Extreme leucocytosis and the risk of serious bacterial infections in febrile children

Objective To determine the clinical significance of extreme leucocytosis (white blood cell (WBC) count > 25 000/mm(3)) as a predictor of serious bacterial infection (SBI) in children. Patients and methods A retrospective case-control study was conducted in a paediatric emergency department in Israel. The study evaluated children aged 3-36 months admitted to the emergency department with fever (> 38 degrees C) who had a complete blood count (CBC). Children with extreme leucocytosis were identified through the laboratory database. Further, for each case patient two consecutive febrile patients with WBC counts of 15 000-24 999/mm(3) (moderate leucocytosis) served as controls (a case-control ratio of 1: 2). Results During the study, 146 patients with extreme leucocytosis were identified and compared with 292 patients with moderate leucocytosis. SBI was found in 57 (39%) patients with extreme leucocytosis compared with 45 (15.4%) control patients (p<0.001). The most commonly found SBI was segmental or lobar pneumonia, which was diagnosed in 41 (28%) patients in the case group compared with 27 (9.2%) patients in the control group (p<0.001, OR 3.83, 95% CI 2.25 to 6.52). Children with extreme leucocytosis were more often treated with antibiotics (52.7% vs 27.7%, p<0.001) and admitted to hospital (98.6% vs 50.68%, p<0.001). Conclusions In febrile children aged 3-36 months, the presence of extreme leucocytosis is associated with a 39% risk of having SBIs. The increased risk for SBI is mainly due to a higher risk for pneumonia.