Introduction: Anticoagulation monitoring during ECMO is challenging, particularly when activated partial thromboplastin time (aPTT) and anti-factor Xa (anti-Xa) are discordant. We aimed to identify factors associated with discordance on ECMO, and to describe heparin adjustments after discordant results.
Methods: Retrospective study of ECMO runs at a tertiary hospital from 7/2019 to 6/2025. Paired aPTT/anti-Xa results drawn ≤2h apart were analyzed. Concordance was defined as both assays low, within goal, or high; discordance as opposing directions. Heparin adjustments within 6h of each pair were classified as appropriate for aPTT (decreased when high, increased when low, not changed when within goal), anti-Xa, both, or inappropriate (not appropriate for either aPTT or anti-Xa). Univariate analyses were conducted using Kruskal–Wallis and chi-squared tests, and a backward stepwise Wald multivariable logistic regression was used to identify independent predictors of discordance.
Results: We analyzed 94 ECMO runs (23 pediatric, 71 adult; 64% VA, 36% VV). Mortality was 47% (29% pediatric, 54% adult). Median heparin dose was 12 U/kg/hr (IQR 6-24). Median aPTT was 64 sec (IQR 47-97) and anti-Xa 0.26 IU/mL (IQR 0.11-0.48). Across 614 paired results, correlation was poor (R²=0.39), with discordance occurring in 53% of pairs. Discordance was associated with VA ECMO (p=0.01) and coagulopathy: lower fibrinogen (p < 0.0001), elevated INR (p < 0.001), and prolonged PT (p < 0.001). However, only adult age (p < 0.001) and VA ECMO (p < 0.001) were independently associated with discordance. When assays were concordant (n=291), 58% of heparin changes were inappropriate. When discordant (n=323), 47% of heparin changes were appropriate for anti-Xa, 33% for aPTT, 19% were inappropriate. In discordant assays, higher aPTT was independently associated with more inappropriate heparin changes (adjB=1.04; 95% CI, 1.01-1.06; p< 0.001).
Conclusions: Discordance between aPTT and anti-Xa was frequent and associated with coagulopathy and VA ECMO. When adjusting heparin during discordance, clinicians more often followed anti-Xa, and inappropriate heparin adjustment was associated with higher aPTT. Discordant assays make heparin management on ECMO highly complex.
