Oxygenation Indices (P/F, S/F, A–a Gradient, OI)

Reference ranges

Severity & critical values

Conversion: mmHg × 0.133 = kPa. A ratio has no units, so the number stays the same in both systems.

Try it: PaO₂ 80 mmHg on FiO₂ 0.40. The P/F is 200.

Only use it when SpO₂ is 97% or lower. S/F 315 matches P/F 300, and S/F 235 matches P/F 200 (ARDS Network derivation, 2007). Try it: SpO₂ 94% on FiO₂ 0.40 gives S/F 235.

At sea level, barometric pressure is 760 mmHg, so the term becomes FiO₂ × 713. At altitude, use the measured local barometric pressure. The 47 mmHg is water vapour pressure at body temperature. The 0.8 is the usual respiratory quotient.

Worked example at sea level, FiO₂ 0.40, PaCO₂ 40 mmHg, PaO₂ 80 mmHg: PAO₂ = 0.40 × 713 − 40 ÷ 0.8 = 235. The A–a gradient is 155 mmHg.

Apply this before you use the ARDS thresholds. When barometric pressure is below 760 mmHg, the corrected ratio comes out lower than the measured one. The same measured ratio can therefore meet an ARDS band at altitude (Matthay 2024, Table 1 footnote).

Example: MAP 20 cm H₂O, FiO₂ 0.60, PaO₂ 60 mmHg gives OI 20. MAP is the mean airway pressure on the ventilator, from an arterial line.

OSI only works when SpO₂ is 97% or lower, so skip it above that. Example: MAP 20, FiO₂ 0.60, SpO₂ 92% gives OSI 13.0.

Low P/F or S/F (hypoxemia)

Wide A–a gradient (above the age-expected value)

• V/Q mismatch, shunt or diffusion limitation: alveolar and arterial PO₂ split apart, so the gradient widens.
• Higher FiO₂: more FiO₂ lifts PAO₂, which widens the gradient even when the lungs are healthy.
• Age: the gradient normally climbs with age. Compare it with the value expected for that patient.

High P/F or S/F

A high value on supplemental oxygen is what you should expect. The oxygen is doing its job. In ventilated patients, though, that same number can mean too much oxygen (hyperoxemia). Wean FiO₂ back to the target range.

Step-by-step approach

1. Is it real? Confirm the sample is arterial, free of air bubbles and analysed promptly. Record the FiO₂ and device, and the PEEP or CPAP setting, with the value.
2. How sick is the patient? Compare the SpO₂ with the target, and check work of breathing, mental state and hemodynamics.
3. Acute or chronic, and what is the trend? Compare with the previous value at the same FiO₂ and PEEP. Trend beats a single value.
4. Calculate the derived values: P/F or S/F, then the A–a gradient if an ABG is available. Use OI or OSI only in ventilated children.
5. Classify with named criteria: the 2023 Global ARDS definition for adults, or PALICC-2 for children.
6. Escalate or disposition: act on the patient's trajectory, not only on the index. Escalate support early when the index falls.

Pattern and what it suggests

ARDS criteria (Global definition, 2023)

1. Timing: onset or worsening of hypoxemic respiratory failure within 1 week of a known risk factor or new respiratory symptoms.
2. Chest imaging: bilateral opacities on radiography or CT, or bilateral B-lines or consolidation on ultrasound, not fully explained by effusion, atelectasis or nodules.
3. Origin: not fully explained by cardiac failure or fluid overload. ARDS can still be diagnosed if a risk factor is also present.
4. Oxygenation: P/F ≤300, or S/F ≤315 when SpO₂ ≤97%, on high-flow nasal oxygen at 30 L/min or more, on NIV or CPAP with PEEP 5 cm H₂O or more, or when intubated with PEEP 5 cm H₂O or more. Above 1,000 m, correct the ratio first.

In resource-limited settings, the modified definition uses S/F ≤315 alone, with no PEEP or minimum flow requirement. Where you can measure PEEP and blood gases, go with the standard definition.

Ventilated children (PALICC-2)

• Diagnosis on invasive ventilation: OI ≥4 or OSI ≥5 (OSI requires SpO₂ ≤97%). When using SpO₂-based criteria, titrate oxygen to SpO₂ 88–97% (PALICC-2 definition statement 1.4.5).
• Severity, invasive ventilation: mild or moderate OI <16 or OSI <12; severe OI ≥16 or OSI ≥12. Stratify at least 4 hours after diagnosis.
• Non-invasive ventilation: diagnosis at P/F ≤300 or S/F ≤250; severe at P/F ≤100 or S/F ≤150, with a full face mask and CPAP or PEEP of 5 cm H₂O or more.

Medications

Doses vary by protocol; follow local protocols and medical direction.

Definitive care & disposition

• ICU: need for high-flow oxygen, NIV or intubation; rising oxygen requirement; rising PaCO₂ with acidosis.
• ECMO center: early discussion for selected patients with reversible severe ARDS in its first 7 days and few futility risk factors, with P/F below 80 mmHg or pH below 7.25 with PaCO₂ ≥60 mmHg, despite optimal ventilation and prone positioning.
• Post-discharge: chronic hypoxemia needs outpatient assessment for long-term oxygen.

Doses are typical references. Follow local protocols, medical direction and institutional policy.

Point-of-care testing

• Pulse oximetry (S/F): you can use it in every setting. It is only valid while SpO₂ is 97% or lower. Skin pigmentation reduces accuracy.
• Blood gas analyzer (P/F, A–a): it needs an arterial sample. Stay inside the temperature and sample limits in the device manual.
• FiO₂ at the bedside: a ventilator or high-flow device gives you a known FiO₂. A nasal cannula or simple mask gives only an estimate, and that estimate depends on how the patient breathes.

Before and during interfacility transport

• Obtain: FiO₂ and device, PEEP or CPAP, SpO₂ and the time of the most recent blood gas.
• Recheck: after every change in oxygen, position or ventilator settings, and at the cadence set by protocol.
• Falling index despite support: stabilize first, then confirm the destination and level of care. Don't hold up a transport that is itself the escalation (for example, to an ECMO or ICU center).

Altitude and flight

Altitude and cabin pressure both pull barometric pressure down, and that drop shows up in PAO₂, PaO₂ and SpO₂. Build the local or cabin barometric pressure into the alveolar equation. Once you're above 1,000 m, multiply P/F and S/F by barometric pressure ÷ 760 before you apply the ARDS thresholds (formula in the Reference section). Plan your oxygen to the target, with cylinder reserve.

Collection, handling and measurement

Common misinterpretations

• A–a gradient is a difference, not a diffusion gradient: it reflects several mechanisms, and diffusion is only one of them.
• P/F is not a measure of lung severity alone: the number moves when FiO₂ and PEEP change.
• OI and OSI are for ventilated children: adult ARDS runs on P/F or S/F under the 2023 definition.

What it measures

Each of these is a derived index. It pairs the oxygen you deliver (FiO₂) with the oxygen in the blood (PaO₂ or SpO₂) to grade gas exchange. This page covers the P/F ratio, the S/F ratio, the A–a gradient, and the OI and OSI used in ventilated children. A single PaO₂ value has its own page.

Physiology & regulation

• Alveolar PO₂ rests on three inputs: FiO₂, barometric pressure and PaCO₂.
• The A–a gradient is the gap between alveolar and arterial PO₂. That gap reflects V/Q mismatch, shunt and diffusion limitation.
• Hypoventilation pushes PaCO₂ up and PAO₂ down, but the gradient stays the same width.
• Ratios shift with FiO₂, so only compare values taken at the same FiO₂ and PEEP.

How it is measured

An arterial blood gas on a heparinized syringe gives the P/F and the A–a gradient, and the sample must be analysed promptly. Continuous pulse oximetry at a stated FiO₂ gives the S/F. The OI and OSI need an arterial line for PaO₂ or SpO₂, plus the mean airway pressure from the ventilator.

Why it matters

These indices grade hypoxemia and ARDS severity. They set when to escalate support and when to refer for ECMO. Read every ratio next to the FiO₂ and PEEP that produced it, because those settings steer each change to oxygen or the ventilator.