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P/F and S/F ratio with ARDS severity (Berlin 2012 and Global 2023)

Also called PaO2/FiO2, P:F ratio, SpO2/FiO2, S:F ratio, Horowitz index, ARDS severity

Oxygenation ratios with Berlin (PaO₂/FiO₂) and Global ARDS definition (PaO₂/FiO₂ or SpO₂/FiO₂) severity bands.

%

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Formula and scoring

P/F = PaO₂ (mmHg) ÷ FiO₂ (fraction). Berlin: mild 200 < P/F ≤ 300, moderate 100 < P/F ≤ 200, severe ≤ 100. Global 2023 adds S/F (only if SpO₂ ≤ 97%): mild 235 < S/F ≤ 315, moderate 148 < S/F ≤ 235, severe ≤ 148; non-intubated ARDS: P/F ≤ 300 or S/F ≤ 315 on HFNO ≥ 30 L/min or NIV/CPAP ≥ 5.

Exam and ward pearls

  • Berlin mortality by severity: mild 27%, moderate 32%, severe 45%.
  • Rice 2007: S/F 235 ≈ P/F 200 and S/F 315 ≈ P/F 300 (S/F = 64 + 0.84 × P/F).
  • Global 2023 lets you diagnose ARDS without an ABG (S/F) and with lung ultrasound, for resource-limited settings.

When not to rely on it

  • Global definition: pulse oximetry is not recommended for diagnosis if a haemoglobin abnormality (methaemoglobinaemia, carboxyhaemoglobinaemia) is suspected.

Source

  1. ARDS Definition Task Force. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012;307(23):2526-33. doi:10.1001/jama.2012.5669 · PMID 22797452
  2. Matthay MA, et al. A new global definition of acute respiratory distress syndrome. Am J Respir Crit Care Med. 2024;209(1):37-47. doi:10.1164/rccm.202303-0558WS · PMID 37487152
  3. Rice TW, et al. Comparison of the SpO2/FIO2 ratio and the PaO2/FIO2 ratio in patients with acute lung injury or ARDS. Chest. 2007;132(2):410-7. doi:10.1378/chest.07-0617 · PMID 17573487

Formula and cut-offs checked against these sources on 8 Nov 2026. Educational tool for clinicians; it does not replace clinical judgement.

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