MPO and acute respiratory distress syndrome: The levels of MPO-DNA complex (Fig. 1A) and cell-free DNA (cf-DNA) (Fig. 1B) in ARDS patients were significantly higher than those in healthy controls, and there is a strong correlation between the level of cf-DNA and PaO2/FiO2 (r = −0.88, P < 0.01) (Fig. 1C).