ROX Index

Q: When should I recalculate the ROX index during therapy?

The ROX index is intended for serial monitoring during the initial hours of high-flow nasal cannula therapy rather than a one-time calculation. Reassess it at regular clinical reviews using the current SpO2, FiO2, and respiratory rate. A value that rises toward 4.88 or higher suggests the patient is responding, while a persistently low or falling value supports earlier consideration of escalation.

Q: Can I use the ROX index in patients with pneumonia only?

It was derived and validated primarily in adults with pneumonia-related acute hypoxemic respiratory failure, so evidence is strongest in that group. Clinicians apply it more broadly in adult hypoxemic respiratory failure on high-flow nasal cannula, but it has not been validated in children and neonates, and it does not replace clinical assessment of work of breathing, comorbidities, or overall trajectory.

Q: How does the ROX index differ from the P/F ratio?

The P/F ratio uses arterial PaO2 divided by FiO2, whereas the ROX index uses peripheral oxygen saturation (SpO2) divided by FiO2 and then by respiratory rate. Adding respiratory rate is what lets the ROX index reflect work of breathing and predict high-flow nasal cannula success, while the P/F ratio is used to grade oxygenation impairment and ARDS severity.

Predicts success of high-flow nasal cannula (HFNC) therapy in acute hypoxemic respiratory failure.

Pulmonology PulmonologyEvidence-BasedUpdated 2026-07

Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.

Clinical Notes

The ROX Index helps predict whether patients with acute hypoxemic respiratory failure receiving HFNC are likely to require intubation.

Calculator Information

Specialty

Pulmonology

📋 Reference Ranges

Reference ranges for this calculator
Classification	Range
High Risk of HFNC Failure	<3.85
Intermediate	3.85–4.87
Likely HFNC Success	≥4.88

What This Calculator Measures

Calculates the ROX index, which combines oxygenation (SpO₂/FiO₂) with respiratory rate to estimate the likelihood that high-flow nasal cannula (HFNC) therapy will succeed or fail in adults with acute hypoxemic respiratory failure.

How to Use

Use only in patients already receiving high-flow nasal cannula (HFNC) therapy.
Enter SpO₂ as a percentage (e.g., 92 for 92%), FiO₂ as a fraction (e.g., 0.6 for 60%), and the respiratory rate.
The index is designed to be reassessed over time during the initial hours of HFNC therapy rather than used as a one-time definitive decision.
Interpret the result together with the patient's clinical trajectory when deciding whether to continue or escalate respiratory support.

How the Calculation Works

ROX index = (SpO2 / FiO2) / respiratory rate. SpO2 is the peripheral oxygen saturation, FiO2 is the inspired oxygen fraction, and respiratory rate is in breaths per minute; higher values predict greater likelihood of high-flow nasal cannula success.

Variables Explained

SpO2

Peripheral oxygen saturation as a percentage.

FiO2

Fraction of inspired oxygen as a decimal (0.21 to 1.0); SpO2 is divided by this value.

Common error: Entering FiO2 as a percentage rather than a decimal alters the ratio.

Respiratory Rate

Respiratory rate in breaths per minute, used as the divisor.

Interpretation Guide

ROX ≥4.88 suggests a high likelihood of HFNC success with low risk of intubation; 3.85–4.87 indicates intermediate risk warranting close monitoring and reassessment; <3.85 indicates a high risk of HFNC failure and supports earlier consideration of escalation of respiratory support.

Clinical Significance

The ROX index combines simple bedside measurements to help clinicians gauge the likelihood of HFNC success in acute hypoxemic respiratory failure, supporting earlier identification of patients who may require invasive mechanical ventilation.

When It Is Useful

Adults with acute hypoxemic respiratory failure receiving HFNC
Serial monitoring of HFNC response during the initial hours of therapy
Supporting decisions about continuing HFNC versus escalating respiratory support

When It Is Not Useful

Patients not receiving HFNC therapy
Children and neonates, where the index has not been validated in the same way as in adults
As a standalone indication for immediate intubation without clinical assessment

Common Mistakes

Using the ROX index to guide intubation decisions in patients on standard oxygen or noninvasive ventilation, since it was derived and validated in adults receiving high-flow nasal cannula.
Relying on a single ROX value instead of reassessing it over the initial hours of therapy, because the index is a dynamic measure and one reading cannot confirm HFNC success or failure.
Entering FiO2 as a percentage rather than a fraction, which shrinks the SpO2/FiO2 component and can drive the index below 3.85 even when the patient is responding well.

Limitations

The ROX index is a dynamic assessment and must be reassessed over time; a single value is not definitive.
It was derived and validated primarily in adults with pneumonia-related acute hypoxemic respiratory failure.
SpO₂-based ratios can be affected by hemoglobin level, peripheral perfusion, and measurement conditions.
It does not replace clinical judgment or account for comorbidities, work of breathing, or overall trajectory.

Worked Example

A 68-year-old man with COVID-19 pneumonia on high-flow nasal cannula has SpO₂ 85% on FiO₂ 0.8 with a respiratory rate of 30 breaths/min.

Inputs:

spo2: 85

fio2: 0.8

rr: 30

Expected Result: ROX = (85 / 0.8) / 30 ≈ 3.54 — high risk of HFNC failure; consider earlier escalation of respiratory support.

Evidence & References

Source:: Original Index Description
Reference:: 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.
Content updated:: 2026-08

Roca O, et al. J Crit Care. 2016;35:200-205.

References

Roca O, 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. (Original index)

Frequently Asked Questions

When should I recalculate the ROX index during therapy?

The ROX index is intended for serial monitoring during the initial hours of high-flow nasal cannula therapy rather than a one-time calculation. Reassess it at regular clinical reviews using the current SpO2, FiO2, and respiratory rate. A value that rises toward 4.88 or higher suggests the patient is responding, while a persistently low or falling value supports earlier consideration of escalation.

Can I use the ROX index in patients with pneumonia only?

It was derived and validated primarily in adults with pneumonia-related acute hypoxemic respiratory failure, so evidence is strongest in that group. Clinicians apply it more broadly in adult hypoxemic respiratory failure on high-flow nasal cannula, but it has not been validated in children and neonates, and it does not replace clinical assessment of work of breathing, comorbidities, or overall trajectory.

How does the ROX index differ from the P/F ratio?

The P/F ratio uses arterial PaO2 divided by FiO2, whereas the ROX index uses peripheral oxygen saturation (SpO2) divided by FiO2 and then by respiratory rate. Adding respiratory rate is what lets the ROX index reflect work of breathing and predict high-flow nasal cannula success, while the P/F ratio is used to grade oxygenation impairment and ARDS severity.

Disclaimer: This calculator is intended for educational and clinical decision support purposes only. The ROX index is one component of respiratory-failure assessment and must be interpreted with serial measurements and full clinical judgment.