Normal White Blood Cell and Neutrophil Counts in a 15-Day-Old Baby
For a 15-day-old term or late-preterm infant, normal total white blood cell (WBC) count ranges from approximately 5,000-21,000/mm³, with absolute neutrophil count (ANC) typically between 1,800-5,400/mm³, though these values vary significantly with the infant's exact postnatal age in hours and clinical context. 1
Age-Specific Reference Ranges
The interpretation of neonatal blood counts is highly dependent on precise postnatal age:
Total WBC count: Reference ranges for healthy neonates at 2 weeks of age typically fall between 5,000-21,000/mm³, though individual variation exists 1
Absolute Neutrophil Count (ANC): Normal values range from approximately 1,800-5,400/mm³ at this age, with significant physiologic variation 1
Lymphocyte predominance: Unlike older infants where lymphocyte predominance is expected (occurring by 7 months), at 15 days neutrophils may still be the predominant cell type, though this is transitioning 2, 1
Critical Interpretation Considerations
Timing matters profoundly - CBC interpretation in neonates requires understanding that discrimination for infection improves dramatically with postnatal age in hours, not just days 3:
- At <1 hour of life, WBC counts have poor discriminatory value (area under ROC curve 0.52) 3
- By ≥4 hours, discrimination improves substantially (area under ROC curve 0.87) 3
- At 15 days, these values have stabilized but still differ from older infant norms 1
Abnormal Values Suggesting Infection
Very low counts are more concerning than elevated counts in neonates at risk for sepsis 3:
- ANC <1,000/mm³: Highly suspicious for bacterial sepsis (likelihood ratio 115 at ≥4 hours of age) 3
- WBC <5,000/mm³: Concerning for serious bacterial or viral infection 3, 4
- WBC >20,000/mm³: Less specific but warrants evaluation, particularly if accompanied by fever 3
Clinical Context Integration
The clinical scenario dramatically affects interpretation 5, 3:
- Maternal risk factors: Inadequate intrapartum antibiotic prophylaxis (<4 hours), maternal fever, or chorioamnionitis lower the threshold for concern 5
- Clinical appearance: Well-appearing infants with borderline values may be observed, while ill-appearing infants require full evaluation regardless of counts 5
- Associated findings: Thrombocytopenia, elevated immature neutrophil proportions (I:T ratio), or bandemia increase concern for sepsis 1, 3
Common Pitfalls to Avoid
- Don't rely on a single CBC value: Serial measurements are more informative than isolated values, particularly for platelet trends 5
- Don't use adult reference ranges: Neonatal values differ substantially from older children and adults 1
- Don't ignore very low counts: Leukopenia and neutropenia are more predictive of serious infection than leukocytosis in this age group 3, 4
- Don't delay antibiotics for borderline values: In a 15-day-old with clinical concern for sepsis, empiric antibiotics should not be withheld based on "normal" CBC alone 5, 4