Factors influencing level of hydrogen peroxide in exhaled breath condensate

Hydrogen peroxide (H 2O 2) in exhaled breath condensate (EBC) has been proposed as a marker for oxidative stress in the airways. The aim of the present study was to evaluate the measurement of H 2O 2 in EBC with or without use of a nose clip, and the influence of mouth rinsing, sampling time and storage. An elevated H 2O 2 level was seen during nasal breathing compared to mouth breathing with nose clip (3.4 pmol/s vs. 2.1 pmol/s, p = 0.02). Breathing through the mouth, using a nose clip, was therefore practiced in all experiments. The H 2O 2 levels were increased when mouth rinsing was performed using an acid buffer (1.4 pmol/s vs. 1.9 pmol/s, p = 0.03). 15 min sampling time decreased the H 2O 2 output by almost 50% compared with 2 min sampling time (1.2 vs. 0.6 pmol/s, p = 0.03). When samples were left unattended for 15 min no change in H 2O 2 concentration in the EBC was seen. We found no significant differences in H 2O 2 levels between samples stored for 4 weeks at − 80 °C and samples analysed directly; however, a significant decrease in the levels was seen for samples stored for 4 weeks at − 20 °C. In conclusion, the method of EBC collection and storage plays an important role in reducing variability within and between individuals.