Association between On-Scene Interval and 24-Hour Mortality among Code Red Emergency Medical Services Patients: A...

On-scene interval (OSI) is a modifiable component of prehospital care, but the interpretation of prolonged OSI is complex because longer scene time may reflect either avoidable delay or necessary life-saving care. This study aimed to evaluate the association between OSI and 24-hour mortality among Code Red emergency medical services (EMS) patients and to identify an operational OSI range that may warrant quality-improvement review.

We conducted a retrospective cohort study of all eligible Code Red EMS activations dispatched with advanced life support teams in Saraburi Province, Thailand, from May 2017 to July 2019. EMS dispatch records, ambulance run sheets, and hospital electronic medical records were used. OSI was defined as the interval from EMS arrival at the scene to departure from the scene. OSI was categorized in 5-minute intervals for operational interpretability and was also modeled continuously using restricted cubic splines. The primary outcome was all-cause mortality within 24 hours of EMS activation. Multivariable logistic regression, adjusted predicted probabilities, and restricted mean survival time were used to evaluate associations.

Of 2,500 Code Red EMS activations screened, 2,321 patients were included. The 24-hour mortality rate was 5.1% (118/2,321). Non-survivors had a longer median OSI than survivors (12 [interquartile range (IQR): 9-16] vs. 9 [IQR: 6-12] minutes; p < 0.001). After adjustment, OSI of 21-25 minutes was associated with higher odds of 24-hour mortality compared with OSI of 5 minutes or less (adjusted OR: 5.61, 95% CI: 1.58-19.97; p < 0.01). Restricted cubic spline analysis suggested a non-linear increase in mortality risk with longer OSI, although estimates at longer intervals were imprecise because of sparse data. Patients with OSI of 20 minutes or longer had 321.7 fewer expected survival minutes within the 24-hour horizon than those with OSI shorter than 20 minutes (95% CI: -530.1 to -113.2; p = 0.002). Low Glasgow Coma Scale score (p <0.001) and prehospital life-saving interventions (p <0.001) were the strongest predictors of death.

Prolonged OSI was associated with 24-hour mortality, with the clearest adjusted signal observed at 21-25 minutes. However, this finding should be interpreted cautiously because patient severity and the need for life-saving interventions were dominant predictors of mortality, and the optimal OSI threshold may differ by clinical condition. OSI should be used as an operational quality-improvement benchmark rather than a standalone prognostic marker or universal cutoff.