Determining Eosinophil Count in Asthma

Induced Sputum Analysis

The American Thoracic Society recommends induced sputum analysis as the gold standard for airway inflammation, which is determined through sputum induction using nebulized hypertonic saline and processed with the mucolytic dithiothreitol, with a success rate of 80-90% in adults and somewhat lower in children 2
The procedure requires pretreatment with short-acting beta-agonists to prevent bronchospasm, and low baseline lung function is a relative contraindication 3
The principal readout is the differential inflammatory cell count, expressed as a percentage, based on manual count of 400 inflammatory cells on a stained cytospin preparation, with total cell count, cell viability, and squamous cell contamination also reported 1
The upper limit of normal for sputum eosinophil differential count is 1.9%, and eosinophilic airway inflammation is defined as sputum eosinophil count >1.9% 5
A halving or doubling of the sputum differential eosinophil count is considered clinically significant, with 90% sensitivity for loss of asthma control after inhaled corticosteroid withdrawal 1
Raised sputum eosinophil counts predict asthma exacerbations, and management strategies targeting eosinophil normalization reduce severe exacerbations by up to 60% 6

Blood eosinophil count is obtained through standard complete blood count with differential, which is more accessible than induced sputum analysis and can be performed in primary care settings 7
Age may affect blood eosinophil counts, particularly over age 50 5

Peripheral blood eosinophil counts may not correlate with tissue eosinophilia, particularly in eosinophilic esophagitis where tissue biopsy remains the gold standard 7
Eosinophil counts respond rapidly to corticosteroids, so timing relative to treatment is critical 6
The dose-response curve for inhaled corticosteroids plateaus at low doses, approximately 200 mcg/day beclomethasone equivalent 1
Use induced sputum analysis when available in specialist settings for precise assessment of airway inflammation and to guide corticosteroid therapy 6