TABLE 1

Factors other than asthma that raise or lower the fractional concentration of exhaled nitric oxide (FeNO)

Factors that increase FeNO	Factors that decrease FeNO
Chronic rhinosinusitis, nasal polyposis, or both: FeNO is increased in patients with allergic rhinitis or nasal polyposis even in the absence of a concomitant asthma diagnosis. Patients with allergic rhinitis have higher FeNO levels than patients with nonallergic rhinitis. Perennial sensitization leads to higher FeNO than seasonal allergens. Atopy: Acute exposure to allergens can increase FeNO by up to 50%. Rhinovirus respiratory infections can increase FeNO by 50% to 150% through inducible nitric oxide synthase upregulation. However, not all viral infections increase FeNO—respiratory syncytial virus and influenza reduce FeNO, and bacterial infections of the respiratory tract do not affect FeNO. Nitrate-containing foods such as beetroot can increase FeNO levels by 20% to 60%. Air pollution (particulate matter and ozone), possibly because of oxidative potential, even in the absence of asthma. Male sex: association is consistent after adjustment for height. HIV infection is associated independently with raised FeNO after adjusting for use of airway medication, blood eosinophil counts, and immunoglobulin E. Increasing age: Effect is greater in people over age 60 or 64, but data conflict in adults and are more consistent in children. Increased height, because of increases in airway mucosal surface area.	Cigarette smoking decreases FeNO by 40% to 60%. Magnitude of reduction correlates with the cumulative lifetime cigarette consumption. The effect of cigarette smoking may be reversible, but has not been reported consistently. Smokers with asthma have higher FeNO levels than healthy smokers. In clinical practice, look for intraindividual changes. Inhaled steroid use. FeNO generally is sensitive to inhaled steroids and therefore will be low in most patients who are adherent to treatment. Alcohol ingestion—Avoid before testing. Spirometry may cause a marginal decrease. The effect is of minor importance in clinical practice, but one preferably should measure FeNO before spirometry. Certain drugs Leukotriene receptor antagonists Prostaglandins: inhaled prostaglandin E2 and iloprost downregulate inducible nitric oxide synthase expression. Physical exercise—Avoid strenuous exercise before testing. UNCERTAIN EFFECT Menstrual cycle FeNO levels may vary, but this is not a consistent finding.

Factors that increase FeNO

Factors that decrease FeNO

Chronic rhinosinusitis, nasal polyposis, or both: FeNO is increased in patients with allergic rhinitis or nasal polyposis even in the absence of a concomitant asthma diagnosis. Patients with allergic rhinitis have higher FeNO levels than patients with nonallergic rhinitis. Perennial sensitization leads to higher FeNO than seasonal allergens.
Atopy: Acute exposure to allergens can increase FeNO by up to 50%.
Rhinovirus respiratory infections can increase FeNO by 50% to 150% through inducible nitric oxide synthase upregulation. However, not all viral infections increase FeNO—respiratory syncytial virus and influenza reduce FeNO, and bacterial infections of the respiratory tract do not affect FeNO.
Nitrate-containing foods such as beetroot can increase FeNO levels by 20% to 60%.
Air pollution (particulate matter and ozone), possibly because of oxidative potential, even in the absence of asthma.
Male sex: association is consistent after adjustment for height.
HIV infection is associated independently with raised FeNO after adjusting for use of airway medication, blood eosinophil counts, and immunoglobulin E.
Increasing age: Effect is greater in people over age 60 or 64, but data conflict in adults and are more consistent in children.
Increased height, because of increases in airway mucosal surface area.

Cigarette smoking decreases FeNO by 40% to 60%. Magnitude of reduction correlates with the cumulative lifetime cigarette consumption. The effect of cigarette smoking may be reversible, but has not been reported consistently. Smokers with asthma have higher FeNO levels than healthy smokers. In clinical practice, look for intraindividual changes.
Inhaled steroid use. FeNO generally is sensitive to inhaled steroids and therefore will be low in most patients who are adherent to treatment.
Alcohol ingestion—Avoid before testing.
Spirometry may cause a marginal decrease. The effect is of minor importance in clinical practice, but one preferably should measure FeNO before spirometry.
Certain drugs

Leukotriene receptor antagonists
Prostaglandins: inhaled prostaglandin E2 and iloprost downregulate inducible nitric oxide synthase expression.

Physical exercise—Avoid strenuous exercise before testing.
UNCERTAIN EFFECT
Menstrual cycle
FeNO levels may vary, but this is not a consistent finding.

Reprinted from Chest 2022; 161(4):906-917, Rupani H, Kent BD. Using fractional exhaled nitric oxide measurement in clinical asthma management, with permission from Elsevier.