Can FeNO be a biomarker in the post-COVID-19 patients monitoring?

The nature of the inflammatory and fibrotic processes found in patients with post-COVID-19 syndrome makes it possible to speculate that in such patients fractional exhaled nitric oxide (FeNO) may be a useful biomarker. Consequently, we set out to verify the consistency of this hypothesis. We consecutively enrolled 68 post-COVID patients after being hospitalized for persistent clinical manifestations within 2 months from disease onset and 29 healthy volunteers as control group. None of post-COVID patients had bronchial asthma or were being treated with a corticosteroid. Only 19 out of 68 post-COVID-19 patients reported a FeNO value > 25 ppb. The mean FeNO value in post-COVID-19 patients was 18.55 ppb (95% CI: 15.50 to 21.58), while in healthy subjects it was 17.46 ppb (95% CI: 15.75 to 19.17). The mean difference was not statistically significant (P = 0.053). However, the mean FeNO value of post-COVID-19 patients was higher in men than in women (20.97 ppb; 95% CI: 16.61 to 25.33 vs 14.36 ppb; 95% CI: 11.11 to 17.61) with a difference between the two sexes that was statistically significant (P = 0.016). Mean FeNO was 14.89 ppb (95% CI: 10.90 to 18.89) in patients who had been treated with systemic corticosteroids because of their COVID-19, and 20.80 ppb (95% CI: 16.56 to 25.04) in those who had not taken them, with a difference that was statistically significant (P = 0.043). The data generated in this study suggest that measurement of FeNO is not useful as a biomarker in post-COVID-19 patient. However, this hypothesis needs solid validation with additional specifically designed studies.