In this cohort, we found similar Vd and CL for vancomycin in patients receiving ECMO when compared to others. These findings are in contrast with neonatal ECMO PK studies in which increased Vd and decreased antibiotic CL were reported [24,25]. Furthermore, it is reasonable to assume that the decreased drug CL observed in neonates was the result of immature hepatic and renal antibiotic metabolic pathways rather than the ECMO circuitry itself [26]. Similarly, the volume of priming fluid for the ECMO system is likely to have a more profound effect on vancomycin Vd in newborns because of a larger priming/blood volume ratio. Thus, **data** in adult patients may differ significantly from those in neonatal studies, underpinning the need for further PK studies that can be used to optimize antibiotic dosing during ECMO in this setting. Finally, ECMO may have different effects on antibiotic PK depending on the class of antibiotic used. In contrast to studies on aminoglycosides, vancomycin Vd was similar in neonates treated with ECMO compared to controls (0.45 ± 0.18 versus 0.39 ± 0.12 L/kg), whereas half-life was significantly shorter during ECMO (8.3 ± 2.2 versus 6.5 versus 2.0 h, P = 0.02) [27].