What is Leukocytosis in Infants Under 8 Months with Kawasaki Disease?
Leukocytosis in infants under 8 months with Kawasaki disease is an elevated white blood cell count (typically >15,000/mm³) with predominance of immature and mature granulocytes, representing a typical acute-phase inflammatory response that occurs in approximately 50% of patients. 1, 2
Definition and Laboratory Characteristics
- Leukocytosis is defined as white blood cell counts >15,000/mm³, occurring in approximately 50% of Kawasaki disease patients during the acute stage 1, 2
- The leukocytosis demonstrates a predominance of immature and mature granulocytes (neutrophils), reflecting the acute inflammatory vasculitis 1
- Leukopenia is rare in Kawasaki disease and should prompt consideration of alternative diagnoses 2
Age-Specific Considerations in Young Infants
- Infants younger than 6 months with Kawasaki disease demonstrate significantly higher white blood cell counts (mean 21,740 ± 11,706/mm³) compared to older children (mean 11,830 ± 4,390/mm³) 3
- Young infants under 6 months have a 35% incidence of incomplete (atypical) presentation, making diagnosis more challenging despite the presence of leukocytosis 3
- Infants under 1 year have a 31.2% incidence of atypical presentation (compared to 7.5% in older children), with mean white blood cell counts of 15,403 ± 6,282/mm³ 4
Clinical Significance and Diagnostic Utility
- Leukocytosis serves as one of the supplemental laboratory criteria for diagnosing incomplete Kawasaki disease when WBC count ≥15,000/mm³ 2
- The combination of leukocytosis with other laboratory abnormalities (thrombocytosis after day 7, elevated CRP/ESR, hypoalbuminemia, elevated ALT, sterile pyuria) supports the diagnosis when clinical criteria are incomplete 2
- In young infants with prolonged unexplained fever and leukocytosis, Kawasaki disease should be strongly suspected even without complete clinical criteria, as these infants have the highest risk of coronary artery involvement (65% in infants <6 months) 3
Common Diagnostic Pitfalls
- Do not dismiss Kawasaki disease in young infants based on incomplete clinical presentation—leukocytosis combined with prolonged fever (≥5 days) warrants echocardiographic evaluation 3, 5
- The presence of leukocytosis may lead to misdiagnosis as bacterial sepsis or meningitis, particularly when combined with CSF pleocytosis (which occurs in 30% of Kawasaki disease patients) 2, 5
- Infants younger than 6 months require heightened clinical suspicion because they present with higher white blood cell counts, more incomplete presentations, and significantly higher rates of coronary artery complications despite having laboratory evidence of inflammation 3
Associated Laboratory Findings
- Leukocytosis typically occurs alongside other acute-phase responses including elevated ESR and CRP (nearly universal), anemia (normochromic, normocytic), and eventual thrombocytosis (peaking in week 3) 1, 2
- Young infants (<6 months) also demonstrate lower hemoglobin (9.98 ± 1.25 g/dL) and higher platelet counts (483 ± 393 × 1000/mm³) compared to older children 3