Causes of Low Eosinophil Count
Low eosinophil counts (eosinopenia) are most commonly caused by acute stress responses, corticosteroid therapy, or acute bacterial/viral infections, though this finding is rarely clinically significant in isolation.
Primary Causes of Eosinopenia
Corticosteroid Effects
- Systemic corticosteroids profoundly suppress eosinophil counts, with effects visible as early as 6 hours after administration and dose-response curves plateating at low doses 1, 2
- After an average initial systemic corticosteroid prescription of 35 mg/day, mean eosinophil counts decrease by 30-36% within one month, depending on baseline levels 2
- Inhaled corticosteroids also significantly reduce blood eosinophils—increasing from medium to high-dose ICS can drop median eosinophil concentrations from 560 to 320 cells/µL 3
- The suppressive effect persists for several weeks after corticosteroid discontinuation, with only 21-26% of patients returning to baseline within one month 2
Acute Stress and Infection
- Acute bacterial or viral infections trigger stress responses that redistribute eosinophils from peripheral blood into tissues 4
- Eosinophil counts <50/μL in hospitalized COPD patients are strongly associated with active infection (91% vs 51.9% in those with counts >150/μL), longer hospital stays (7 vs 4 days), and lower 12-month survival (82.4% vs 90.7%) 4
- The acute phase response during severe infections causes bone marrow suppression of eosinophil production 4
Endogenous Cortisol Excess
- Cushing's syndrome and other conditions causing elevated endogenous cortisol produce sustained eosinopenia through the same mechanisms as exogenous corticosteroids 1
Clinical Significance and Interpretation
When Eosinopenia Matters
- In hospitalized patients with acute exacerbations, eosinophil counts <50/μL serve as a practical biomarker for bacterial infection requiring antibiotic therapy and predict worse outcomes 4
- Low eosinophil counts may indicate immunosuppression or severe systemic illness requiring investigation of the underlying cause 4
Common Pitfalls to Avoid
- Do not interpret eosinophil counts without considering recent corticosteroid exposure—the timing of corticosteroid discontinuation is critical, as suppressive effects persist for weeks after stopping therapy 2
- Do not assume a single low eosinophil measurement represents a stable state, as eosinophil levels show substantial variability with seasonal differences of approximately 20% between summer and winter months 5
- Current smoking decreases eosinophil levels by 23%, while nasal polyps increase levels by 38%—these intrinsic and extrinsic factors must be considered when interpreting any eosinophil count 5
Technical Considerations
- Time from blood collection to laboratory analysis affects eosinophil counts, with longer delays reducing measured values and increasing variability, particularly at higher baseline counts 5
- Laboratory-to-laboratory differences exist despite high correlation (Spearman's R = 0.89), so serial measurements should ideally use the same laboratory 5
When to Investigate Further
- Persistent eosinopenia (<50 cells/μL) without obvious cause (corticosteroids, acute infection, or stress) warrants evaluation for bone marrow disorders, though this is uncommon 6
- In the context of other cytopenias, consider bone marrow evaluation to exclude myelodysplastic syndromes or other hematologic disorders 6