ROX Index Calculator

Combine oxygen saturation, FiO2 and respiratory rate into the ROX index and read the high-flow failure risk.

What it does

The ROX index combines pulse oximetry, the fraction of inspired oxygen and respiratory rate into one number. It was built for adults with pneumonia and acute hypoxemic respiratory failure who are being treated with high-flow nasal cannula, where the clinical question is whether the support is working or whether intubation is becoming more likely.

The arithmetic has two steps. First divide SpO2 by FiO2 written as a decimal, which gives the saturation to oxygen ratio. Then divide that result by the respiratory rate. A saturation of 92% on 50% oxygen with a rate of 28 gives 92 / 0.50 = 184, then 184 / 28 = 6.57.

The original 2019 study followed 191 patients and found that a ROX of 4.88 or more at 2, 6 or 12 hours was linked with a lower risk of intubation. Values below 2.85 at 2 hours, 3.47 at 6 hours and 3.85 at 12 hours pointed toward failure. A later systematic review and meta-analysis supported the index as a predictor, while noting that cutoff values vary between studies.

The time matters because the index is a trend tool. A value taken immediately after high-flow starts says less than a series taken at 2, 6 and 12 hours. A rising ROX is reassuring; a flat or falling value despite stable support deserves a closer look at the work of breathing, the chest image and the underlying cause.

It is not a substitute for clinical judgement. The index uses pulse oximetry, so poor perfusion, nail polish, carbon monoxide or methemoglobin can distort the saturation. It was not designed for children, for chronic respiratory failure or for patients who are already intubated, and a single number should never delay an urgent airway decision.

How to use

1. Record the SpO2, FiO2 and respiratory rate at the same moment.
2. Enter the SpO2 as a percentage.
3. Enter the FiO2 as a percentage, not as a decimal fraction.
4. Enter the respiratory rate in breaths per minute.
5. Select how many hours have passed since high-flow started.
6. Read the ROX value, the risk band and the failure cutoff for that time point.

Formula

SpO2

Peripheral oxygen saturation in percent.

FiO2

Fraction of inspired oxygen as a decimal, so 50% is 0.50.

Respiratory rate

Breaths per minute at the time of the reading.

ROX

A unitless index; higher values are better.

The SpO2 to FiO2 part is a saturation ratio, not a pressure ratio like P/F. That is why ROX can be calculated from a pulse oximeter and does not need an arterial blood gas. The cutoffs are time specific: 2.85 at 2 hours, 3.47 at 6 hours and 3.85 at 12 hours for failure, with 4.88 or more as the reassuring threshold at all three time points. Do not apply the 12 hour cutoff to a 2 hour reading.

Worked example

Reference table

Limits

• Developed for adults with pneumonia and acute hypoxemic respiratory failure on high-flow nasal cannula, not for every oxygen device.
• Uses pulse oximetry, which can be inaccurate with poor perfusion, nail polish, carbon monoxide or methemoglobin.
• Not validated for children, chronic respiratory failure or patients already on invasive ventilation.
• Cutoffs vary between studies, and the trend over time is more informative than one reading.

Common mistakes

• Entering FiO2 as 0.50 in a field that expects percent, or as 50 in a hand calculation that expects a decimal.
• Mixing values taken at different times or after a change in oxygen flow.
• Applying the 2 hour failure cutoff to a 12 hour reading.
• Reading a single low value as a mandate for intubation instead of assessing the whole patient.
• Using ROX in a patient on a device or diagnosis for which it was not studied.

Try it yourself

Quick checks. Answers sit tucked under each question.

Questions

Sources

• Roca et al: an index combining respiratory rate and oxygenation to predict high-flow outcome, PubMed
• ROX index systematic review and meta-analysis in pneumonia with acute hypoxemic respiratory failure, PubMed

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