17th January 2024

Diagnosis

To diagnose ARDS, the following clinical and radiographic criteria need to be met (4):

• Acute onset time of <1 week
• Bilateral opacities on chest x-ray
• Arterial to inspired oxygen ratio (PₐO2/FᵢO2) of ≤300 on positive end-expiratory pressure (PEEP) or continuous positive airways pressure (CPAP) of ≥5cm H2O
• Presentation cannot be fully explained by cardiac failure or fluid overload

ARDS can be stratified by severity according to oxygenation defined by PₐO2/FiO2 ratio:

• Mild – PₐO2/FᵢO2 200-300mmHg
• Moderate – 100-199mmHg
• Severe – <100mmHg

Investigations

Often, the diagnosis of ARDS can be made on the basis of basic clinical and chest x-ray findings, and the priority should be initiating supportive management in a timely fashion. However, there is still an important role in continuing investigations alongside supportive care, both to differentiate between ARDS and acute pulmonary oedema and to establish the underlying cause. Some of the below investigations can be useful in certain situations:

• Chest x-ray – an essential investigation, as diffuse pulmonary infiltrates on a plain radiograph, are part of the diagnostic criteria for ARDS.
• Lung ultrasound – increasingly, point of care lung ultrasound use in critical care can be used to make the radiological diagnosis in the place of x-ray, looking for B-lines associated with an irregular pleural morphology with spared areas (5).
• Arterial blood gas monitoring – frequent ABG monitoring helps determine the PaO2 to FiO2 ratio for both diagnosis and to determine progression and oxygen requirements.
• Transthoracic echocardiogram (TTE) – the role of TTE here is to rule out the differential diagnosis of pulmonary oedema due to left ventricular dysfunction and/or valvular disease.
• Bronchoscopy – useful where the underlying aetiology is unclear, as samples can be taken and sent for culture, biochemistry and cytology testing.
• Lung biopsy – this is a very invasive test and not routinely done for all patients; however, biopsy can be a useful investigation where there is a lot of diagnostic uncertainty regarding aetiology of the ARDS. Histopathologically, the diagnosis of ARDS can be confirmed by the appearance of diffuse alveolar damage (DAD).

Image Source: Wikimedia Commons (2017). ARDSSevere, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:ARDSSevere.png (Accessed: 6 June 2022).