained by effusions/nodules/collapse, respiratory failure not fully explained by cardiac failure or fluid overload, and hypoxemia with PaO₂/FiO₂ ≤300 mmHg with minimum PEEP of 5 cmH₂O. 1, 2

Diagnostic Criteria (Berlin Definition)

The diagnosis requires all four of the following components to be present simultaneously:

1. Timing

• Onset within 1 week of a known clinical insult OR new or worsening respiratory symptoms 1, 2, 3

2. Chest Imaging

• Bilateral opacities on chest radiograph or CT scan that are not fully explained by effusions, lobar/lung collapse, or nodules 2, 3
• A reference set of chest radiographs exists to improve inter-observer reliability, though clinician interpretation remains a limitation 4, 3

3. Origin of Edema

• Respiratory failure not fully explained by cardiac failure or fluid overload 1, 2
• If no ARDS risk factor is apparent, objective evaluation (echocardiography) is required to exclude hydrostatic edema 3
• The pulmonary artery wedge pressure criterion from older definitions has been removed 3

4. Oxygenation Defect

• PaO₂/FiO₂ ≤300 mmHg with minimum PEEP of 5 cmH₂O 1, 2
• This must be measured on arterial blood gas, not estimated from pulse oximetry 5

Severity Classification

Once ARDS is diagnosed, classify severity based on degree of hypoxemia (all with minimum PEEP 5 cmH₂O): 1, 2, 4

• Mild ARDS: 200 mmHg < PaO₂/FiO₂ ≤ 300 mmHg (mortality ~27%)
• Moderate ARDS: 100 mmHg < PaO₂/FiO₂ ≤ 200 mmHg (mortality ~32%)
• Severe ARDS: PaO₂/FiO₂ ≤ 100 mmHg (mortality ~45%)

Essential Diagnostic Workup

Arterial Blood Gas

• Mandatory to determine PaO₂/FiO₂ ratio and assess for metabolic or respiratory acidosis 6, 5
• Pulse oximetry alone is insufficient for diagnosis 5

Chest Imaging

• Chest radiograph or CT showing bilateral opacities 2, 3
• Standard chest radiographs are poor predictors of severity but necessary for diagnosis 6

Cardiac Assessment

• Echocardiography should be performed if no clear ARDS risk factor exists to exclude cardiogenic pulmonary edema 3
• Look for absence of signs suggesting fluid overload (increased vascular pedicle width, cardiothoracic ratio) 6

Laboratory Studies

• Complete blood count, serum electrolytes, renal and liver function to assess for multiple organ dysfunction 6
• Blood cultures if infection suspected 6

Common Pitfalls and Caveats

Under-Recognition Remains Common

• ARDS continues to be significantly under-recognized even with the Berlin Definition, with delayed diagnosis being common 7
• Complexity of ARDS biology and low specificity of diagnostic criteria contribute to missed diagnoses 7

Radiographic Interpretation Challenges

• Clinician interpretation of chest radiographs and origin of edema shows poor reliability 4
• Radiographic findings may be asymmetric, patchy, or focal rather than the classic diffuse bilateral pattern 6

ARDS Mimics

• Conditions like diffuse interstitial lung diseases, diffuse pulmonary infections, and drug-induced lung injury can present identically and fall within the syndrome definition 6
• These "ARDS-mimics" may require specific treatments (corticosteroids, antimicrobials, drug withdrawal) rather than standard ARDS management 6
• Consider bronchoscopy or additional diagnostic testing when clinical presentation is atypical 6

Pathologic Correlation

• Only a minority of patients meeting Berlin criteria actually have diffuse alveolar damage on autopsy 6, 8
• This highlights the heterogeneity captured by clinical diagnostic criteria 8

Practical Diagnostic Algorithm

1. Identify acute respiratory failure with dyspnea, tachypnea, hypoxemia requiring mechanical ventilation or high-flow oxygen 6, 5

2. Obtain arterial blood gas to calculate PaO₂/FiO₂ ratio with PEEP ≥5 cmH₂O 1, 2

3. Review chest imaging for bilateral opacities 2, 3

4. Confirm timing: onset within 1 week of known insult or worsening symptoms 2, 3

5. Exclude cardiac causes: use echocardiography if no clear ARDS risk factor or if cardiac failure suspected 3

6. Classify severity based on PaO₂/FiO₂ ratio to guide treatment intensity 1, 2

7. Consider ARDS mimics requiring specific therapy, particularly if no typical risk factors present 6