Differential WBC Count Interpretation in a 7-Month-Old Infant
In a 7-month-old infant, interpret the differential WBC count by recognizing that normal total WBC ranges from approximately 6,000-17,500/mm³, with lymphocyte predominance being physiologic at this age, and that elevated counts (>15,000/mm³) suggest bacterial infection while very low counts (<5,000/mm³) may indicate viral illness. 1
Age-Specific Normal Values
At 7 months of age, the WBC differential differs significantly from older children and adults:
- Total WBC count: Normal range is typically 6,000-17,500/mm³, though this represents extrapolation from neonatal and older infant data 2
- Lymphocyte predominance: Infants at this age normally have lymphocyte predominance rather than neutrophil predominance seen in adults 3
- Physiologic variation: WBC counts vary significantly with gestational age at birth, postnatal age, and clinical context 3, 2
Clinical Interpretation Framework
High WBC Counts (>15,000/mm³)
Bacterial infection is the primary concern when WBC counts exceed 15,000/mm³:
- WBC >15,000/mm³ combined with fever >39°C increases risk of occult bacteremia and serious bacterial infection 4
- Granulocyte elevation (>10,000-15,000/mm³) has 84-97% specificity for bacterial respiratory infections 5
- Extreme leukocytosis (≥35,000/mm³) indicates serious disease in 26% of cases and bacteremia in 10% 6
Low WBC Counts (<5,000/mm³)
Viral infections are most likely with low counts:
- WBC <5,000/mm³ occurs in 8-27% of children with influenza A 1
- Very low counts (<1,500/mm³) with lymphopenia may indicate severe infection requiring immediate evaluation 1
- Low counts do not rule out bacterial infection, as sensitivity is poor for distinguishing bacterial from viral etiologies 5
Context-Dependent Interpretation
Febrile Infants
When evaluating a 7-month-old with fever:
- Temperature >39°C (102.2°F) + WBC >15,000/mm³: Consider empiric antibiotics and blood culture for occult bacteremia risk 4
- Temperature >38.4°C (101.1°F) + WBC >6,100/mm³: Higher risk for serious bacterial infection, particularly during summer months (June-September) 4
- Lymphocyte counts alone have no value in distinguishing bacterial from viral infections 5
Timing of Sample Collection
Age of sample in hours significantly affects interpretation in young infants:
- WBC discrimination improves dramatically after 4 hours of age in neonates 2
- Very low absolute neutrophil counts (ANC <1,000) at ≥4 hours have likelihood ratio of 115 for sepsis 2
- For a 7-month-old, this timing consideration is less critical than in neonates but still relevant 2
Critical Pitfalls to Avoid
Do not rely solely on WBC counts for clinical decisions:
- Low sensitivity: Normal or low WBC counts do not exclude bacterial infection 5
- High specificity: Very high WBC (>20,000/mm³) or granulocyte counts (>15,000/mm³) strongly suggest bacterial etiology but miss many cases 5
- Lymphocyte counts are not discriminatory between viral and bacterial infections 5
Specimen quality matters:
- Minimize sampling errors by ensuring proper collection technique 3
- Consider factors affecting interpretation: stress response, inflammation, medications 3
Practical Algorithm
For a 7-month-old with abnormal WBC differential:
- WBC >15,000/mm³ with fever: Obtain blood culture, consider urinalysis/urine culture, evaluate for focal bacterial infection 4
- WBC <5,000/mm³: Consider viral testing (influenza, RSV), monitor closely for deterioration 1
- WBC ≥35,000/mm³: High suspicion for serious bacterial disease; obtain cultures and consider empiric antibiotics 6
- Normal WBC with clinical concern: Do not be falsely reassured; pursue appropriate diagnostic workup based on clinical presentation 5