There are certain factors that can influence the amount of nitric oxide that a patient exhales. Active eosinophilic inflammation associated with allergic asthma is one of them, but there are other factors that could lead this levels to be raised which could lead to a false positive result, including:

• Tall height
• Male sex
• Rhinitis
• Eosinophilic bronchitis
• High nitrate diet
• Rhinovirus infection
• Low BMI

Equally, there are factors that can dampen the exhaled nitric oxide, leading to a false negative FeNO result, including:

• Cigarette smoke
• Intense physical activity
• Young children