Reference equations for FeNO measurements in healthy individuals

We aimed to create reference equations for FeNO, from a sample of healthy individuals with no respiratory diseases. We selected healthy adults from a population-based sample, included in the national study ICAR – Control and Burden of Asthma and Rhinitis (PTDC/SAU-SAP/119192/2010, funded by Fundacao Ciencia e Tecnologia). Healthy was defined as absence of asthma or COPD, with FEV1/FVC ≥ 0.7. Atopy was defined by positive skin prick tests and smoking status with exhaled CO levels. FeNO was measured with NIOX MINO (Aerocrine AB, Sweden). Reference equations were modeled using stepwise multiple linear regression for the dependent variable ln(FeNO). The independent variables included in the models were gender, age, standing height, atopy and smoking status. Models with and without atopy were computed. We9ve included 147 healthy adults (59% females) with mean (SD) age of 44.8 (16) years being 20% active smokers and 31% atopic. Median (min-max) FeNO was 15.0 (5-57) ppb. Significant variables included were height, age, atopy and smoking status, but not gender. The best reference equation was ln (FeNO) = -0.737 + height (cm) * 0.019 + Age (years) * 0.007 + Atopy (1-Yes, 0-No) * 0.235 + Smoking status (1-Smoker, 0-Never-smoker or Ex-smoker) * -0.518, with an R2= 0.259. The reference equation without atopy was ln (FeNO) = -0.238 + height (cm) * 0.0197+ Age (years) * 0.006 + Smoking status (1-Smoker, 0- Never-smoker or Ex-smoker) * -0.465 with an R2= 0.192. The highest proportion of explained FeNO variability in healthy adults was given by the reference equation with atopy (the highest to the best of our knowledge). For easier inclusion in clinical practice, we also propose a reference equation without atopy evaluation.