The Fractional Exhaled Nitric Oxide (FeNO) test is a straightforward, non-invasive breath test designed to measure the degree of inflammation within a person’s airways. It quantifies the amount of nitric oxide (NO) present in the air exhaled from the lungs. This procedure provides medical professionals with an objective measurement of underlying airway inflammation, which is often difficult to assess through physical symptoms alone. The test delivers immediate results, supporting the diagnosis and management of certain respiratory conditions.
What is FeNO and What Does the Test Measure?
FeNO stands for Fractional Exhaled Nitric Oxide. Nitric oxide is a gaseous biomarker naturally produced by cells lining the airways of the lungs. When the airways become inflamed, these cells, specifically the epithelial cells, often produce significantly higher amounts of this gas.
The elevated nitric oxide production is a direct response to eosinophilic inflammation, which is common in allergic diseases. Eosinophils are a type of white blood cell involved in allergic and asthma responses. Measuring the FeNO level quantifies the intensity of this eosinophil-driven inflammation in the lungs.
The Simple Procedure of Taking the Test
The FeNO test uses a portable, handheld monitor, typically found in a doctor’s office or specialty clinic. The patient inhales deeply through the device’s mouthpiece, usually taking in nitric oxide-free air. The test involves exhaling slowly and steadily into the device for about ten seconds.
The exhalation must maintain a consistent pressure and flow rate to ensure the measurement accurately reflects the gas concentration in the lower airways. Unlike tests such as spirometry, the patient is asked to exhale gently rather than forcefully. Because the procedure is quick and painless, it is well-tolerated by both adults and children. To ensure the most accurate reading, patients are usually asked to avoid smoking, vigorous exercise, or consuming foods high in nitrates for a few hours before the test.
Why Doctors Use the FeNO Test
Doctors use the FeNO test primarily as a supportive tool in the diagnosis and management of asthma. It helps differentiate between asthma driven by eosinophilic inflammation, which responds well to inhaled corticosteroids, and other non-eosinophilic forms. A high FeNO score suggests a patient is more likely to benefit from steroid-based anti-inflammatory therapy.
The test also serves as a valuable tool for monitoring treatment effectiveness. When a patient begins treatment involving inhaled corticosteroids (ICS), a significant drop in their FeNO level indicates the medication is effectively reducing airway inflammation. If a patient’s FeNO level remains high or increases, it may suggest the current dose is insufficient, adherence issues, or poorly controlled inflammation. Regular FeNO monitoring helps doctors personalize drug dosage, aiming for the lowest effective dose necessary to suppress airway inflammation.
Understanding Your FeNO Results
FeNO results are reported in parts per billion (ppb), representing the concentration of nitric oxide in the exhaled air. These measurements are categorized into three ranges, though adult and child ranges differ slightly. A low FeNO level, typically below 25 ppb for adults, suggests minimal eosinophilic inflammation and makes an allergic form of asthma less likely.
Intermediate results, generally between 25 and 50 ppb in adults, require careful interpretation alongside a patient’s symptoms and medical history. A high FeNO level (greater than 50 ppb for adults) strongly indicates significant eosinophilic airway inflammation and suggests the patient is likely to respond positively to anti-inflammatory treatments. The FeNO result is just one piece of the puzzle, and a doctor must combine it with physical exams and other lung function tests to create a complete clinical picture.