Which gas is used to evaluate a patient for asthma?

• A. CO2
• B. Helium
• C. Exhaled NO
• D. O2

Answer: (C)

Measuring exhaled nitric oxide shows the level of airway inflammation, which is a hallmark of asthma. In asthma, inflammatory signals increase production of nitric oxide in the airway lining, so more NO appears in the breath. This noninvasive test helps identify eosinophilic airway inflammation and guides therapy, since higher exhaled NO often indicates better responsiveness to inhaled corticosteroids and can be used to monitor control over time. By contrast, CO2 reflects ventilation, O2 reflects oxygenation, and helium is mainly used as a tracer gas for certain lung volume tests, not for assessing asthma inflammation. So the gas used to evaluate asthma is exhaled nitric oxide.