Activated partial thromboplastin time (aPTT) is the standard for monitoring bivalirudin but demonstrates a nonlinear response at higher drug concentrations. The objective of this study was to assess the relationship between bivalirudin dose and aPTT in patients receiving extracorporeal membrane oxygenation (ECMO) to determine a threshold where aPTT unresponsiveness occurs. Two hundred fourteen adults receiving bivalirudin during ECMO between 2018 and 2022 were included. Piecewise regression in a linear mixed effects model was used to determine a bivalirudin dose threshold of 0.21 mg/kg/hr for aPTT unresponsiveness. For doses of less than 0.21 mg/kg/hr (n = 135), every 0.1 mg/kg/hr dose increase led to an aPTT increase of 11.53 (95% confidence interval [CI] = 9.85–13.20) seconds compared to only a 3.81 (95% CI = 1.55–6.06) seconds increase when dose was greater than or equal to 0.21 mg/kg/hr (n = 79) (p_interaction < 0.001). In multivariable logistic regression, venovenous configuration (odds ratio [OR] = 2.83, 95% CI = 1.38–5.77) and higher fibrinogen concentration (OR = 1.22, 95% CI = 1.05–1.42) were associated with greater odds of unresponsiveness, whereas older age (OR = 0.79, 95% CI = 0.63–0.98), kidney dysfunction (OR = 0.48, 95% CI = 0.25–0.92), and a higher baseline aPTT (OR = 0.89, 95% CI = 0.82–0.97) were associated with lower odds. Alternative methods are necessary to ascertain bivalirudin’s hemostatic impact when doses exceed 0.21 mg/kg/hr during ECMO.