Critical Care
ROX Index
Predicts high-flow nasal oxygen success.
Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.
Background
The ROX index predicts the success or failure of high-flow nasal oxygen (HFNO) in patients with acute hypoxaemic respiratory failure. It is calculated as the ratio of oxygen saturation to inspired oxygen fraction, divided by the respiratory rate: ROX = (SpO₂/FiO₂)/RR. A higher value reflects good oxygenation achieved at a lower oxygen requirement and respiratory rate, signalling that HFNO is working. It is typically measured at intervals (for example around 2, 6 and 12 hours) so the trend, not a single reading, guides decisions.
Interpreting the result
Higher values are reassuring: a ROX of 4.88 or above (classically at 12 hours) predicts HFNO success and a low likelihood of needing intubation, whereas a value below about 3.85 flags a high risk of HFNO failure. Values between roughly 3.85 and 4.87 are indeterminate and warrant a repeat measurement and close observation. A static or falling ROX over successive checks is a warning sign that should prompt consideration of escalation, even if individual values sit in the middle zone.
Worked example
A patient on HFNO has SpO₂ 95%, FiO₂ 0.6 and a respiratory rate of 24/min: ROX = (95/0.6)/24 = 158.3/24 ≈ 6.6. This is comfortably above 4.88, predicting that HFNO is likely to succeed.
Critical actions
ROX = (SpO₂/FiO₂)/RR. Enter FiO₂ as a fraction (e.g. 0.5, not 50). A ROX ≥4.88 at 12h predicts success; values should be trended, not used once.
Pearls / pitfalls
- Enter FiO₂ as a fraction (e.g. 0.5, not 50), or the index will be wrong by orders of magnitude.
- Trend the value across time points rather than relying on one reading — a falling ROX is more concerning than a single low value.
- Do not let a reassuring number delay intubation in a patient who is visibly tiring or deteriorating clinically.
- It was validated mainly in pneumonia and hypoxaemic failure; extrapolation to other causes (e.g. hypercapnic failure) is uncertain.
Evidence & validation
Derived and validated by Roca and colleagues in patients with pneumonia-related hypoxaemic respiratory failure, the ROX index has been reproduced in further cohorts, including during the COVID-19 pandemic, and is widely used to support HFNO monitoring although it is not a substitute for clinical judgement.
Frequently asked questions
How is the ROX index calculated?
ROX = (SpO₂/FiO₂)/respiratory rate, with FiO₂ entered as a fraction. A higher value means better oxygenation at a lower oxygen requirement and respiratory rate.
What ROX value predicts HFNO success?
A ROX of 4.88 or above, classically measured at 12 hours, predicts a low risk of HFNO failure. Values below about 3.85 indicate a high risk of needing intubation.
Why measure ROX at several time points?
A single value can mislead. Trending it at intervals such as 2, 6 and 12 hours shows whether the patient is improving or deteriorating, which is more informative than any one reading.
Does a good ROX mean intubation can be avoided?
It makes HFNO success more likely, but the index supports rather than replaces clinical judgement. A patient who is tiring or deteriorating should be escalated regardless of a reassuring number.
Can the ROX index be used outside pneumonia?
It was validated mainly in pneumonia and hypoxaemic respiratory failure. Its performance in other settings, such as hypercapnic failure, is less certain, so interpret cautiously.
References
- Roca O, Caralt B, Messika J, et al. An index combining respiratory rate and oxygenation to predict outcome of nasal high-flow therapy. Am J Respir Crit Care Med. 2019;199(11):1368–1376.
- Roca O, Messika J, Caralt B, et al. Predicting success of high-flow nasal cannula in pneumonia patients with hypoxemic respiratory failure: the ROX index. J Crit Care. 2016;35:200–205.
About the creator
Roca O et al.
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