Extracorporeal membrane oxygenation (ECMO) provides critical support to patients in severe cardiac and respiratory failure, but it requires anticoagulation to prevent complications like bleeding and thrombosis. Heparin, the primary anticoagulant utilized, is monitored by activated partial thromboplastin time (aPTT) and anti-Factor Xa (AntiXa) levels. Discordance between the two assays complicates its titration and the impact on patient outcomes is not well-established. This study examines the prevalence of discordance, its impact on heparin dosing, and the association of bleeding, thrombosis, ICU-free days, and mortality in pediatric ECMO patients.