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SpO₂/FiO₂ Ratio (S/F)

Non-invasive oxygenation ratio from pulse oximetry, used in the 2023 global ARDS definition.

For healthcare professionalsRespiratoryv1.0.1

For healthcare professionals / clinical use. Not a self-diagnosis tool.

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Enter your measurements, then press Calculate.

Methodology

Global definition of ARDS (Matthay 2024); Rice 2007

S/F = SpO₂ (%) ÷ FiO₂ (fraction)

S/F ≤ 315 corresponds approximately to P/F ≤ 300 when SpO₂ is 97% or less.

Who it is for
Adults with SpO₂ of 97% or less (the ratio is unreliable above 97%).
Version
1.0.1

How this calculator works

Where the formula comes from

  • Rice and colleagues (2007) derived the S/F–P/F relationship in 672 patients from an ARDS Network trial and validated it in 402 patients from a second trial, using SpO₂ values of 97% or less.[1]
  • S/F = 64 + 0.84 × P/F (r = 0.89); an S/F of 235 matched a P/F of 200 and 315 matched 300.[1]

Where accuracy falls (published evidence)

  • In Rice and colleagues (2007), an S/F of 315 had 91% sensitivity and 56% specificity for P/F ≤ 300; an S/F of 235 had 85% sensitivity and 85% specificity for P/F ≤ 200.[1]
  • A 2022 meta-analysis found pulse oximetry probably overestimates oxygen saturation in people with high levels of skin pigmentation.[2]

What guidelines say

  • The 2024 global ARDS definition accepts SpO₂/FiO₂ ≤ 315, when SpO₂ is 97% or less, to identify hypoxaemia.[3]

Sources

  1. 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
  2. Shi C, Goodall M, Dumville J, et al. The accuracy of pulse oximetry in measuring oxygen saturation by levels of skin pigmentation: a systematic review and meta-analysis. BMC Med. 2022;20(1):267. doi:10.1186/s12916-022-02452-8
  3. Matthay MA, Arabi Y, Arroliga AC, 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

Compiled from the published sources above. Every statement was agreed independently by three AI research tools (Claude, ChatGPT, DeepSeek) and its source verified in PubMed or Crossref by the Hakeemly editorial team. Not reviewed by a doctor unless stated. Agreed on

Worked example: how to calculate S/F ratio

Inputs

SpO₂
92 %
FiO₂ (inspired oxygen)
40 %

Result

SpO₂/FiO₂
230

315 or less: meets the oxygenation criterion of the global ARDS definition (with SpO₂ ≤ 97%)

Calculation:

92/0.4 = 230

An illustration with sample values, worked out with the same method the tool above uses.

Limitations

  • Pulse oximetry can overestimate saturation in people with darker skin.
  • At 315, sensitivity was 91% but specificity 56% for P/F ≤ 300 (Rice 2007).

References

  1. Matthay MA, Arabi Y, Arroliga AC, 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
  2. 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
  3. Shi C, Goodall M, Dumville J, et al. The accuracy of pulse oximetry in measuring oxygen saturation by levels of skin pigmentation: a systematic review and meta-analysis. BMC Med. 2022;20(1):267. doi:10.1186/s12916-022-02452-8

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