FENO (Fractional Exhaled Nitric Oxide) Test
Fractional Exhaled Nitric Oxide (FENO) is a non-invasive, real-time diagnostic breath test used to measure the concentration of nitric oxide (NO) gas in a patient's exhaled breath, reported in parts per billion (ppb). Nitric oxide is naturally produced by the epithelial cells lining the airways in response to inflammatory cytokines, particularly those associated with eosinophilic (allergic-type) inflammation. When the airways are inflamed — as seen in conditions like allergic asthma or eosinophilic bronchitis — nitric oxide production increases significantly, making elevated FENO levels a reliable biomarker of underlying airway inflammation.
The test is performed by having the patient inhale deeply to full lung capacity and then exhale slowly and steadily through a handheld or tabletop device at a controlled flow rate of approximately 50 mL/second. The device analyzes the exhaled breath in real time and delivers an immediate result, typically within seconds to minutes. No special preparation, bronchodilator, or invasive procedure is required, making it well-tolerated by both adults and children as young as 4 to 5 years of age.
Clinically, FENO serves multiple important purposes. It aids in confirming an eosinophilic component in patients presenting with respiratory symptoms suggestive of asthma, helps differentiate asthma phenotypes (eosinophilic vs. non-eosinophilic), guides the initiation or adjustment of inhaled corticosteroid (ICS) therapy, monitors a patient's response to anti-inflammatory treatment over time, and can detect non-adherence to prescribed corticosteroid inhalers, as FENO levels tend to rise when steroid use is discontinued.
According to American Thoracic Society (ATS) guidelines, a FENO level below 25 ppb in adults (below 20 ppb in children) suggests that eosinophilic inflammation is unlikely, a level between 25 and 50 ppb is considered intermediate and requires interpretation within the broader clinical context, and a level above 50 ppb in adults (above 35 ppb in children) strongly indicates significant eosinophilic airway inflammation. However, FENO should never be used in isolation — it is most valuable when interpreted alongside a patient's clinical history, symptoms, spirometry results, and response to therapy, as various factors such as smoking, recent corticosteroid use, or active respiratory infections can falsely lower FENO readings, while allergic rhinitis or atopic dermatitis can elevate them independent of asthma.