The ROX index (Respiratory rate-OXygentation) has been described as a predictor of the requirement for invasive mechanical ventilation (iMV) in pneumonia patients receiving high-flow nasal oxygen (HFNO). Our aim was to assess the utility of the ROX index in predicting the need for iMV in patients with community-acquired pneumonia (CAP). <b>Methods:</b> This is a retrospective cohort study with CAP-diagnosed subjects who required admission in a tertiary hospital in Colombia between January 2009 and December 2019. The ROX index was estimated as the ratio of SpO2/FiO2 to respiratory rate ((SpO2/FiO2)/RR) using pulse oximetry and arterial blood gases (ABG) measurements. We analyzed the ROX index9s performance in all subjects with CAP and in the subgroups of CAP with oxygen delivery systems different from HFNO FiO2<28% and FiO2>=28%. <b>Results:</b> A total of 1811 subjects were included in this study; 159 (8.7%) required iMV. The ROX index was a good predictor of iMV requirement with an AUROC of 0.704 (95% CI, 0.653-0.756; p < 0.001) when calculated using pulse oximetry and 0.773 (95% CI, 0.714-0.832; p < 0.001) when calculated by ABG. The estimated cutoff point was 13.8 and lower values were associated with higher risk for requiring iMV. ROX index from pulse oximetry when a FiO2 <0.28% was administered showed an AUROC for iMV of 0.571 (95% CI, 0.488-0.654; p = 0.091) in contrast to administering a FiO2 ≥0.28% AUROC for iMV of 0.813 (95% CI, 0.763-0.864; p < 0.001). <b>Conclusion:</b> The ROX index is a good predictor of iMV requirement in patients hospitalized with CAP. The performance increases when a FiO2 ≥0.28 is administered through other oxygen delivery systems different from HFNO. It is an easy-to-use bedside score.
