Many physicians use a white blood cell count to screen for infection in infants with fever. Studies have found that white blood cell count analysis is not accurate in screening infants for bacterial meningitis and urinary tract infection; its efficacy in screening for bacteremia also has been questioned. Because infants' immune systems are immature, data from studies of older children cannot be extrapolated. Bonsu and Harper reviewed the records of infants younger than 90 days with a temperature of at least 38°C (100.4°F) who were treated in an urban pediatric emergency department.

Infants with acute leukemia were excluded from the study. If a blood culture contained a pathogen known to cause bacteremia in this age group, bacteremia was coded. Of the 3,810 infants included in the analysis, a bacterial pathogen was identified in 38 children. The median total peripheral white blood cell count among infants with bacteremia was 13,900 cells per mm³, which was not significantly different from the count among infants without bacteremia.

The authors conclude that using a white blood cell count as the basis for the decision to obtain a blood culture probably will cause significant errors. Because of the potentially disastrous results of misdiagnosis, blood cultures should be obtained in all infants who are being evaluated for sepsis.