From the Guidelines
Eosinophils in induced sputum are a crucial biomarker for diagnosing and monitoring airway inflammation, particularly in asthma and eosinophilic bronchitis, with a normal range of less than 3% of total inflammatory cells. The assessment of eosinophilic airway inflammation using induced sputum provides additional, clinically important information about ICS responsiveness and preventable future risk of exacerbations 1.
Key Points
- Values above the 3% threshold indicate eosinophilic airway inflammation, which often responds well to corticosteroid therapy.
- To collect induced sputum, patients inhale nebulized hypertonic saline (typically 3-5%) for increasing intervals (usually 5-20 minutes), then expectorate sputum for analysis.
- This procedure should be performed by trained healthcare professionals with emergency equipment available, as it may trigger bronchospasm in susceptible individuals.
- Pre-treatment with a short-acting bronchodilator like salbutamol (200-400 mcg) is recommended to prevent this complication 1.
Clinical Implications
- Eosinophil counts in induced sputum can guide treatment decisions, particularly for corticosteroid therapy in asthma, and help distinguish between different inflammatory phenotypes of respiratory diseases.
- A raised sputum eosinophil count is predictive of the development of an exacerbation, and management strategies aimed at normalizing sputum eosinophil counts have been associated with up to 60% reduction in severe asthma exacerbations 1.
- The sputum differential eosinophil count is consistently reduced by corticosteroids and is increased by allergen challenge, before asthma exacerbations, and after prednisone reduction 1.
From the Research
Eosinophils in Induced Sputum
- Eosinophils in induced sputum can be used as a marker of airway inflammation in asthmatic patients 2
- The presence of eosinophils in induced sputum can predict an early decrease in peak expiratory flow (PEF) after reduction of inhaled corticosteroids 2
- Induced sputum eosinophilia is common in patients with asthma, especially those not using inhaled corticosteroids 3
Prediction of Sputum Eosinophilia
- Clinical variables such as low forced expiratory volume in 1 second (FEV1)% predicted, raised serum eosinophil, positive smoking history, and negative family history of asthma can predict sputum eosinophilia 4
- The combination of these clinical variables can provide a simple and practical alternative to assessment of airway eosinophilia in the clinical setting 4
Effects of Corticosteroids on Eosinophils
- Systemic corticosteroids can reduce blood eosinophil counts in patients with asthma 5
- The reduction in eosinophil counts can last for several weeks after discontinuation of systemic corticosteroids 5
Tailoring Asthma Interventions
- Tailoring asthma interventions based on sputum eosinophils can reduce the frequency of asthma exacerbations in adults with asthma 6
- This approach may be beneficial in reducing hospitalizations and the need for oral corticosteroids 6
- However, there is insufficient data to assess the effectiveness of this approach in children 6