The prediction of 24-h mortality by the respiratory rate and oxygenation index compared with National Early Warning Score in emergency department patients: an observational study

BACKGROUND: The ROX index combines respiratory rate and oxygenation to predict the response to oxygen therapy in pneumonia. It is calculated by dividing the patient's oxygen saturation, by the inspired oxygen concentration, and then by the respiratory rate (e.g. 95%/0.21/16 = 28). Since this index includes the most essential physiological variables to detect deterioration, it may be a helpful risk tool in the emergency department (ED). Although small studies suggest it can predict early mortality, no large study has compared it with the National Early Warning Score (NEWS), the most widely validated risk score for death within 24 h. AIM: The aim of this study was to compare the ability of the ROX index with the NEWS to predict mortality within 24 h of arrival at the hospital. METHODS: This was a retrospective observational multicentre analysis of data in the Netherlands Emergency Department Evaluation Database (NEED) on 270 665 patients attending four participating Dutch EDs. The ROX index and NEWS were determined on ED arrival and prior to ED treatment. RESULTS: The risk of death within 24 h increased with falling ROX and rising NEWS values. The area under the receiving operating characteristic curves for 24-h mortality of NEWS was significantly higher than for the ROX index [0.92; 95% confidence interval (CI), 0.91-0.92 versus 0.87; 95% CI, 0.86-0.88; P  < 0.01]. However, the observed and predicted mortality by the ROX index was identical to mortality of 5%, after which mortality was underestimated. In contrast, up to a predicted 24-h mortality of 3% NEWS slightly underestimates mortality, and above this level over-estimates it. The standardized net benefit of ROX is slightly higher than NEWS up to a predicted 24-h mortality of 3%. CONCLUSION: The prediction of 24-h mortality by the ROX index is more accurate than NEWS for most patients likely to be encountered in the ED. ROX may be used as a first screening tool in the ED.