Eosinophil Level: Absolute vs. Relative Value
Eosinophil levels are reported as absolute counts (cells per unit volume), not as relative percentages or ratios to other laboratory findings. This is the standard across clinical practice and research, though the specific unit of measurement has evolved over time.
Standard Reporting Methods
Tissue Eosinophils (Esophageal Biopsies)
- Peak eosinophil counts are expressed as absolute numbers per high-power field (hpf) or per 0.3 mm², not as percentages 1.
- The diagnostic threshold for eosinophilic esophagitis is ≥15 eosinophils per 0.3 mm² (equivalent to ≥15 eosinophils/hpf), representing an absolute count in the most densely populated field 1.
- Modern digital microscopy has standardized this to 0.3 mm² to address historical variability in "high-power field" sizes across different microscopes 1.
- The British Society of Gastroenterology specifically recommends expressing eosinophil density as absolute counts per 0.3 mm² rather than converting to larger areas like per mm² 1.
Blood Eosinophils (Peripheral Blood)
- Peripheral blood eosinophils are reported as absolute counts (cells/μL or ×10⁹/L), not as percentages of total white blood cells 1, 2.
- Normal absolute eosinophil counts range from 50-500 cells/μL 2.
- While differential counts may show eosinophils as a percentage of total WBCs, clinical decision-making relies on the absolute count 1.
- Blood eosinophilia is defined by absolute thresholds (commonly >350-800 cells/μL depending on the reference), not by relative proportions 1.
Clinical Implications
Why Absolute Counts Matter
- Absolute counts provide standardized, reproducible measurements that allow comparison across institutions and studies 1.
- Treatment response is monitored using absolute eosinophil reductions, not relative changes 1.
- For asthma management, specific absolute thresholds (e.g., 150 cells/μL, 300 cells/μL) guide biologic therapy decisions 3, 4, 5.
Important Caveats
- The size of the measurement area significantly affects interpretation - historical "high-power fields" varied up to twofold between microscopes, which is why standardization to 0.3 mm² is now recommended 1.
- Peak (maximum) counts are used rather than average counts when evaluating tissue eosinophilia, as inflammation is often patchy and focal 1.
- Blood eosinophil counts show substantial variability due to intrinsic factors (age, sex, atopy), extrinsic factors (smoking, medications, season), and technical factors (time from collection to analysis) 3, 6.
Practical Reporting Standards
For esophageal biopsies: Report the peak absolute eosinophil count per 0.3 mm² from the most densely populated area, along with other histologic features 1.
For peripheral blood: Report the absolute eosinophil count in cells/μL or ×10⁹/L, noting that 10-50% of patients with eosinophilic esophagitis may have elevated peripheral counts (typically modest 2-fold elevations) 1.