FeNO Testing
Last updated: September 2024
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The effect of smoking is not absolute, therefore, patients may still show elevated FeNO levels, but these may not be indicative of actual inflammation in the lung. 1,3
Current smoking rates amongst asthmatic patients in the USA and UK range from 17-35%, despite its association with an accelerated decrease in lung function and increase in severe symptoms. 4,5 Smoking has been well-documented as a factor which causes a decrease in endogenous nitric oxide (NO) production in the lung epithelium, which has been shown to decrease FeNO levels by 30-60% in smoking asthmatics compared to non-smoking asthmatics. 2,6 This can mean that a patients FeNO level may not be a clear representation of the inflammation present in the lungs, which could pose a challenge with asthma management.
Fractional exhaled nitric oxide (FeNO) testing is a quick measure of lung inflammation and works by measuring the level of exhaled NO in the patient’s breath, which is typically upregulated in some types of asthma.7,8 In Type 2 asthma patients, the pathway of increased nitric oxide levels is facilitated by an increase in inflammatory cytokines such as IL-4 and IL-13, leading to an overproduction of airway NO.9 This is used to help diagnose asthma, monitor disease progression, and identify which patients will respond to anti-inflammatory treatment and doses, with increases in FeNO being associated with a deterioration in disease control.10-12
The challenge faced when measuring FeNO level in patients who smoke is that the process of endogenous NO production is dysregulated in the lung.13
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Smoking is associated with decreased endogenous NO production and increased oxidative stress, which can cause damage to the lung epithelium.3,13 When cigarette smoke is inhaled into the lung, this causes a reduction in cytokine-induced inducible NO synthase (iNOS) protein expression in lung epithelial cells, which downregulates the inflammatory process, causing a decrease in endogenous NO production.14, 16
Studies have shown that FeNO levels in asthma patients who smoke are significantly lower than in patients who have never smoked, however, FeNO can still be used to differentiate asthmatic patients from non-asthmatic patients in never smoking and current smoking patients 1,2.
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