Study objective Previous studies on the construct validity of the Emergency Severity Index (ESI) were focused on outcome measures which could not be obtained directly at triage. A study was conducted to the construct validity of the ESI by measuring the association between the ESI triage categories and patients' vital signs at triage.

Methods A prospective observational study was conducted at an emergency department (ED) in the Netherlands. All patients who entered the ED between 20 July 2009 and 21 August 2009 were eligible for inclusion in the study. Patients' vital signs, triage category, age, gender, referrer and main complaint were registered. Vital signs were scored according to the Worthing Physiological Scoring System (WPSS) and the numerical pain rating scale. The data were analysed using ordinal logistic regression analyses.

Results An association was found between ESI triage categories and patients' vital signs at triage. Patients in WPSS categories ‘urgent’ and ‘alert’ were more likely triaged into the urgent triage categories (ESI triage categories 1 and 2) than patients with normal WPSS scores. However, no associations were found between pain scores and ESI triage categories.

Conclusion This study supports the validity of the ESI as it showed that patients' vital signs are associated with the ESI triage categories. However, a revision of the ESI guidelines concerning pain assessments is necessary.