What are the causes of low eosinophil (white blood cell) counts?

Causes of Low Eosinophil Counts

Low eosinophil counts (eosinopenia) are most commonly caused by acute stress responses, corticosteroid therapy, or acute bacterial/viral infections, and typically do not require specific treatment as they resolve when the underlying condition is addressed.

Primary Causes of Eosinopenia

Corticosteroid Effects

• Systemic corticosteroids are the most common pharmacologic cause of low eosinophil counts, causing rapid redistribution of eosinophils from peripheral blood into tissues within hours of administration 1
• Inhaled corticosteroids can also significantly reduce blood eosinophil levels, with median reductions from 560 to 320 cells/µL when doses are increased from medium to high in asthma patients 1
• The eosinophil-lowering effect occurs through multiple mechanisms including decreased bone marrow production, increased apoptosis, and sequestration in lymphoid tissues 1

Acute Stress and Critical Illness

• Acute physiologic stress from severe infections, trauma, surgery, or burns causes rapid eosinopenia through endogenous cortisol release and catecholamine effects 2
• Patients hospitalized with acute bacterial infections or sepsis commonly present with eosinophil counts <50/µL 2
• This stress-induced eosinopenia typically resolves within days as the acute illness improves 2

Acute Infections

• Acute bacterial and viral infections commonly suppress eosinophil production and cause peripheral eosinopenia 2
• In hospitalized COPD exacerbation patients, eosinophil counts <50/µL were strongly associated with bacterial or viral infection (91% of cases) 2
• The degree of eosinopenia correlates with infection severity and inflammatory burden (C-reactive protein ≥20 mg/L) 2

Clinical Significance and Prognostic Implications

Infection Risk Marker

• Eosinophil counts <50/µL in hospitalized patients identify those at higher risk for bacterial infection requiring antibiotic therapy 2
• These patients had significantly longer hospital stays (median 7 vs 4 days) compared to those with eosinophil counts >150/µL 2
• Low eosinophil counts were associated with reduced 12-month survival (82.4% vs 90.7%) in hospitalized COPD patients 2

Medication-Induced Considerations

• When evaluating eosinophil counts, always consider recent corticosteroid exposure (systemic or high-dose inhaled) as this profoundly affects interpretation 1
• "Normal values" of blood eosinophils must be interpreted in the context of individual corticosteroid doses 1
• Increases in inhaled corticosteroid doses may result in blood eosinophil concentrations that would formally preclude treatment with biologics targeting the IL-5 pathway 1

Important Clinical Caveats

Transient Nature

• Eosinopenia is typically a transient finding that resolves when the precipitating cause (infection, stress, medication) is removed 2
• Unlike eosinophilia, isolated eosinopenia rarely indicates primary hematologic disease and does not require hematology referral 3

Measurement Variability

• Technical factors including time from blood collection to analysis can affect eosinophil measurements, with longer delays reducing measured counts 4
• Laboratory-to-laboratory differences exist, though correlation between laboratories is generally high (R = 0.89) 4

When to Investigate Further

• Persistent eosinopenia lasting >3 months after resolution of acute illness warrants consideration of chronic corticosteroid excess (endogenous Cushing's syndrome) or bone marrow disorders 3
• However, isolated chronic eosinopenia without other cytopenias or clinical concerns rarely requires extensive workup 3

Practical Management Approach

• For acute eosinopenia in hospitalized patients: Consider this a marker of acute infection/stress requiring treatment of the underlying condition rather than the eosinopenia itself 2
• Document all corticosteroid use (systemic and inhaled) when interpreting eosinophil counts 1
• In patients with eosinophil counts <50/µL and clinical signs of infection, prioritize antibiotic therapy based on clinical presentation 2
• Repeat eosinophil measurement after resolution of acute illness to confirm normalization 4