Can a chronic parasitic infection cause neutropenia (low neutrophil count) and lymphocytosis (high lymphocyte count) in a 15-year-old boy?

Parasitic Infection and Neutrophil/Lymphocyte Differential in Adolescents

The differential counts described (40% neutrophils, 52% lymphocytes) do NOT represent neutropenia and are actually within normal range for a 15-year-old, making parasitic infection an unlikely explanation for these values. The absolute neutrophil count must be calculated to determine if true neutropenia exists.

Understanding the Laboratory Values

The percentages alone are misleading and do not indicate pathology. To assess for neutropenia, you must calculate the absolute neutrophil count (ANC) by multiplying the total white blood cell count by the neutrophil percentage 1.

• True neutropenia is defined as ANC <1000 cells/µL, with severe neutropenia at <500 cells/µL 1, 2
• A 40% neutrophil differential is actually normal for adolescents and adults (normal range 40-70%) 1
• The 52% lymphocyte count is also within normal limits for this age group 1

Can Parasitic Infections Cause Neutropenia?

Parasitic infections rarely cause true neutropenia; they more commonly cause eosinophilia when tissue invasion occurs 1. The key hematologic findings with parasitic infections include:

• Eosinophilia is the hallmark finding in parasitic infections involving a tissue phase, not neutropenia 1
• Lymphocytic predominance may suggest viral rather than parasitic etiology 1
• Neutrophil counts are typically normal or elevated with invasive bacterial pathogens, not decreased 1

What Could Actually Cause Low Neutrophil Counts in Adolescents?

If true neutropenia exists (which requires ANC calculation), the most common causes in a 15-year-old include:

Acquired Causes (Most Common)

• Viral infections are the most frequent cause of acquired neutropenia in children, followed by drug-induced and autoimmune neutropenia 3
• Bacterial sepsis can paradoxically lower neutrophil counts compared to normal values 1
• Medications (antibiotics, anticonvulsants, anti-inflammatory drugs) 3, 4

Chronic/Congenital Causes

• Congenital neutropenia typically presents with acute, life-threatening invasive bacterial and fungal infections, not chronic parasitic infections 3
• Autoimmune neutropenia 3
• Bone marrow infiltration or failure 5

Critical Next Steps

You must obtain the following immediately:

1. Calculate the absolute neutrophil count from the complete blood count: multiply total WBC by neutrophil percentage 1
2. Check the eosinophil count specifically - this is the key marker for parasitic infections with tissue invasion 1
3. Review the total white blood cell count - normal WBC with these percentages suggests no neutropenia exists 1
4. Examine the peripheral blood smear for morphologic abnormalities 4

Clinical Pitfalls to Avoid

Do not confuse relative percentages with absolute counts - this is the most common error in interpreting differentials 1. A patient can have 40% neutrophils but still have a normal or even elevated ANC if the total WBC is high.

The fact that no family members are sick argues strongly against an infectious cause of true neutropenia 3. Congenital neutropenia would have manifested earlier with severe infections, and acquired infectious neutropenia (especially viral) often affects multiple household contacts 3.

If true severe neutropenia exists (ANC <500 cells/µL), this patient requires urgent evaluation as approximately 10-20% of such patients will develop bloodstream infections 2, 1. However, the described percentages do not suggest this scenario without knowing the total WBC count.