Objective: to estimate the incidence of reactive thrombocytosis among febrile young infants and to asses the utility of platelet count as a potential predictor of serious bacterial infection (SBI).
Design: retrospective study between January 2005 and December 2008.
Setting: tertiary care pediatric unit.
Participants: all infants 29 to 89 days of age, admitted with rectal temperature > 38oC without a focus of infection.
Main outcome measures: the results of the sepsis evaluation on admission were recorded. SBI included all cases of occult bacteremia, urinary tract infection, bacterial meningitis, pneumonia, bacterial gastroenteritis and infections of the soft tissues and bones.
Results: of the 408 infants studied, 103 (25.2%) had SBI. Platelet count was significantly higher in infants with SBI compared to those without (median 513000 /mm3 [interquartile range 455,000-598,000/mm3] vs median 398000/mm3; [interquartile range 313,000-463,000/mm3]; P<0.001). Thrombocytosis had only moderate ability in predicting SBI (area under the curve: 0.74, 95 % CI 0.70-0.79). The combination of platelet count >450,000/mm3, WBC >15,000/mm3, Creactive protein >2 mg/dL, and pyuria >10 WBC/hpf would lead to misclassification of 4 infants with SBI (3.9% of SBIs; negative likelihood ratio 0.08).
Conclusion: reactive thrombocytosis was a frequent finding in young infants with SBI. Thrombocytosis >450,000 cells/mm3, in combination with leucocytosis, elevated CRP and pyuria, may help in early recognition of febrile young infants at risk for SBI.