Background The Manchester Triage System (MTS) determines an inappropriately low level of urgency (undertriage) to a minority of children. The aim of the study was to assess the clinical severity of undertriaged patients in the MTS and to define the determinants of undertriage.

Methods Patients who had attended the emergency department (ED) were triaged according to the MTS. Undertriage was defined as a ‘low urgent’ classification (levels 3, 4 and 5) under the MTS; as a ‘high urgent’ classification (levels 1 and 2) under an independent reference standard based on abnormal vital signs (level 1), potentially life-threatening conditions (level 2), and a combination of resource use, hospitalisation, and follow-up for the three lowest urgency levels.

In an expert meeting, three experienced paediatricians used a standardised format to determine the clinical severity. The clinical severity had been expressed by possible consequences of treatment delay caused by undertriage, such as the use of more interventions and diagnostics, longer hospitalisation, complications, morbidity, and mortality.

In a prospective observational study we used logistic regression analysis to assess predictors for undertriage.

Results In total, 0.9% (119/13,408) of the patients were undertriaged. In 53% (63/119) of these patients, experts considered undertriage as clinically severe. In 89% (56/63) of these patients the high reference urgency was determined on the basis of abnormal vital signs. The prospective observational study showed undertriage was more likely in infants (especially those younger than three months), and in children assigned to the MTS ‘unwell child’ flowchart (adjusted OR<3 months 4.2, 95% CI 2.3 to 7.7 and adjusted ORunwell child 11.1, 95% CI 5.5 to 22.3).

Conclusion Undertriage is infrequent, but can have serious clinical consequences. To reduce significant undertriage, the authors recommend a systematic assessment of vital signs in all children.