Background White blood cell count (WBC) is a widely used marker for the prediction of serious bacterial infection (SBI); however, previous research has shown poor performance. This study aims to assess the value of WBC in the prediction of SBI in children at the emergency department (ED) and compare its value with C reactive protein (CRP) and absolute neutrophil count (ANC).

Methods This study is an observational multicentre study including febrile children aged 0–18 years attending 1 of 12 EDs in 8 European countries. The association between WBC and SBI was assessed by multivariable logistic regression, adjusting for age, CRP and duration of fever. Additionally, diagnostic performance was assessed by sensitivity and specificity. Results were compared with CRP and ANC.

Results We included 17 082 children with WBC measurements, of which 1854 (10.9%) had an SBI. WBC >15 had an adjusted OR of 1.9 (95% CI 1.7 to 2.1) for prediction of SBI, after adjusting for confounders. Sensitivity and specificity were 0.56 (95% CI 0.54 to 0.58) and 0.74 (0.73 to 0.75) for WBC >15, and 0.32 (0.30 to 0.34) and 0.91 (0.91 to 0.91) for WBC >20, respectively. In comparison, CRP >20 mg/L had a sensitivity of 0.87 (95% CI 0.85 to 0.88) and a specificity of 0.59 (0.58 to 059). For CRP >80 mg/L, the sensitivity was 0.55 (95% CI 0.52 to 057) and the specificity was 0.91 (0.90 to 0.91). Additionally, for ANC >10, the sensitivity was 0.55 (95% CI 0.53 to 0.58) and the specificity was 0.75 (0.75 to 0.76). The combination of WBC and CRP did not improve performance compared with CRP alone.

Conclusion WBC does not have diagnostic benefit in identifying children with an SBI compared with CRP and should only be measured for specific indications.