Activated clotting time (ACT) is widely used to monitor intraoperative anticoagulation with unfractionated heparin (UFH) during vascular surgery, but the most informative ACT-derived metric and the optimal approach for weight-based UFH dosing remain uncertain. The recent introduction of new point-of-care ACT analyzers has raised questions regarding the consistency of ACT response. This study aimed to assess the relationship between UFH dose and ACT response and to identify the most informative ACT-derived metric and the most appropriate body weight metric for dose normalization.

Single-center retrospective observational study.

Tertiary university hospital.

Adult patients undergoing vascular surgery requiring intraoperative UFH administration.

UFH was administered intraoperatively. ACT was measured before and 5 minutes after the first UFH bolus using the GEM Hemochron 100 system.

Anticoagulation response was assessed using post-UFH ACT and the ACT ratio (post/baseline). UFH dose was normalized to actual, adjusted, and ideal body weight. Associations were evaluated using Pearson correlation and multivariable linear regression adjusted for baseline ACT and sex. Weight-normalized UFH dose was significantly associated with both outcomes. Correlations were stronger for the ACT ratio than for post-UFH ACT. In multivariable models, UFH dose remained independently associated with both outcomes. Model performance was similar for actual and adjusted body weight, whereas ideal body weight did not improve fit.

Following implementation of a new point-of-care ACT analyzer, the expected UFH dose-response relationship was preserved. The ACT ratio showed stronger associations than absolute ACT values. Actual body weight appears sufficient for routine UFH dosing.