In asthma, clinical symptoms and spirometry are insensitive in reflecting the underlying airway inflammation. Nitric oxide (FeNO) is now recognised as a reliable surrogate marker of eosinophilic airway inflammation. Fractional Exhaled Nitric Oxide (FeNO) is a breath test that measures exhaled Nitric oxide which is simple to perform and it’s non invasive. This test is predominantly carried out in secondary care, but is gradually being introduced in primary care.

Unfortunately FeNO is not a specific test for asthma alone. Patients with allergic rhinitis, for example, may have an increased FeNO, which reflects the airway inflammation of the nose rather that, the airways. There are also many other circumstances where a high FeNO reading may not necessarily be due to asthma. So, there must be caution when using FeNO to support or refute a diagnosis of asthma.

Anyone performing and interpreting FeNO must be appropriately trained. A low FeNO (less than 25ppb) is useful because this is not suggestive of eosinophilic inflammation. A high FeNO (more than 50ppb) is also useful because this is suggestive of eosinophilic inflammation, and can be used as objective evidence for eosinophilic asthma. An intermediate FeNO (25 – 50ppb) is less conclusive.

If the patient with eosinophilic asthma is successfully treated with inhaled or oral steroids, the expectation is that they should have a low FeNO. This is a useful test if you are assessing concordance with therapy.