- Enter the arterial PaO₂ in mm Hg from the blood gas.
- Enter the FiO₂ as a percent (e.g. type 40 for 40% oxygen, 100 for 100%); the tool converts it to a fraction (÷100) before dividing.
- Optionally enter the applied PEEP/CPAP (cm H₂O). The Berlin ARDS categories require PEEP ≥ 5 cm H₂O; a lower value is flagged.
- The P/F ratio and the Berlin severity grade (mild, moderate, or severe — or no ARDS) update automatically.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Use the P/F ratio in any patient with acute hypoxemic respiratory failure to quantify oxygenation impairment and grade severity. It is the oxygenation arm of the Berlin Definition of ARDS, applied once the other criteria are met: acute onset within one week of a known insult, bilateral opacities on chest imaging, and respiratory failure not fully explained by cardiac failure or fluid overload. The ratio is then measured with at least 5 cm H₂O of PEEP (or CPAP) to assign mild, moderate, or severe ARDS. It is also a fast bedside index of hypoxemia for triage and trending even outside a formal ARDS diagnosis.
Appropriate population
Critically ill adults with acute hypoxemia from pneumonia, sepsis, aspiration, pancreatitis, transfusion, or other ARDS-precipitating insults — particularly when an arterial blood gas and a known FiO₂ are available. In nephrology, it helps gauge whether a hypoxemic patient with AKI or sepsis-associated CKD can tolerate fluids or needs a conservative, restrictive strategy and earlier renal replacement for volume control.
When NOT to rely on it
The Berlin ARDS categories require PEEP/CPAP ≥ 5 cm H₂O and assume a reliably measured FiO₂; an uncertain FiO₂ on low-flow nasal cannula or face mask makes the ratio unreliable. P/F also varies with PEEP, recruitment, body position, and altitude, so a single value is a snapshot, not a fixed phenotype. A low ratio still requires the full Berlin criteria (acute onset, bilateral infiltrates, not explained by cardiac failure/fluid overload) before ARDS is diagnosed; pure cardiogenic edema or hydrostatic overload can mimic it.
Pearls & Pitfalls
FiO₂ goes in as a percent, divided by 100
Room air is FiO₂ 21% (0.21); a non-rebreather or fully delivered oxygen is 100% (1.0). Enter the percent here and the tool divides by 100. The classic Berlin thresholds are P/F ≤ 300 (mild), ≤ 200 (moderate), ≤ 100 (severe) — for orientation, a PaO₂ of 80 on 50% O₂ gives 80 ÷ 0.50 = 160, i.e. moderate ARDS.
S/F is a non-invasive surrogate
When no arterial gas is available, the SpO₂/FiO₂ (S/F) ratio approximates P/F: S/F ≈ 235 corresponds to P/F ≈ 200 and S/F ≈ 315 corresponds to P/F ≈ 300. Keep SpO₂ ≤ 97% so the pulse oximeter stays on the steep part of the curve. S/F is useful for screening and trending but the formal Berlin Definition still rests on the arterial P/F ratio.
Pitfalls
(1) A low P/F is not ARDS by itself — the full Berlin criteria (acute onset ≤ 1 week, bilateral infiltrates, not fully explained by cardiac failure or fluid overload) must be met. (2) The ratio is only valid with a known, stable FiO₂ and PEEP/CPAP ≥ 5 cm H₂O; with PEEP < 5 the Berlin grade does not apply. (3) The number is sensitive to ventilator settings, recruitment, prone position, and altitude, so trend it on consistent settings. (4) Garbage in, garbage out: a guessed FiO₂ on low-flow oxygen or a poorly drawn gas invalidates the result.
Why Use It
The P/F ratio turns a raw PaO₂ — which is meaningless without knowing how much oxygen the patient is receiving — into a single, comparable number that grades the severity of hypoxemic respiratory failure. As the oxygenation criterion of the Berlin Definition, it stratifies ARDS into mild, moderate, and severe categories that track tightly with mortality (roughly 27%, 32%, and 45%) and that determine management: lower tidal-volume lung-protective ventilation, higher PEEP, prone positioning, and consideration of neuromuscular blockade or ECMO in severe disease. For the nephrologist, the same grade informs fluid strategy — a conservative, restrictive approach improves lung function in ARDS, which means earlier and more aggressive volume control (including renal replacement) in the hypoxemic patient with AKI or sepsis-associated CKD. It is a universal, validated, bedside index in internal medicine, nephrology, and critical care.
P/F Ratio — PaO₂/FiO₂ & Berlin ARDS Severity
Enter the arterial PaO₂ and the FiO₂ (as a percent) to compute the P/F ratio and grade ARDS severity by the Berlin Definition. Optionally enter the applied PEEP/CPAP to confirm the ≥ 5 cm H₂O requirement for the ARDS categories.
⚕ ARDS Definition Task Force (Berlin Definition). JAMA. 2012;307(23):2526–2533. The P/F ratio grades oxygenation severity but does not by itself diagnose ARDS — the Berlin Definition also requires acute onset ≤ 1 week, bilateral infiltrates, respiratory failure not fully explained by cardiac failure/fluid overload, and PEEP/CPAP ≥ 5 cm H₂O. For licensed clinicians; not a substitute for individualized assessment.
Next Steps
Use the P/F ratio and Berlin grade to confirm the diagnosis and escalate care.
- P/F > 300: no ARDS by the oxygenation criterion. Reassess for other causes of hypoxemia and optimize oxygenation; trend the ratio if the clinical picture is evolving.
- P/F 201–300 (mild): confirm the full Berlin criteria, start lung-protective ventilation (≈ 6 mL/kg predicted body weight, plateau pressure < 30), and apply a conservative fluid strategy.
- P/F 101–200 (moderate): as above plus higher PEEP and a low threshold for prone positioning; consider neuromuscular blockade for severe dyssynchrony.
- P/F ≤ 100 (severe): prone positioning, higher PEEP, neuromuscular blockade, and early referral for ECMO at an experienced center. In AKI/sepsis-associated CKD, restrict fluids and consider earlier renal replacement for volume control.
- Pair this with the A-a gradient to localize the cause of hypoxemia and the minute ventilation & tidal volume tool to set lung-protective targets.
Evidence & References
Formula
| Quantity | Formula |
|---|---|
| P/F ratio | PaO₂ (mm Hg) ÷ (FiO₂% / 100) |
| Example | PaO₂ 80 on 50% O₂ → 80 ÷ 0.50 = 160 |
| S/F surrogate | S/F ≈ 235 ↔ P/F ≈ 200 · S/F ≈ 315 ↔ P/F ≈ 300 |
Berlin Definition — ARDS Severity (PEEP/CPAP ≥ 5 cm H₂O)
| P/F ratio | Category | Approx. mortality |
|---|---|---|
| > 300 | No ARDS / normal–mild impairment | — |
| 201–300 | Mild ARDS | ~27% |
| 101–200 | Moderate ARDS | ~32% |
| ≤ 100 | Severe ARDS | ~45% |
The Berlin Definition also requires acute onset within one week, bilateral opacities on imaging, and respiratory failure not fully explained by cardiac failure or fluid overload. The oxygenation grade must be measured with PEEP or CPAP ≥ 5 cm H₂O.
References
- ARDS Definition Task Force; Ranieri VM, Rubenfeld GD, Thompson BT, et al. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012;307(23):2526–2533. doi:10.1001/jama.2012.5669.
- Rice TW, Wheeler AP, Bernard GR, 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):410–417. doi:10.1378/chest.07-0617.
- Fan E, Del Sorbo L, Goligher EC, et al. An Official ATS/ESICM/SCCM Clinical Practice Guideline: Mechanical Ventilation in Adult Patients with Acute Respiratory Distress Syndrome. Am J Respir Crit Care Med. 2017;195(9):1253–1263. doi:10.1164/rccm.201703-0548ST.
