A fractional exhaled nitric oxide test (often called a FeNO test) measures the amount of nitric oxide in your breath.

Nitric oxide is a gas in the atmosphere, but the body also produces it when there’s inflammation in the airways. A higher amount of nitric oxide indicates inflammation or swelling in the airways and can help diagnose conditions that result from a sensitivity to allergens — conditions like:

Here’s a look at how nitric oxide tests work and what you can expect.

A nitric oxide test is safe, simple, and typically takes less than 5 minutes to complete.

The test is similar to other pulmonary function tests and involves blowing into a handheld device. You’ll complete testing in your doctor’s office, and you’ll likely receive your results on the same visit.

To get started, your doctor places a clip over your nose and you’ll place your mouth over a mouthpiece. Next, you’ll inhale deeply and then exhale into the device until you hear a beep. You’ll repeat this process a few times.

Throughout the test your doctor monitors your breathing from a computer.

A nitric oxide test doesn’t require much preparation. But it’s best to avoid these an hour before the test so that they don’t impact your results:

  • caffeine
  • alcohol
  • smoking
  • nitrate-rich foods, including beets and green leafy vegetables

The results of your test can determine whether there’s swelling or inflammation in your airways. Higher than normal results indicate inflammation.

Exhaled nitric oxide is measured in parts per billion. A higher than normal level is more than 40 parts per billion for adults, and more than 25 parts per billion for children and adolescents.

Along with a fractional exhaled nitric oxide test, your doctor might use other tools for diagnosis — they may also listen to your breathing with a stethoscope and looking for signs of wheezing when you exhale.

A doctor might ask about other symptoms, too. Symptoms that can commonly happen along with high nitric oxide levels include:

  • coughing
  • chest tightness
  • difficulty breathing

A doctor might also order the following pulmonary function tests:

1. Spirometry

This test measures the amount of air you breathe in and out of your lungs. This helps gauge how well your lungs work.

Similarly to a nitric oxide test, you’ll inhale and exhale into a mouthpiece connected to a machine.

2. Challenge test

If certain activities or substances trigger symptoms, the doctor might suggest a challenge test.

You’re first exposed to a possible trigger — such as physical activity or an allergen — and then you’ll take a spirometry test.

3. Bronchodilators

It can be difficult to complete lung function tests on children. So, if a young child has signs of asthma, doctors might first prescribe a bronchodilator. These medications open the airways and make it easier to breathe.

If the medication improves their symptoms, asthma is a likely diagnosis.

4. Pulse oximetry tests

This test involves wearing a pulse oximeter on your finger to measure the oxygen in your red blood cells.

Normal oxygen levels indicate healthy lung function, but low levels can indicate a respiratory issue.

5. Imaging tests

Keep in mind that symptoms of allergic conditions can mimic other conditions like:

The doctor might use an X-ray or CT scan of your chest and sinuses to rule out other conditions, or order an upper endoscopy or phlegm sample to rule out acid reflux and infections, respectively.

A nitric oxide test is safe, so there’s a low risk of side effects. But repeatedly breathing in and out might cause lightheadedness in some people.

Coverage for a nitric oxide test depends on whether your insurance provider considers the test “medically necessary” for a diagnosis. Contact your insurance provider to confirm coverage.

If your policy covers testing, your out-of-pocket responsibility might include copays to your provider and medical deductibles. The deductible is what you pay out-of-pocket before your insurance provider covers diagnostic testing.

The cost of testing without insurance can range from $2,000 to $3,000.

If a nitric oxide test helps confirm a diagnosis, a doctor will next determine the best course of treatment depending on the severity.

Treatment might include an inhaled corticosteroid to reduce airway inflammation and quick relief bronchodilators to open up the airways. If allergies trigger your symptoms, the doctor might prescribe allergy medication or recommend allergy shots as well.

If you’ve been diagnosed with an allergic condition, the doctor can also adjust your medication to better manage symptoms. This can include adding an inhaler with a corticosteroid or recommending biologic therapy if you have severe symptoms.

Some allergic and respiratory conditions can make it difficult to breathe and do some types of physical activity. A nitric oxide test is often the first step to diagnosing one of these conditions, and it can also assess whether treatment is working.

This simple noninvasive test is relatively quick, and in most cases, you can get a diagnosis before leaving your doctor.