Rapid response system (RRS) efficacy and national early warning score (NEWS) performances have largely been reported in inpatient settings, with few such reports undertaken in outpatient settings.This study aimed to investigate NEWS validity in predicting poor clinical outcomes among outpatients who had activated the RRS using single-parameter criteria.A single-center retrospective cohort studyFrom April 1, 2014 to November 30, 2017 in an urban 350-bed referral hospital in JapanWe collected patient characteristics such as activation triggers, interventions, arrival times, dispositions, final diagnoses, and patient outcomes. Poor clinical outcomes were defined as unplanned intensive care unit transfers or deaths within 24 hours. Correlations between the NEWS and clinical outcomes at the time of deterioration and disposition were analyzed.Among 31 outpatients, the NEWS value decreased significantly after a medical emergency team intervention (median, 8 vs 4, P < .001). The difference in the NEWS at the time of deterioration and at disposition was significantly less in patients with poor clinical outcomes (median 3 vs 1.5, P = .03). The area under the curve (AUC) for the NEWS high-risk patient group at the time of deterioration for predicting hospital admission was 0.85 (95% confidence interval [CI], 0.67-1.0), while the AUC for the NEWS high-risk patient group at disposition for predicting poor clinical outcomes was 0.83 (95% CI, 0.62-1.0).The difference between the NEWS at the time of deterioration and at disposition might usefully predict admissions and poor clinical outcomes in RRS outpatient settings.