Due to the obvious nature of the inflammatory and fibrotic processes identified in patients with the post-COVID-19 syndrome, it is conceivable to postulate that fractional exhaled nitric oxide (FeNO) may be a valuable biomarker in such patients. As a result, researchers set out to test the consistency of the notion. They included 68 post-COVID patients who had been hospitalized for persistent clinical symptoms within 2 months of illness start and 29 healthy volunteers as a control group. None of the post-COVID patients developed bronchial asthma or were using corticosteroids. 

Only 19 of 68 post-COVID-19 patients had a FeNO level of more than 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 participants it was 17.46 ppb (95% CI: 15.75 to 19.17). The difference in mean (P=0.053) was not statistically significant. However, males had a higher mean FeNO value than women (20.97 ppb; 95% CI: 16.61 to 25.33 versus 14.36 ppb; 95% CI: 11.11 to 17.61), with a statistically significant difference (P=0.016). Patients who had been treated with systemic corticosteroids because of their COVID-19 had a mean FeNO of 14.89 ppb (95% CI: 10.90 to 18.89), compared to 20.80 ppb (95% CI: 16.56 to 25.04) in those who had not. The findings implied that FeNO measurement was not effective as a biomarker in post-COVID-19 patients. The notion, however, required further testing using more well-planned experiments.