Situation: A 46-year-old woman with severe community-acquired pneumonia and septic shock is in the intensive care unit (ICU). On day 2 she is intubated and diagnosed with acute respiratory distress syndrome (ARDS): her chest X-ray shows bilateral opacities, and an echocardiogram shows normal heart function. She is 160 cm tall and weighs 82 kg. Her ventilator is set with a positive end-expiratory pressure (PEEP) of 10 cm H2O and a fraction of inspired oxygen (FiO2) of 0.60. Her arterial oxygen tension (PaO2) is 72 mmHg. Using the PaO2/FiO2 ratio (round off to the nearest whole number), how is the severity of her ARDS classified?

FiO2 is written as a decimal: 72 ÷ 0.60 = 120. In intubated ARDS with PEEP of at least 5 cm H2O, a ratio of 201 to 300 is mild, 101 to 200 is moderate, and 100 or less is severe, so 120 is moderate. A ratio below 150 is also a common threshold for prone positioning, which should not be confused with the severe category.

The PaO₂/FiO₂ (P/F) ratio is calculated by dividing the arterial oxygen tension by the fraction of inspired oxygen expressed as a decimal. For this patient, 72 mmHg divided by 0.60 equals 120. Because she is intubated and receiving a PEEP of 10 cm H₂O, which meets the minimum PEEP requirement of 5 cm H₂O specified by the Berlin Definition, the severity classification applies directly.

According to the Berlin Definition, ARDS severity is stratified as mild when the P/F ratio is 201–300, moderate when it is 101–200, and severe when it is 100 or less, provided PEEP or CPAP is at least 5 cm H₂O. A ratio of 120 falls within the 101–200 range, classifying this patient as having moderate ARDS. The option listing “Ratio 120; moderate” is therefore correct.

The Berlin Definition, published by the ARDS Definition Task Force, established these mutually exclusive categories to improve reliability and prognostic validity compared with the earlier American-European Consensus Conference definition ^[1]. The definition requires four criteria: acute onset within one week of a known clinical insult, bilateral opacities on chest imaging not fully explained by effusions or collapse, respiratory failure not fully explained by cardiac failure or fluid overload, and the oxygenation impairment quantified by the P/F ratio on a minimum PEEP of 5 cm H₂O ^[1]. In this scenario, the bilateral opacities and normal echocardiogram satisfy the radiographic and cardiac exclusion criteria, so the P/F ratio becomes the determining factor for severity.

Watch out! A P/F ratio below 150 is often cited as a threshold for considering prone positioning in moderate-to-severe ARDS, but this is a treatment trigger, not a severity category boundary. The severe category begins at 100 or less, not below 150. Confusing the prone positioning threshold with the severe classification is a common error.

Key point! The P/F ratio must be calculated with FiO₂ as a decimal. Using 0.60 rather than 60 is essential; dividing by 60 would yield 1.2, which is not a valid P/F ratio and would lead to an incorrect interpretation. The option “Ratio 1.2; severe” reflects this decimal placement error.

The P/F ratio remains the global standard for defining and classifying ARDS severity, although it does not incorporate the level of ventilatory support beyond the minimum PEEP requirement . A systematic review and meta-analysis evaluated a PEEP-incorporated P/F ratio (PaO₂/FiO₂ × PEEP, or PFP ratio) as a potentially more physiologically meaningful index, but the Berlin Definition continues to use the unadjusted P/F ratio for classification . For licensure examinations, the unadjusted P/F ratio with the Berlin Definition cutoffs is the expected framework.

In clinical practice, the P/F ratio is measured via arterial blood gas analysis. A related line of research has examined whether the SpO₂/FiO₂ (S/F) ratio can substitute for the P/F ratio when arterial blood gas is unavailable, particularly in resource-limited settings or for serial monitoring . However, the S/F ratio is a surrogate and does not replace the P/F ratio for formal ARDS diagnosis and severity classification under the Berlin Definition .

ARDS affects approximately 200,000 patients annually in the United States and accounts for about 10% of ICU admissions globally, underscoring the importance of accurate severity classification for prognosis and management decisions . Severity categories correlate with mortality and guide adjunctive therapies such as prone positioning, neuromuscular blockade, and extracorporeal membrane oxygenation consideration in severe cases .

Severity category	P/F ratio (mmHg)	Minimum PEEP/CPAP	This patient
Mild	201–300	≥5 cm H₂O	Not applicable
Moderate	101–200	≥5 cm H₂O	120 — matches
Severe	≤100	≥5 cm H₂O	Not applicable

The calculation is straightforward: 72 ÷ 0.60 = 120, and 120 lies between 101 and 200, placing the patient in the moderate ARDS category. The option “Ratio 43; severe” appears to reflect an arithmetic error or a misapplication of a different formula and does not correspond to any recognized ARDS severity calculation.