How to Calculate SpO2/FiO2 Ratio

The SpO2/FiO2 Ratio Calculator computes the ratio of peripheral oxygen saturation (SpO2) to the fraction of inspired oxygen (FiO2) as a noninvasive surrogate for assessing lung function, especially in acute respiratory failure. The formula is:

Where:

• \( \text{SpO2 (%)} \): Peripheral oxygen saturation measured by pulse oximetry.
• \( \text{FiO2 (%)} \): Fraction of inspired oxygen, typically ranging from 21% (room air) to 100%.

Enter the SpO2 and FiO2 values, and calculate the result.

Note: FiO2 is entered as a percentage and converted to a decimal for calculation.

Using the SpO2/FiO2 Ratio Calculator

This calculator is useful for clinicians assessing hypoxemia in patients with acute lung injury (ALI) or acute respiratory distress syndrome (ARDS) when arterial blood gas is unavailable.

Input the SpO2 (%) and FiO2 (%). The calculator outputs the ratio.

Example 1: For SpO2 of 95% and FiO2 of 40%.

1. SpO2: 95
2. FiO2: 40
3. Calculation: \( 95 / (40 / 100) = 237.50000 \)
4. Result: 237.50000

Example 2: For SpO2 of 98% and FiO2 of 21%.

1. SpO2: 98
2. FiO2: 21
3. Calculation: \( 98 / (21 / 100) \approx 466.66667 \)
4. Result: 466.66667

Use this tool for quick assessments in critical care settings.

Common FAQ

Below are frequently asked questions about SpO2/FiO2 Ratio:

• Q: What is the SpO2/FiO2 Ratio?
  A: It's a noninvasive estimate of oxygenation, used as a surrogate for PaO2/FiO2 in diagnosing ARDS or ALI.
• Q: What are typical values?
  A: Normal is around 400-500; <315 suggests mild ARDS (corresponding to PaO2/FiO2 <300), <235 moderate (<200).
• Q: Why use percentage for FiO2?
  A: FiO2 is commonly expressed as a percentage in clinical settings (e.g., 40% oxygen).
• Q: How accurate is this calculator?
  A: It provides precise results based on the formula, rounded to 5 decimal places.
• Q: Can negative values be used?
  A: No, input values must be positive; invalid inputs leave the result blank.

Advice on Interpreting Results

Interpreting results for adults (Rice, 2007):

• An S/F ratio of 315 approximates a P/F ratio of 300.
• An S/F ratio of 235 correlates with a P/F ratio of 200.

For use in ARDS:

S/F Ratio Thresholds for Specific ARDS Categories (per the "global definition" of ARDS)

Category	S/F Ratio Criteria*
Nonintubated ARDS	≤315 on HFNC with flow of ≥30 L/min or NIV/CPAP with ≥5 cm H₂O end-expiratory pressure
Intubated ARDS	Mild: >235 and ≤315
	Moderate: >148 and ≤235
	Severe: ≤148
Modified definition for resource-limited settings	≤315**

*If SpO₂ ≤97%

**Neither positive end-expiratory pressure nor a minimum flow rate of oxygen is required for diagnosis in resource-limited settings.

Keep in mind that the S/F ratio is just one of several criteria used in the diagnosis and severity classification of ARDS.