Measuring Airway Eosinophils in Asthmatic Children

Measuring Airway Eosinophils in Asthmatic Children

Andrew Bush
European Respiratory Disease 2007 - Issue I
Published: October 2008
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It is clear that assessing asthma on the basis of history, physical examination and physiological testing – as recommended by most current national and international guidelines – is no longer state of the art.1 The inclusion of some measure of airway inflammation into the decision-making process improves control of asthma, usually – but not invariably – without the need for increased doses of inhaled corticosteroids, and may:2-5

• predict relapse when reducing or stopping asthma therapy;6,7
• predict impending asthma exacerbations;8 and
• improve the diagnosis of asthma.9

Surrogates used for inflammation in clinical practice include induced sputum,2,6 fractional exhaled nitric oxide (FeNO)3,4,6-8 and bronchial hyperreactivity (BHR).5 The following aspects characterise the perfect ‘inflammometer’:

• inexpensive;
• easy to maintain and calibrate;
• completely non-invasive;
• easy to use, no co-operation needed;
• direct measurement of all relevant aspects of inflammation;
• rapid availability of answers; and
• evidence of beneficial clinical outcomes.

In clinical practice, although direct invasive methods such as bronchoscopy, bronchoalveolar lavage (BAL) and endobronchial biopsy have been used on a ‘one-off’ basis to phenotype severe asthma and plan treatment,10,11 for most purposes non-invasive methods must be used. The most direct method is induced-sputum cytology, in particular eosinophil count. A randomised, double-blind, controlled study in adults demonstrated improved control of asthma if sputum eosinophil counts were normalised.2 In other words, increasing treatment in an asymptomatic patient if there was a raised sputum eosinophil count, and reducing treatment – irrespective of symptoms – if the sputum eosinophil count was <2.5%, leads to better outcomes with no increase in inhaled-corticosteroid dose than conventional guideline-driven management.12 Similarly, the absence of eosinophils from induced sputum predicted successful dose reduction in a group of asthmatic children.6 However, sputum induction is timeconsuming, may not be safe in severe airflow obstruction and is unsuccessful in around one-quarter of children.13 Hence, FeNO has been increasingly used as a surrogate for airway inflammation in clinical practice. This is not just because of ease of measurement in the clinic, but because it is becoming increasingly possible to monitor FeNO daily in the community.14-18 Guidelines for the measurement of exhaled nitric oxide have been published,19 and it is advised that the expiratory flow rate should be kept at 50ml/second (FeNO50).

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