Blood eosinophil count in asthma

Asthma, a chronic respiratory disease, is characterized by symptoms like wheezing, chest tightness, coughing, and shortness of breath, resulting from factors including airway inflammation, bronchoconstriction, hyperresponsiveness, and airway remodeling (1).

Eosinophilic asthma, a subtype of asthma characterized by heightened levels of eosinophils in the airways, peripheral blood, and sputum, involves a complex interplay of factors that contribute to its development and progression (1):

• clinical trials have utilized peripheral blood eosinophil counts of ≥150 cells/µL, ≥300 cells/µL, or ≥400 cells/µ, as well as sputum eosinophil levels exceeding 2 to 3%, to describe eosinophilic asthma
• normally, eosinophils undergo apoptosis and are cleared by macrophages without causing inflammation
  • note thought that disruptions in this apoptotic process can prolong the presence of eosinophils in the airways, exacerbating the manifestations of eosinophilic asthma
• while the exact prevalence of eosinophilic asthma is uncertain, it is estimated to represent about 50% of all severe asthma cases
• blood eosinophilia is correlated with an increased risk of asthma exacerbation, so with increasing blood eosinophil counts, exacerbation becomes more severe (2)
• eosinophils, as specialized granulocytes, play a critical role in driving various inflammatory responses
  • development of eosinophils begins with the differentiation of progenitor and hematopoietic stem cells into eosinophil/mast cell progenitors (EoMCPs) within the bone marrow
    • EoMCPs then give rise to eosinophil progenitors (EoPs), which mature into eosinophils
    • development and maturation of eosinophils from CD34⁺ hematopoietic precursors in the bone marrow are influenced by cytokines such as IL-3, IL-5, and granulocyte-macrophage colony-stimulating factor (GM-CSF)
      • among these, IL-5 plays a crucial role in initiating the development, maturation, and survival of eosinophils in the peripheral tissues, although it is not absolutely essential to eosinophil development

The primary causes of eosinophilic asthma include (1):

• exposure to allergens such as pollen, dust mites, and pet dander;
• genetic predisposition;
• psychological stressors; obesity; and
• environmental triggers like pollutants and respiratory infections

• these environmental and physiological factors trigger an immune response, leading to chronic inflammation in the airways

Blood/Serum Eosinophil Count

• eosinophilic asthma is often indicated by peripheral blood eosinophil counts that surpass specific benchmarks, such as >150 cells/μL, >300 cells/μL, or >400 cells/μL (1):
  • elevated counts are frequently linked to the severity of asthma exacerbations
  • is a discernible correlation between eosinophil counts in the sputum and blood
  • treatments like corticosteroids have been observed to reduce blood eosinophilia

NICE have included blood eosinophil count as an investigation in the diagnosis of asthma (3):

• "..the committee agreed that peripheral blood eosinophil counts could be part of the battery of tests used to establish a diagnosis of asthma, and that it could be useful as a rule-in test. They noted that peripheral blood eosinophil count is a simple measure to obtain, and it can be routinely collected in full-blood count in adults, although the need for venepuncture makes the test less easy to carry out in children..."
• "..concluded that peripheral blood eosinophils could be of use as a diagnostic test for asthma but noted that the current evidence did not support a specific cut-off for diagnosis. There are slight variations between the normal ranges quoted by different laboratories and their consensus view was that the upper limit of the relevant lab’s range should be used.."

NICE suggest a significant blood eosinophil count in the diagnosis of asthma is (4):

• if the eosinophil count is above the laboratory reference range when considering in adults and young people (aged over 16 years old)
• if the eosinophil count is more than 0.5 x 10⁹ per litre (500 cells/μL) in children aged 5 to 16 years

Duration of course of steroid therapy in asthma based on blood eosinophil count (5):

• a study (n=110) found tailoring length of prednisolone course to eosinophil levels (3 days if <300 cells/µL; 5 days if ≥300 cells/µL) was non-inferior to usual care (5 days for all) in terms of treatment failure (10.9% v 7.3%, respectively; 3.6% difference [95%CI -8.9% to 16.2%])
• concluded that "..eosinophil-guided therapy safely reduced systemic corticosteroid exposure in non-eosinophilic exacerbations while maintaining non-inferior outcomes.."

Reference:

1. Hussain M, Liu G. Eosinophilic Asthma: Pathophysiology and Therapeutic Horizons. Cells. 2024 Feb 23;13(5):384
2. Hirano T, Matsunaga K. Measurement of Blood Eosinophils in Asthma and Chronic Obstructive Pulmonary Disease. Intern Med. 2023 Jan 1;62(1):21-25.
3. NICE (November 2024). Final Asthma: diagnosis,monitoring and chronic asthma management(update);[G] Evidence reviews for diagnostic accuracy of eosinophil blood count measures in the diagnosis of asthma.
4. NICE (November 2024). Asthma: diagnosis, monitoring and chronic asthma management (BTS, NICE, SIGN)
5. Yii A, Tay TR, Lee KCH, et al. Blood eosinophil-guided systemic corticosteroid duration in adults hospitalised for asthma exacerbation: a randomised, controlled, open-label, non-inferiority trial. Thorax Published Online First: 03 December 2025.