From the Guidelines
Eosinophils in induced sputum are a crucial marker for diagnosing and monitoring respiratory conditions, particularly asthma and eosinophilic airway diseases, with elevated levels (>3%) indicating eosinophilic airway inflammation that often responds well to corticosteroid therapy. The normal level of eosinophils in induced sputum is generally less than 3% of total cells 1. To collect induced sputum, patients inhale nebulized hypertonic saline (typically 3-5%) for increasing intervals (5-20 minutes), then cough deeply to produce sputum from the lower airways. This sample is processed in a laboratory where cells are counted and classified.
Key Points
- Induced sputum analysis is particularly useful for distinguishing between different asthma phenotypes, guiding treatment decisions for inhaled or oral corticosteroids, and monitoring response to therapy 1.
- The presence of high eosinophil counts suggests type 2 inflammation and predicts good response to corticosteroids and biologics targeting IL-5, IL-4, or IL-13 pathways.
- This test is more specific than blood eosinophil counts for airway inflammation but requires specialized laboratory facilities and trained personnel to perform correctly 1.
Clinical Application
- Assessment of eosinophilic airway inflammation using induced sputum provides additional, clinically important information about ICS responsiveness and preventable future risk of exacerbations 1.
- Minimization of eosinophilic airway inflammation should be considered as an additional criterion for control of the underlying disease activity and for reduction of future risk, especially in patients with more severe asthma 1.
- In patients with cough due to asthma, induced sputum analysis can help identify those who may benefit from more aggressive antiinflammatory therapy, such as higher dose inhaled corticosteroids or oral steroid therapy 1.
From the Research
Eosinophils in Induced Sputum
- Eosinophils in induced sputum can be used as a marker of airway inflammation and to assess the severity of asthma 2, 3, 4
- Studies have shown that eosinophil counts in induced sputum are higher in asthmatic patients compared to healthy controls 2, 4
- The percentage of eosinophils in induced sputum can be used to predict the response to treatment with inhaled corticosteroids 2, 3, 4
Correlation with Clinical Parameters
- Eosinophil levels in induced sputum have been shown to correlate with clinical parameters such as days of acute exacerbations, use of rescue medication, emergency visits, nighttime cough, and days of school absence 4
- A significant negative correlation has been found between reduction in sputum eosinophil levels and improvement in FEV1 4
- Eosinophil levels in induced sputum can be used to monitor asthma severity and response to treatment in children 4
Treatment Response
- Inhaled corticosteroids have been shown to reduce eosinophil counts in induced sputum 2, 3, 5, 4
- The reduction in eosinophil counts has been correlated with improvement in clinical parameters and lung function 2, 3, 4
- Systemic corticosteroids have also been shown to reduce blood eosinophil counts in patients with asthma 6