When you have asthma, you don’t always know if your symptoms are getting worse because your airways are inflamed-or if something else is going on. That’s where FeNO testing comes in. It’s a simple, quick breath test that tells your doctor how much inflammation is in your lungs right now. No needles. No blood draws. Just breathe into a small device, and in under a minute, you get a number that helps guide your treatment.
What FeNO Testing Actually Measures
FeNO stands for Fractional Exhaled Nitric Oxide. It’s not measuring your lung capacity like spirometry. Instead, it’s detecting nitric oxide gas in your breath-a chemical your body produces when there’s type 2 inflammation in your airways. This kind of inflammation is common in allergic asthma and often responds well to inhaled steroids.
In healthy people, FeNO levels are low-usually below 20 parts per billion (ppb). But if your level is 25 ppb or higher (20 ppb for kids), it’s a strong sign that eosinophilic inflammation is active. That means your immune system is overreacting in your lungs, swelling up the lining and making it harder to breathe. High FeNO doesn’t mean you have asthma for sure-but it gives your doctor objective proof that inflammation is driving your symptoms.
Why It’s Better Than Guesswork
For years, doctors relied on symptoms like wheezing, coughing, and shortness of breath to adjust asthma meds. But symptoms can be misleading. You might feel fine one day and have a flare-up the next, even if your inhaler hasn’t changed. Or you might think your asthma is under control, but your airways are still quietly inflamed.
FeNO testing cuts through the guesswork. A 2023 study from Morgan Scientific showed that when FeNO levels are high, the chance of an accurate asthma diagnosis increases up to seven times compared to using symptoms and lung function tests alone. It’s especially helpful when:
- Your symptoms don’t match your lung function test results
- You’re not responding well to your current inhaler
- You’re being considered for biologic therapy (like dupilumab or benralizumab)
- You’ve had multiple asthma attacks in the past year
One patient in Manchester, who’d been told she had chronic bronchitis for seven years, finally got a correct asthma diagnosis after her FeNO level came back at 48 ppb. Her symptoms had been dismissed as ‘just a smoker’s cough.’ The test changed everything.
How the Test Works
It’s easier than you think. You’ll be asked to:
- Inhale deeply through a filter that removes outside air pollution and nitric oxide
- Exhale slowly and steadily into a handheld device at a flow rate of 50 mL per second
- Hold the breath for about 10 seconds
The device measures the nitric oxide in your breath and gives you a number in ppb. The whole thing takes less than 30 seconds. Kids as young as five can do it. No training needed. No discomfort.
But there are rules to get accurate results:
- Avoid eating, drinking (except water), smoking, or exercising for at least one hour before the test
- Don’t do it right after using your rescue inhaler
- It should be done before spirometry, not after
Smokers often have lower FeNO readings-sometimes 30-50% lower-because smoking suppresses nitric oxide production. That doesn’t mean they’re not inflamed; it just means the test might underestimate their inflammation. Your doctor will take that into account.
FeNO vs. Other Tests
Many people wonder how FeNO compares to blood tests or sputum analysis.
FeNO vs. Spirometry: Spirometry measures how much air you can blow out and how fast. It’s great for spotting airflow obstruction, but it doesn’t tell you why it’s happening. FeNO doesn’t replace spirometry-it complements it. Together, they give a fuller picture. One study found combining both improved diagnostic accuracy to 76%, far better than either test alone.
FeNO vs. Blood Eosinophils: Blood tests can show elevated eosinophils, which are white blood cells linked to allergic inflammation. But FeNO is more directly tied to what’s happening in your airways. Studies show FeNO correlates better with sputum eosinophils (the gold standard for airway inflammation) than blood tests do.
FeNO vs. Sputum Analysis: Getting a sputum sample requires coughing up mucus, which many patients find unpleasant. It also needs lab processing, so results take days. FeNO gives instant results and is painless.
But here’s the catch: FeNO only detects type 2 inflammation. If your asthma is triggered by stress, cold air, or non-allergic causes, your FeNO might be normal-even if you’re having attacks. That’s why it’s never used alone.
What the Numbers Mean
FeNO results are broken into three ranges:
- Low (under 25 ppb in adults): Unlikely to have active eosinophilic inflammation. Your asthma may be controlled, or it might be non-type 2 asthma.
- Medium (25-50 ppb): Moderate inflammation. Likely to benefit from inhaled corticosteroids. Your doctor may increase your dose.
- High (over 50 ppb): Significant inflammation. Strong indicator that your current treatment isn’t working. May need a higher steroid dose or biologic therapy.
These aren’t hard rules. A 60-year-old smoker with 30 ppb might not need treatment changes, while a 12-year-old non-smoker with the same level might need a dose increase. Context matters.
How Often Should You Get Tested?
For most people on regular asthma medication, testing every 3 to 4 months is enough to track trends. But if your doctor is adjusting your treatment-like adding a new inhaler or switching to a biologic-you might get tested every 4 to 6 weeks until your levels stabilize.
One study found that using FeNO to guide treatment cuts asthma flare-ups by up to 50%. That’s huge. Fewer ER visits. Fewer missed days of work or school. Less need for oral steroids.
Real Patient Experiences
On asthma forums and community boards, patients are split on FeNO-but the ones who’ve had it consistently report big wins.
A 32-year-old teacher in Leeds said: ‘I thought my inhaler was working fine. But my FeNO was 62 ppb. My doctor said, “You’re still inflamed.” She upped my dose. Three months later, my FeNO dropped to 18. I haven’t had a cough since.’
Others aren’t so lucky. Some say their insurance denied coverage. One user on HealthUnlocked wrote: ‘I had three asthma attacks in six months. My allergist ordered FeNO. Insurance said it wasn’t medically necessary. I paid £120 out of pocket.’
In the UK, NICE guidelines say FeNO should be available in all asthma clinics. In the US, only 58% of commercial insurance plans cover it without restrictions. Access still varies wildly.
The Future of FeNO Testing
FeNO testing is evolving fast. In early 2023, the FDA cleared the first smartphone-connected FeNO device. For $299, patients can now test at home and send results directly to their doctor. This could be a game-changer for people who live far from clinics or have trouble getting appointments.
Researchers are also testing whether FeNO can predict who will respond to expensive biologic drugs. Early data suggests patients with high FeNO levels are far more likely to benefit from drugs like dupilumab. That means fewer people wasting time and money on treatments that won’t work.
But challenges remain. Standardizing readings across different machines is still a problem. And more research is needed on how FeNO behaves in diverse populations-especially in non-white patients, where data is still limited.
Who Should Get FeNO Testing?
FeNO isn’t for everyone. But it’s highly recommended if:
- You’re diagnosed with asthma but your symptoms don’t match your treatment response
- You’re having frequent attacks despite using your inhaler
- Your doctor suspects you have allergic asthma
- You’re being considered for biologic therapy
- You’re a child who can’t do spirometry well
If you’ve had asthma for years and your treatment hasn’t changed, ask your doctor if FeNO testing could help. It’s not magic-but it’s one of the clearest windows we have into what’s happening inside your airways.
Final Thoughts
FeNO testing turns subjective symptoms into objective data. It doesn’t replace your doctor’s judgment-it enhances it. For many, it’s the missing piece that finally explains why their asthma won’t settle down.
It’s not perfect. It won’t catch every kind of asthma. Insurance coverage is still patchy. But when used right, it reduces flare-ups, avoids unnecessary treatments, and helps you take back control of your breathing.
If you’re struggling with asthma and feel like your care is stuck in the dark, ask about FeNO. It might be the simplest test you’ve ever taken-and the most important one you’ll ever get.
What does a high FeNO level mean for asthma?
A high FeNO level (over 25 ppb in adults, over 20 ppb in children) indicates active type 2 airway inflammation, often linked to eosinophilic asthma. This means your airways are swollen and irritated, and you’re likely to respond well to inhaled corticosteroids. High levels also suggest a higher risk of future asthma attacks if left untreated.
Can FeNO testing diagnose asthma on its own?
No. FeNO testing cannot diagnose asthma alone. It’s a biomarker for inflammation, not a diagnostic tool for asthma itself. Many people with asthma have normal FeNO levels, especially if their asthma isn’t driven by type 2 inflammation. Diagnosis requires combining FeNO with symptoms, lung function tests, and medical history.
How accurate is FeNO testing compared to other methods?
FeNO has high specificity (75-95%), meaning it’s very good at confirming inflammation when it’s present. But its sensitivity is lower (52-75%), so it can miss some cases. When combined with spirometry and symptom tracking, accuracy jumps to 76%. It’s more reliable than blood eosinophil counts for detecting airway inflammation and far more practical than sputum analysis.
Why might my FeNO result be normal even if I have asthma?
Not all asthma is caused by type 2 inflammation. If your asthma is triggered by cold air, exercise, stress, or non-allergic factors, your body may not produce excess nitric oxide. Smokers also often have artificially low FeNO levels. In these cases, your asthma is real-but FeNO won’t show it. Other tests and clinical judgment are needed.
How often should I get a FeNO test?
For most people on maintenance therapy, testing every 3 to 4 months is sufficient. If your treatment is being adjusted-like changing your inhaler or starting a biologic-you may need testing every 4 to 6 weeks until your levels stabilize. Your doctor will tailor the schedule based on your symptoms and response to treatment.
Can I do FeNO testing at home?
Yes. Since early 2023, FDA-cleared home FeNO devices like Breathometer Asthma are available for purchase. These connect to smartphones and let you track your levels over time. Results can be shared with your doctor remotely. While convenient, they’re not a replacement for clinical evaluation and should be used alongside professional care.
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