Normal White Blood Cell Count and Neutrophil Percentage for a 15-Day-Old 35-Week Preterm Infant
For a 15-day-old infant born at 35 weeks gestation, the normal total white blood cell count ranges from 5,000 to 20,000/mm³, with neutrophils typically comprising 30-60% of the differential, though lymphocytes are beginning to predominate during this transitional period. 1
Age-Specific Reference Values
Total White Blood Cell Count
- The reference range for total WBC count in neonates is 5,000-20,000/mm³ (5-20 Gpt/L), established due to considerable individual variation in healthy newborns without infection 1
- This range applies broadly across the neonatal period, though individual values can vary significantly 1
Neutrophil Values
- Absolute neutrophil count (ANC): Normal values are typically ≥1,500/mm³ 2
- Neutrophil percentage: At 15 days of age, neutrophils may still represent the predominant white cell type, though this is transitioning 3, 1
- The physiologic left shift (immature neutrophils) decreases progressively during the first days of life 1
Lymphocyte Transition
- Critical developmental milestone: Segmented neutrophils and lymphocyte counts cross over after the 5th day of life, resulting in relative lymphocytosis 1
- At 15 days of age, lymphocyte predominance is not yet fully expected, and neutrophils may still be the predominant cell type during this transitional period 3
Clinical Context for Interpretation
Gestational Age Considerations
- A 35-week infant is classified as a late preterm infant, which places them at higher risk for complications compared to term infants 4
- These infants require closer monitoring and lower thresholds for clinical concern 3
Important Clinical Pitfalls to Avoid
Do not rely on a single CBC value for clinical decision-making. The American Academy of Pediatrics recommends serial measurements, particularly when evaluating for sepsis 3
Maternal risk factors should lower your threshold for concern when interpreting blood counts, including inadequate intrapartum antibiotic prophylaxis, maternal fever, or chorioamnionitis 3
Normal CBC values should not provide false reassurance. Empiric antibiotics should not be withheld based on "normal" CBC alone in a 15-day-old with clinical concern for sepsis 3
Abnormal Values Requiring Further Evaluation
- Total WBC <5,000 or >30,000/mm³ 2
- Absolute neutrophil count <1,500/mm³ 2
- Band-to-polymorphonuclear cell ratio >0.2 2
However, the sensitivity and specificity of these abnormal values for predicting sepsis are limited (41% and 73% respectively), with a positive likelihood ratio of only 1.52 2, 5
Practical Application
When evaluating a 15-day-old 35-weeker, integrate the CBC results with clinical assessment rather than using laboratory values in isolation 3, 5. Up to 99% of asymptomatic neonates may have at least one abnormal neutrophil value during serial monitoring, yet remain clinically well 5. The clinical examination and maternal/perinatal risk factors are more important than isolated laboratory values in determining management 3, 5.