What does a relative neutropenia of 37% indicate?

Understanding Relative Neutropenia of 37%

Relative neutropenia of 37% indicates a decreased percentage of neutrophils in the white blood cell differential count while the absolute neutrophil count may still be within normal limits. 1

Definition and Clinical Significance

Neutrophils normally constitute 50-70% of the total white blood cell count in adults. When the percentage falls below this range (as in 37%), it's considered relative neutropenia. This differs from absolute neutropenia, which is defined by the actual number of neutrophils per volume of blood:

• Normal: ANC > 1500 cells/mm³
• Mild neutropenia: ANC 1000-1500 cells/mm³
• Moderate neutropenia: ANC 500-1000 cells/mm³
• Severe neutropenia: ANC < 500 cells/mm³ 1, 2

Clinical Interpretation

A relative neutropenia of 37% can occur in several scenarios:

1. Normal absolute count with altered differential: The total white blood cell count is normal, but there's a shift in the differential with increased lymphocytes, monocytes, or eosinophils, causing the neutrophil percentage to decrease.

2. Viral infections: Many viral infections cause a relative lymphocytosis, which decreases the neutrophil percentage while the absolute neutrophil count remains adequate.

3. Early bacterial infection: In some cases, relative neutropenia may be seen in the early stages of bacterial infection before the bone marrow responds with increased neutrophil production.

4. Medication effect: Certain medications can cause a shift in the differential without affecting the absolute neutrophil count.

Assessment Approach

When evaluating relative neutropenia of 37%, consider:

1. Calculate the absolute neutrophil count: ANC = WBC × (% neutrophils ÷ 100)

   • This is crucial to determine if there's also absolute neutropenia, which carries greater clinical significance

2. Review the complete blood count:

   • Are other cell lines affected?
   • Is the total WBC count normal, elevated, or decreased?
   • Are there abnormal cells present?

3. Consider clinical context:

   • Recent infections (particularly viral)
   • Medication history
   • Underlying medical conditions
   • Duration of the finding (acute vs chronic)

Clinical Implications

The clinical significance of relative neutropenia depends on the absolute neutrophil count:

• If ANC > 1500 cells/mm³: Generally low risk for infections despite the relative neutropenia 1
• If ANC 1000-1500 cells/mm³: Minimal increased risk of infection
• If ANC 500-1000 cells/mm³: Moderate risk of infection
• If ANC < 500 cells/mm³: Significant risk of bacterial and fungal infections 3

Common Pitfalls

• Focusing only on percentage: Always calculate the absolute neutrophil count to determine true infection risk
• Overlooking transient causes: Many cases of relative neutropenia are transient and resolve without intervention
• Missing underlying conditions: Relative neutropenia can be a clue to underlying disorders that require further investigation
• Unnecessary interventions: Relative neutropenia with normal ANC rarely requires specific treatment

Follow-up Recommendations

• If ANC is normal: Monitor with repeat CBC in 2-4 weeks if clinically indicated
• If ANC is low: More frequent monitoring and investigation for underlying causes
• If accompanied by fever or signs of infection: Prompt evaluation and possible empiric antibiotic therapy, especially if ANC < 500 cells/mm³ 4

Remember that relative neutropenia of 37% is a laboratory finding that requires clinical correlation and calculation of the absolute neutrophil count to determine its true significance.