SpOâ‚‚/FiOâ‚‚ Ratio Calculator
What is the SpO₂/FiO₂ Ratio?
The SpO₂/FiO₂ ratio, also called the S/F ratio, is a noninvasive oxygenation index calculated by dividing peripheral oxygen saturation measured by pulse oximetry by the fraction of inspired oxygen. It can be used as an alternative oxygenation measure when arterial blood gas measurement is unavailable or when pulse oximetry is used within the applicable criteria of the 2024 Global Definition of ARDS.
Formula
S/F Ratio = SpO₂ (%) ÷ FiO₂ (fraction)
FiO₂ must be expressed as a decimal fraction. For example, an FiO₂ of 50% is converted to 0.50. Therefore, if SpO₂ is 90% and FiO₂ is 50%, the S/F ratio is 180.
2024 Global Definition of ARDS
The 2024 Global Definition of ARDS allows the SpO₂/FiO₂ ratio to be used as an alternative to PaO₂/FiO₂ when SpO₂ is 97% or lower. The corresponding oxygenation thresholds are:
>235 to ≤315
Mild oxygenation category
>148 to ≤235
Moderate oxygenation category
≤148
Severe oxygenation category
These thresholds describe oxygenation categories within the Global Definition of ARDS. The S/F ratio alone does not establish an ARDS diagnosis. ARDS assessment also requires the appropriate clinical context, timing, imaging findings, and respiratory-support criteria.
Respiratory Support Context
For nonintubated ARDS in the standard Global Definition, an S/F ratio ≤315 with SpO₂ ≤97% is used in patients receiving HFNO at a flow rate of at least 30 L/min or NIV/CPAP with at least 5 cmH₂O end-expiratory pressure.
For intubated ARDS, the oxygenation severity categories require invasive mechanical ventilation with a minimum PEEP of 5 cmH₂O. In resource-limited settings, the modified definition allows S/F-based assessment without requiring a specific respiratory-support device, minimum oxygen flow, or PEEP.
Rice Relationship
Rice et al. demonstrated a relationship between the S/F and P/F ratios in patients with acute lung injury or ARDS. The reported regression relationship was:
S/F = 64 + 0.84 × P/F
The corresponding estimated P/F ratio can therefore be calculated as:
Estimated P/F = (S/F − 64) ÷ 0.84
The estimated P/F value is derived from the Rice relationship and should not be considered a direct measurement of PaO₂/FiO₂. When an arterial blood gas is available, the measured PaO₂/FiO₂ ratio should be used for clinical assessment.
Important Clinical Considerations
The S/F ratio is not valid for ARDS oxygenation assessment when SpO₂ is above 97%, because the relationship between SpO₂ and PaO₂ becomes less reliable at higher saturation levels. Pulse oximetry measurements can also be affected by factors such as poor signal quality, motion, hypoperfusion, hypothermia, and abnormal hemoglobin states.
Measurements should ideally be obtained when the patient is comfortably at rest with an adequate pulse-oximetry waveform. The 2024 Global Definition also recommends allowing sufficient time after changes in position, FiO₂, or oxygen flow before assessing oxygenation.
Clinical note: This calculator provides an oxygenation index and derived estimates. It does not independently diagnose ARDS or replace clinical assessment, arterial blood gas analysis, or established diagnostic criteria.
References
- Rice TW, Wheeler AP, Bernard GR, Hayden DL, Schoenfeld DA, Ware LB. 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. PMID: 17573487.
- 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. PMID: 37487152.
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Appcardio may contain errors. Always verify the calculation and its accuracy before clinical use.