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What is Nitric Oxide?

Nitric oxide is a gas produced by cells involved in the inflammation associated with allergic or eosinophilic asthma.

What is FeNO?

A Fractional exhaled Nitric Oxide (FeNO) test is a way of measuring the amount of nitric oxide in an exhaled breath. This test can help with the diagnosis of asthma by showing the level of inflammation in the lungs.

Clinical Utility of FeNO

FeNO can provide a noninvasive adjunct for the initial diagnosis of asthma with the ATS and NICE recommending it as part of their current guidelines and diagnostic algorithms.

Abbreviations: ATS, American Thoracic Society; FeNO, fractional ex- haled nitric oxide; GINA, Global Initiative for Asthma; ICS, inhaled corticosteroid; NICE, National Institute for Health and Care Excellence.

The ATS guidelines define high, intermediate, and low FeNO levels in adults as >50 ppb, 25 to 50 ppb, and <25 ppb, respectively. While in children, high, medium, and low FeNO levels are described as >35 ppb, 20 to 35 ppb, and <20 ppb (Table 1). The ATS recommends the use of FeNO to support a diagnosis of asthma where objective evidence is needed, particularly in the diagnosis of eosinophilic inflammation. The ATS describes that high FeNO levels (>50 ppb in adults and >35 ppb in children), when interpreted in the clinical context, indicate that eosinophilic inflammation is present with corticosteroid responsiveness in symptomatic patients, while low levels (<25 ppb in adults and <20 ppb in children) make this unlikely and intermediate levels should be interpreted with caution.

Current NICE guidelines, which use lower FeNO cut-off levels than ATS (Table 1), recommend the use of FeNO as part of the diagnostic work up where a diagnosis of asthma is being considered in adults or where there is diagnostic uncertainty in children. FeNO levels are again interpreted in a clinical context and further testing, such as bronchial provocation testing may aid the diagnosis by demonstrating airway hyperresponsiveness. GINA guidelines acknowledge the role of FeNO in identifying eosinophilic inflammation in asthma but do not currently see a role for FeNO in asthma diagnostic algorithms.  The Scottish Consensus defines cut-offs according to steroid exposure with positive values of >40 ppb in steroid-naive patients and >25 ppb for patients on ICS.