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ROX Index Calculator (High-Flow Nasal Cannula Success)

Calculate the ROX index — SpO₂/FiO₂ divided by respiratory rate — to predict high-flow nasal cannula (HFNC) success or the need for intubation, with the 2, 6 and 12 hour thresholds.

SpO₂
FiO₂
Respiratory rate
Time on high-flow oxygen
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How it works

ROX = (SpO₂ / FiO₂) / respiratory rate, with SpO₂ in % and FiO₂ as a fraction. ROX ≥ 4.88 at 2, 6 or 12 hours predicts HFNC success; ROX < 2.85 at 2 h, < 3.47 at 6 h or < 3.85 at 12 h predicts HFNC failure (need for intubation).

The ROX index (Roca et al., American Journal of Respiratory and Critical Care Medicine 2019) was validated in adults with pneumonia and acute hypoxemic respiratory failure treated with high-flow nasal cannula. A value of 4.88 or more at 2, 6 or 12 hours was linked to a lower risk of needing intubation, while values below the time-specific thresholds identified patients likely to fail. Values in between call for close reassessment; the index supports — but does not replace — clinical judgement about timely intubation.

Worked example

After 6 hours on high-flow oxygen a patient has SpO₂ 92% on FiO₂ 50% (0.50) and respiratory rate 28: SpO₂/FiO₂ = 92 / 0.50 = 184, and ROX = 184 / 28 = 6.57 — low risk of HFNC failure (≥ 4.88).

Sources: Roca O et al. An index combining respiratory rate and oxygenation to predict outcome of nasal high-flow therapy. Am J Respir Crit Care Med 2019

Frequently asked questions

What is a good ROX index?

4.88 or more at 2, 6 or 12 hours after starting high-flow nasal cannula predicts success (a lower risk of intubation).

What ROX value predicts failure?

Below 2.85 at 2 hours, below 3.47 at 6 hours or below 3.85 at 12 hours was associated with HFNC failure in the validation study.

Should FiO₂ be entered as a percentage?

Yes — enter it as a percentage (e.g. 50) and the calculator converts it to a fraction (0.50) for the formula.

Can the ROX index be used in COVID-19?

It has been studied in COVID-19 pneumonia with broadly similar thresholds, but cut-offs vary between studies, so use it as one input to the decision.

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Clinical decision tool for education and for use by trained clinicians alongside clinical judgement — not a diagnosis and not a substitute for medical assessment. Scores validated in specific populations may not apply to every patient.