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ROX Index for High-Flow Nasal Cannula

The ROX index = (SpO₂ / FiO₂) / respiratory rate predicts high-flow nasal cannula (HFNC) outcome in acute hypoxemic respiratory failure. ROX ≥4.88 at 2, 6 or 12 hours predicts HFNC success; values <2.85 (2 h), <3.47 (6 h) or <3.85 (12 h) predict HFNC failure and need for intubation.

SpO₂ (%)
FiO₂ (% or fraction) (%)
Respiratory rate (/min)
Time on HFNC: 2 hours; 6 hours; 12 hours

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Interpretation

Result	Meaning
≥4.88	HFNC success likely (2, 6 or 12 h)
<2.85 at 2 h / <3.47 at 6 h / <3.85 at 12 h	HFNC failure likely
In between	Reassess serially

Formula

ROX = (SpO₂ % ÷ FiO₂ fraction) ÷ respiratory rate

Clinical pearls

Validated in pneumonia-related hypoxemic respiratory failure on HFNC; less validated in COPD, COVID-19 or immunocompromised patients.
SpO₂ above ~97% plateaus on the dissociation curve and can inflate ROX; titrate oxygen to SpO₂ 92–96%.

Examples

rox spo2 95 fio2 50% rr 25 rox index sats 88 on fio2 0.8 resp rate 35 at 6 hours ROX at 2h: SpO2 92, FiO2 60, RR 30

FAQ

What is a good ROX index?

A ROX of 4.88 or higher at 2, 6 or 12 hours of HFNC is associated with a lower risk of intubation.

What ROX predicts HFNC failure?

Values below 2.85 at 2 hours, 3.47 at 6 hours or 3.85 at 12 hours.

Evidence

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;199:1368–76.

Pulmonary & Critical Care · Reviewed 2026-10-14 · Methodology