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Fractional Exhaled Nitric Oxide Test

A FeNO test assesses how much fractional nitric oxide is exhaled in your breath. Nitric oxide is a signalling molecule produced by the cells lining the airways, and its levels can be elevated in the presence of inflammation, such as in conditions like asthma.

Also Known As… a fractional exhaled nitric oxide test is often referred to by its initials FeNO


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Everything you need to know

A FeNO test assesses how much fractional nitric oxide is exhaled in your breath.

Nitric oxide is a signalling molecule produced by the cells lining the airways, and its levels can be elevated in the presence of inflammation, such as in conditions like asthma.

The test helps in the diagnosis of asthma and can also be used to assess the level of inflammation and monitor how well asthma is being controlled. If you’re living with severe asthma, a FeNO test is also a useful way to assist with asthma management as it can help a healthcare professional check if your medication is working.

FeNO levels can also provide information on the type of asthma a person may have. Allergic asthma is often associated with elevated FeNO levels. FeNO testing can assist healthcare professionals in making decisions about the type and dose of medications needed for asthma management. It can help optimise treatment plans for individual patients.

FeNO levels can sometimes help distinguish between asthma and other respiratory conditions that may have similar symptoms. This can contribute to a more accurate diagnosis.


If you have an upcoming FeNO test, then there are some things you can do to help improve the accuracy of the test results.

That includes:

  • Avoid smoking before the FeNO test as it can exacerbate symptoms suggestive of asthma.
  • Avoid eating nitrate-rich food an hour before the test, this includes leafy vegetables such as spinach and kale. The team also recommends avoiding beetroot, caffeine, and alcohol as this can also impact the test results.
  • Follow your healthcare provider’s instructions regarding any medications you are taking. Some medications, such as inhaled corticosteroids, can influence FeNO levels. Your doctor may advise you to temporarily discontinue certain medications or adjust the timing of your doses.
  • If you manage to avoid smoking and eating certain foods, then you should receive accurate FeNO results.


To begin the FeNO test, the patient will first breathe into a plastic mouthpiece connected to a machine that analyses the air you exhale. You will be instructed on how to properly perform the test. This typically involves taking a deep breath and then exhaling slowly and steadily into the mouthpiece. The technician will guide you through the process.Patients may be asked to repeat this a few times just to determine the correct test results.

As you exhale, the device measures the amount of nitric oxide in your breath. This measurement is expressed as parts per billion (ppb) and represents the concentration of nitric oxide in the air you breathe out. The test may be repeated several times to ensure accuracy and consistency of the results. Each exhalation is usually separated by a short period of normal breathing.


As a FeNO test only takes a short amount of time, the doctor will usually be able to receive the results within a few minutes of the test being completed.

Your doctor will interpret the results in the context of your symptoms, medical history, and other diagnostic information.

A positive FeNO test would show high levels of nitric oxide and airway inflammation. The result should also offer a FeNO measurement to show the extent of the inflammation; this is especially useful to a new patient as it can show how severe their asthma is.

For patients who received a positive result in the FeNO test, the next step is to introduce a treatment plan to help their symptoms. There are asthma medicines that will lower the nitric oxide levels in their breath, by decreasing the inflammation in their airways, and remove the other symptoms associated with mild asthma.

For those suffering from severe or chronic asthma, further treatment may be required.

It is important to note that the interpretation of FeNO levels requires expertise, and your healthcare provider will use the results in conjunction with other clinical information to guide diagnosis and treatment decisions. If you have any questions or concerns about the test, don’t hesitate to get in touch.

How to pay for your treatment

If you’re… paying for yourself

Did you know you don’t need private medical insurance to come to St John & St Elizabeth Hospital? As a self-pay patient, you can access safe, outstanding quality health care at times to suit you.

For scans and tests, as well as to see most consultants, you’ll still need to be referred by a medical professional like your GP, but as a self-pay patient, the process is more straightforward. You won’t need authorisation from an insurance provider, and you’ll have greater choice of consultant and appointment times.

If you’re… insured

St John & St Elizabeth Hospital is approved by all major medical insurance companies. If you have a personal private health insurance policy, or your company provide it for you, you can use it to pay for your care from your initial consultation through to treatment, surgery and aftercare such as physiotherapy. Not all private health insurance plans cover the same things. It’s very important to check exactly what you are covered for with your insurance provider.