OBJECTIVE:This prospective observational study aimed to assess the validity of the Manchester Triage System (MTS) for children with chronic illnesses who presented to the emergency department (ED) with infectious symptoms.METHODS:Children (<16 years old) presenting to the ED of a university hospital between 2008 and 2011 with dyspnea, diarrhea/vomiting, or fever were included. Chronic illness was classified on the basis of International Classification of Diseases, Ninth Revision, Clinical Modification, codes. The validity of the MTS was assessed by comparing the urgency categories of the MTS with an independent reference standard on the basis of abnormal vital signs, life-threatening working diagnosis, resource utilization, and follow-up. Overtriage, undertriage, and correct triage were calculated for children with and without a chronic illness. The performance was assessed by sensitivity, specificity, and diagnostic odds ratios, which were calculated by dichotomizing the MTS into high and low urgency.RESULTS:Of the 8592 children who presented to the ED with infectious symptoms, 2960 (35%) had a chronic illness. Undertriage occurred in 16% of children with chronic illnesses and in 11% of children without chronic illnesses (P < .001). Sensitivity of the MTS for children with chronic illnesses was 58% (95% confidence interval [CI]: 53%–62%) and was 74% (95% CI: 70%–78%) for children without chronic illnesses. There was no difference in specificity between the 2 groups. The diagnostic odds ratios for children with and without chronic illnesses were 4.8 (95% CI: 3.9–5.9) and 8.7 (95% CI: 7.1–11), respectively.CONCLUSIONS:In children presenting with infectious symptoms, the performance of the MTS was lower for children with chronic illnesses than for children without chronic illnesses. Nurses should be particularly aware of undertriage in children with chronic illnesses.