Objective Febrile infants≤3 months old constitute a vulnerable group at risk of serious infections (SI). We aimed to (1) study the test performance of two clinical assessment tools—the National Institute for Health and Care Excellence (NICE) Traffic Light System and Severity Index Score (SIS) in predicting SI among all febrile young infants and (2) evaluate the performance of three low-risk criteria—the Rochester Criteria (RC), Philadelphia Criteria (PC) and Boston Criteria (BC) among well-looking febrile infants.

Methods A retrospective validation study was conducted. Serious illness included both bacterial and serious viral illness such as meningitis and encephalitis. We included febrile infants≤3 months old presenting to a paediatric emergency department in Singapore between March 2015 and February 2016. Infants were assigned to high-risk and low-risk groups for SI according to each of the five tools. We compared the performance of the NICE guideline and SIS at initial clinical assessment for all infants and the low-risk criteria—RC, PC and BC—among well-looking infants. We presented their performance using sensitivity, specificity, positive, negative predictive values and likelihood ratios.

Results Of 1057 infants analysed, 326 (30.8%) were diagnosed with SI. The NICE guideline had an overall sensitivity of 93.3% (95% CI 90.0 to 95.7), while the SIS had a sensitivity of 79.1% (95% CI 74.3 to 83.4). The incidence of SI was similar among infants who were well-looking and those who were not. Among the low-risk criteria, the RC performed with the highest sensitivity in infants aged 0–28 days (98.2%, 95% CI 90.3% to 100.0%) and 29–60 days (92.4%, 95% CI 86.0% to 96.5%), while the PC performed best in infants aged 61–90 days (100.0%, 95% CI 95.4% to 100.0%).

Conclusions The NICE guideline achieved high sensitivity in our study population, and the RC had the highest sensitivity in predicting for SI among well-appearing febrile infants. Prospective validation is required.