FeNO Levels: What They Mean for Asthma and Airway Inflammation

When your doctor talks about FeNO levels, fractional exhaled nitric oxide, a non-invasive test that measures inflammation in your airways. It's not just a number—it's a direct look at what’s happening inside your lungs, especially if you have asthma. High FeNO means your airways are swollen and irritated, often from allergic triggers or uncontrolled inflammation. Low FeNO? That usually means your treatment is working—or that something else is causing your symptoms.

FeNO testing is different from spirometry. While spirometry checks how well you can breathe out, FeNO tells you why you might be struggling. It’s used by doctors to decide if inhaled steroids are the right move, whether your asthma is allergic in nature, or if you’re actually responding to your meds. People with allergic asthma often have elevated FeNO levels because their immune system overreacts to pollen, dust, or pet dander, releasing nitric oxide as part of the inflammation process. This test helps cut through guesswork. It’s not for everyone, but if you’re on asthma meds and still wheezing, or if your doctor suspects your symptoms aren’t asthma at all, FeNO gives clarity.

Related concepts like airway inflammation, the underlying process that narrows breathing passages and causes coughing, tightness, or wheezing, and asthma diagnosis, the process of confirming asthma using clinical history, lung tests, and biomarkers like FeNO are tied directly to FeNO results. You can’t diagnose asthma with FeNO alone, but it’s a powerful tool when combined with symptoms and breathing tests. It’s also useful for spotting non-adherence—many patients think they’re using their inhaler properly, but if FeNO stays high, the meds aren’t reaching where they need to.

FeNO levels are measured with a simple breath test. You breathe into a handheld device at a steady pace, and it analyzes the air you exhale. No needles, no fasting, no discomfort. Results come back in minutes. Normal levels vary by age and smoking status, but generally, above 25 ppb in adults suggests active airway inflammation. For kids, the cutoff is lower. If your level is 50 ppb or higher, your doctor is likely to recommend or adjust inhaled corticosteroids. If it drops after treatment, that’s a good sign your meds are working.

FeNO also helps rule out other conditions. Chronic cough? High FeNO points to eosinophilic asthma. Low FeNO might mean vocal cord dysfunction or GERD. It’s not a magic bullet, but it’s one of the few tools that gives real-time insight into lung inflammation without imaging or blood draws. You’ll find posts here that dig into how FeNO fits into asthma management, what causes false highs or lows, and how it compares to other biomarkers like eosinophils in blood or sputum. You’ll also see how it connects to real-world issues—like why some people don’t respond to steroids, or how environmental triggers raise FeNO even when symptoms feel mild.