ROX index (high-flow nasal oxygen)
Predicts success or failure of high-flow nasal oxygen in hypoxaemic respiratory failure
Sources last checked: 2026-10-13
ROX index (high-flow nasal oxygen)
Predicts success or failure of high-flow nasal oxygen in hypoxaemic respiratory failure
Derived and validated in pneumonia patients on high-flow nasal oxygen (Spain and France). Thresholds depend on the time since HFNO start; a single value never replaces clinical assessment of work of breathing.
Interpretation
| < 2.85 — high risk of HFNO failure | Below the failure threshold at every time point studied (2 h < 2.85, 6 h < 3.47, 12 h < 3.85). In Roca 2019, values below these thresholds predicted HFNO failure (need for intubation). |
|---|---|
| 2.85–3.84 — check the time point | Below the failure threshold if measured at 6 h (< 3.47) or 12 h (< 3.85) after starting HFNO; indeterminate at 2 h. |
| 3.85–4.87 — indeterminate | Neither the success nor the failure threshold reached; the result is indeterminate at this time point. |
| ≥ 4.88 — lower risk of intubation | Associated with HFNO success when measured at 2, 6 or 12 h. |
What is the ROX index (high-flow nasal oxygen)?
The ROX index is the ratio of SpO₂/FiO₂ to respiratory rate. Roca and colleagues developed it in Spain for patients with pneumonia on high-flow nasal oxygen and validated it in a multicentre cohort in Spain and France, where values ≥ 4.88 at 2, 6 or 12 hours predicted success and low time-specific values predicted the need for intubation.
When to use it
- Serial assessment of patients with hypoxaemic respiratory failure on high-flow nasal oxygen
- Estimating the risk of HFNO failure (need for intubation) at the time points studied by Roca 2019
- Documenting response to HFNO at standard time points (2, 6, 12 h)
When not to use it
- Hypercapnic respiratory failure (e.g. COPD exacerbation) — not the validation population
- Patients on low-flow oxygen or non-invasive ventilation
- When SpO₂ is unreliable (poor perfusion, dark skin pigmentation with some oximeters, carbon monoxide)
How it is calculated
ROX = (SpO₂ % ÷ FiO₂ as fraction) ÷ respiratory rate
Clinical pearls and limitations
- Derived and validated in pneumonia patients on high-flow nasal oxygen (Spain and France). Thresholds depend on the time since HFNO start; a single value never replaces clinical assessment of work of breathing.
- SpO₂/FiO₂ carried more weight than respiratory rate in the original analysis.
- Failing patients showed a smaller rise in ROX over the first 12 hours, so the trend matters as much as one value.
- SpO₂ values above 97 % flatten the oxyhaemoglobin curve and make SpO₂/FiO₂ less informative.
- A reassuring ROX does not override clinical signs of exhaustion such as accessory muscle use or paradoxical breathing.
Frequently asked questions
What is a good ROX index?
A value ≥ 4.88 at 2, 6 or 12 hours after starting HFNO was associated with a lower risk of intubation.
When does ROX predict HFNO failure?
Values below 2.85 at 2 hours, below 3.47 at 6 hours and below 3.85 at 12 hours were highly specific predictors of failure.
How is ROX calculated?
Divide SpO₂ (%) by FiO₂ as a fraction, then divide by the respiratory rate. For SpO₂ 92 %, FiO₂ 0.6 and RR 30: (92 / 0.6) / 30 = 5.1.
Can ROX be used in COVID-19?
Several cohorts applied it in COVID-19 pneumonia with broadly similar thresholds, but the original validation was in bacterial and viral pneumonia before the pandemic.
Evidence and guidelines
- Primary source: 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 utility of the ROX index. J Crit Care 2016;35:200-205 (derivation).
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