Definition, causes and differential diagnosis of ARDS

Many past paper questions ask about the causes and differential diagnosis of " a diffuse bilateral infiltrate on CXR." There are many scenarios available.  It is important to be able to generate a lot of differentials in this sort of question.

  • Question 14 from the first paper of 2021(again, non specific "diffuse bilateral infiltrate")
  • Question 4 from the second paper of 2015 puts the infiltrate into an APML patient receiving ATRA (what you might call "differentiation syndrome")
  • Question 20 from the first  paper of 2014 talks about some non specific "diffuse bilateral infiltrate."
  • Question 17 from the second paper of 2011 puts the infiltrates into a patient recovering from a bone marrow transplant. 
  • Question 10 from the second paper of 2010 puts this radiological finding in the context of a recent cardiac arrest.

ARDS: The Berlin definition

  • Acute onset of hypoxemia with PaO2 / FiO2 ratio < 300
  • Bilateral infiltrates on chest X-ray
  • Not completely explained by heart failure or fluid overload
 ARDS Severity   PaO2/FiO2 on PEEP of 5   Mortality 
Mild 200 – 300 27%
Moderate 100 – 200 32%
Severe < 100 45%

In 2012, learned scholars came together and revised the old ARDS definition, making some changes to the inclusion and exclusion criteria.

  • There is no longer such a thing as "acute lung injury" when you have a PaO2/FiO2 ratio of 200-300.
    We call it "mild ARDS" instead.
  • Onset must be acute: within 7 days of whatever pathology is thought to be the cause
  • There is no need to exclude heart failure - you can have as much heart failure as you like, so long as its severity is insufficient to by itself explain the bilateral pulmonary infiltrates.

Differential diagnosis of pulmonary infiltrate in ICU patients

An excellent article from Silvia Blanco and Antoni Torres ( actually contains a brilliant table of differentials, which is incorporated into the table below.

Differential Diagnosis for Diffuse Bilateral Pulmonary Infiltrates


  • Pulmonary haemorrhage
  • Cardiogenic pulonary oedema


  • Bacterial
  • Viral
  • Fungal
  • PJP


  • Lymphangitis
  • Infiltrative neoplasm


  • ARDS
  • Idiopathic pneumonia syndrome


  • Eosinophilic pneumonitis
  • COP
  • Alveolar haemorrhage
  • Methotrexate-induced


  • Goodpastures (haemorrhagic)
  • Rheumatoid pneumonitis
  • Graft vs host disease in BMT
  • Engraftment syndrome
  • ATRA syndrome


  • Bilateral atelectasis
  • Pulmonary contusions
  • Chemical pneumonitis

Idiopathic pneumonia syndrome and ATRA syndrome(nowadays referred to as "differentiation syndrome") have been added since Question 4 from the second paper of 2015 introduced them into the list of differentials.


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