What does an eosinophil count measure?

What Eosinophils Measure

Eosinophils are white blood cells that serve as biomarkers for type 2 inflammatory processes, allergic conditions, parasitic infections, and certain tissue-invasive diseases—their elevation signals the need to investigate these underlying causes systematically. 1

Primary Clinical Significance

Eosinophil counts measure the presence and intensity of several distinct pathophysiologic processes:

Type 2 Inflammatory Disease

• Eosinophils are a reasonable surrogate marker for type 2 immune responses, particularly in respiratory conditions like asthma and chronic rhinosinusitis with nasal polyps. 1
• Blood eosinophil counts correlate significantly with tissue eosinophilia in nasal polyps (r=0.353 for CT severity, r=0.444 for endoscopic scores). 1
• In asthma, sputum eosinophil counts are consistently reduced two- to sevenfold by corticosteroids and predict loss of asthma control after treatment withdrawal with 90% sensitivity. 1
• Eosinophilic airway inflammation (sputum eosinophils >1.9%) is present in 66-100% of asthma patients and up to 40% of patients with isolated chronic cough. 1

Allergic and Atopic Conditions

• In adults with mild eosinophilia, 50-80% have concurrent atopic conditions such as allergic rhinitis, atopic dermatitis, or asthma. 2
• Eosinophils accumulate in diseased tissue and their numbers generally correlate with clinical symptom severity in allergic diseases. 3
• Seasonal pollen exposure can transiently increase peripheral eosinophil counts in atopic individuals. 2

Parasitic and Infectious Causes

• In returning travelers or migrants, helminth infections account for 19-80% of eosinophilia cases, making this the most common identifiable cause in endemic-exposed populations. 2
• Only tissue-invasive helminthic parasites cause eosinophilia, limiting its utility as a general screening tool for all parasitic infections. 2
• Many helminth-infected patients do not have eosinophilia, so normal counts do not exclude parasitic infection. 2

Tissue Damage and End-Organ Involvement

• Eosinophils release toxic granule proteins (major basic protein, eosinophil cationic protein, eosinophil peroxidase) that can damage surrounding tissues, making elevated counts a marker for potential organ injury. 3
• In eosinophilic esophagitis, tissue eosinophil counts ≥15 per 0.3 mm² define the diagnosis, though peripheral blood eosinophilia occurs in only 10-50% of adult cases. 1, 2
• Persistent eosinophilia ≥1.5 × 10⁹/L for >3 months or counts >5.0 × 10⁹/L at any time carry significant risk of morbidity and mortality from cardiac, pulmonary, or neurologic damage. 2

Reporting Standards and Interpretation

Blood Eosinophils

• Peripheral blood eosinophils are reported as absolute counts (cells/μL or ×10⁹/L), with normal ranges of 50-500 cells/μL. 4
• Mild eosinophilia is defined as 0.5-1.5 × 10⁹/L, moderate-to-severe as ≥1.5 × 10⁹/L. 2
• Blood eosinophil counts ≥300 cells/μL predict COPD exacerbations with 71% sensitivity and 64% specificity in endemic parasitic areas. 5

Tissue Eosinophils

• Peak (maximum) eosinophil counts per high-power field or per 0.3 mm² are used for tissue diagnosis, as inflammation is often patchy and focal. 4
• For eosinophilic esophagitis, the diagnostic threshold is ≥15 eosinophils per 0.3 mm² in the most densely populated area. 1, 4
• Standardization to 0.3 mm² addresses historical variability in microscope field sizes across institutions. 4

Critical Clinical Caveats

• Do not rely on peripheral eosinophil counts alone to diagnose tissue eosinophilic diseases like eosinophilic esophagitis—histologic confirmation via biopsy is the gold standard. 2, 4
• Do not assume eosinophilia alone is adequate screening for helminth infection, as many infected patients have normal counts. 2
• In COPD patients from parasitic-endemic areas, blood eosinophil counts ≥300 cells/μL identify higher exacerbation rates and hospital admissions but paradoxically lower mortality. 5
• Eosinopenia (low eosinophil counts) in COVID-19 patients indicates disease severity, whereas rising counts during recovery predict better prognosis. 6