Frequently Asked Questions

Q: Is the FeNO test suitable for children?

FeNO (Fractional Exhaled Nitric Oxide) testing is suitable for patients age 4 and above. As measurement requires patient cooperation, some children below the age of 7 may require additional coaching and encouragement. FeNO testing can play an important role in the diagnosis and management of their disease.¹ In several recent studies, asthma therapy directed by FeNO testing has been shown to reduce asthma exacerbations in children when compared with symptom-based care or usual care.^2-4

Q: What do I do if the patient’s FeNO value is on the border, for example, 24 ppb?

Use caution and clinical judgement when evaluating a single FeNO value without any previous measurements. Evaluate all clinical data (symptoms, spirometry, inhaled corticosteroid dosing, medication adherence and other information) before making any treatment decisions.

Q: Should I replace spirometry with FeNO?

FeNO testing is not a replacement for spirometry. FeNO directly reflects the extent of airway inflammation, while spirometry measures airflow limitation. They both provide important information about different aspects of the disease. For example, FeNO can provide information on the level of underlying airway inflammation, but it cannot tell you the degree of airflow obstruction.

Measuring FeNO levels assists in the diagnosis of asthma and helps identify patients with Type 2 inflammation.⁵ For treatment optimisation, recent publications indicate that adjusting inhaled corticosteroids (ICS) dose according to FeNO value gives reductions in exacerbations up to 50%.^2,6 FeNO measurement is recommended in addition to spirometry in national and international guidelines. ^5,7

Q: What about patients who have a high FeNO value but do not have asthma?

There are other causes of elevated exhaled nitric oxide (NO).⁸ After carefully excluding asthma with other tests, significant allergic rhinitis and other atopic disorders should be considered. Also, food allergies and bowel diseases have been reported to increase exhaled NO.⁹ A diet rich in nitrate-containing foods may also temporarily raise exhaled NO.⁷

Q: How long does it take for FeNO to respond to treatment with corticosteroids?

A study done in 2012 showed that after starting a treatment with ICS, the FeNO levels follow an exponential decay curve. The t1/2 (half-life of exponential decay) for FeNO after starting treatment with ICS was 2.5 to 3 days. FeNO had reached a new plateau by 2 weeks.¹⁰ Thus, 10-14 days after starting ICS would be the earliest you may wish to consider rechecking FeNO levels.

Q: How long before an exacerbation will FeNO increase?

This is unknown, as long-term studies specifically looking at variation in FeNO levels and its time course in relation to an exacerbation have not been performed.¹¹

Q: What about asthma patients with no symptoms and a FeNO value in the medium range?

Results from spirometry poorly correlate with FeNO values since spirometry measures airway obstruction and FeNO testing measures airway inflammation.^2,6 It is beneficial to measure inflammation, because if left untreated or uncontrolled, chronic inflammation in the lungs may lead to the progression of airflow limitation and reduction in airway reversibility over time.¹²

Q: How often should a FeNO test be conducted?

Generally, patients seen by asthma specialists should have a baseline FeNO measurement, particularly those who are not well controlled.⁵ Once treatment with ICS is started, FeNO levels should be monitored according to the patient’s symptoms while optimising the ICS dose.⁷ In studies that showed a reduction in exacerbations, measuring FeNO every 2 to 4 months and titrating ICS until FeNO value was in the low/normal range resulted in a 50% reduction in exacerbations when compared with protocols that used symptom-based treatments (ACQ – Asthma Control Questionnaire) or usual care.^2,6

Q: What is the explanation for high FeNO results in patients diagnosed with asthma who are asymptomatic with long-term normal spirometry results?

FeNO values in the intermediate range (25-50 ppb) should be interpreted cautiously and with reference to the clinical context (i.e. patient assessment, other asthma assessment tools, ICS dose, patient adherence, etc).⁷