What would cause an elevated white blood cell (WBC) count with neutrophilia in an 11-year-old female patient?

Acute Bacterial Infection

An 11-year-old female with WBC 18,000/mm³ and 89% neutrophils most likely has an acute bacterial infection requiring immediate evaluation for the source and consideration of antibiotic therapy. 1, 2

Diagnostic Interpretation

The laboratory findings strongly indicate bacterial infection based on established thresholds:

• WBC count of 18,000/mm³ exceeds the 14,000 cells/mm³ threshold, which has a likelihood ratio of 3.7 for documented bacterial infection 3, 1
• Neutrophil percentage of 89% approaches the 90% threshold, which carries a likelihood ratio of 7.5 for bacterial infection 3, 2
• The combination of elevated WBC with marked neutrophilia creates high probability for bacterial pathology even without fever 3, 2

Key Laboratory Considerations

Assess for left shift immediately by obtaining the absolute band count and band percentage from the manual differential 1:

• Absolute band count ≥1,500 cells/mm³ has the highest likelihood ratio (14.5) for bacterial infection 3, 1
• Band percentage ≥16% has a likelihood ratio of 4.7 for bacterial infection 3, 1
• Manual differential is essential—automated analyzers miss critical band forms 1

Most Likely Bacterial Sources in Children

The most common bacterial infections causing this degree of leukocytosis and neutrophilia in an 11-year-old include 2:

• Respiratory tract infections (pneumonia, severe pharyngitis)
• Urinary tract infections (pyelonephritis)
• Skin and soft tissue infections (cellulitis, abscess)
• Gastrointestinal infections (appendicitis, bacterial enterocolitis)

Immediate Diagnostic Workup

Perform targeted testing based on clinical symptoms 2:

• Blood cultures if systemic infection or sepsis suspected 2
• Urinalysis and urine culture if dysuria, flank pain, or abdominal symptoms present 2
• Chest imaging if respiratory symptoms (cough, tachypnea, chest pain) present 2
• Abdominal imaging if localized abdominal pain, peritoneal signs, or persistent GI symptoms 3

Non-Infectious Causes to Consider

While bacterial infection is most likely, evaluate for these alternative causes 2, 4:

• Medications: lithium, beta-agonists, epinephrine 2
• Acute stress response: surgery, trauma, severe emotional stress 4
• Inflammatory conditions: though less likely to cause this degree of neutrophilia in isolation 4

Critical Clinical Pitfalls

Do not delay evaluation based on absence of fever—bacterial infections can present without fever, particularly with significant leukocytosis and neutrophilia 3, 2

Do not rely on automated differential alone—manual review is mandatory to accurately assess band forms and detect left shift 1

Do not treat empirically without identifying the source—targeted diagnostic testing should guide antibiotic selection and duration 2

Do not ignore this as "stress leukocytosis" in a child—while stress can cause mild WBC elevation, a count of 18,000/mm³ with 89% neutrophils warrants thorough infection workup 4, 5

Specificity of Findings

High WBC counts and granulocyte percentages have excellent specificity for bacterial infection in children 5:

• WBC >15,000/mm³ has 86% specificity for bacterial vs. viral infection 5
• WBC >20,000/mm³ has 95% specificity for bacterial infection 5
• This patient's WBC of 18,000/mm³ falls in the range where bacterial etiology is highly probable 5