Study objective: To determine the correlation between activated clotting time (ACT) or activated partial thromboplastin time (aPTT) and plasma heparin concentration.
Design: Two-phase prospective study.
Setting: University-affiliated community hospital.
Patients: Thirty patients receiving continuous-infusion intravenous heparin.
Interventions: Measurement of ACT, aPTT and plasma heparin concentrations.
Measurements and main results: Linear and log linear correlations were determined between clotting time tests and heparin concentrations. Linear correlations yielded r values of 0.58 for ACT (p=0.008) and 0.89 for aPTT (p=0.0001). Log linear correlations yielded r values of 0.60 for ACT (p=0.005) and 0.88 for aPTT (p=0.0001). A decision analysis was performed to determine possible consequences of dosage adjustments based on either test in relationship to the decision based on plasma heparin concentration. The decision analysis based on ACT disagreed with corresponding decisions based on plasma heparin concentration in 15 of 30 patients; 13 disagreements may have increased the risk of bleeding, and the other 2 may have increased the risk of thrombosis. Decisions based on aPTT disagreed with corresponding decisions based on plasma heparin concentration in 13 of 30 patients; 2 disagreements may have increased the risk of bleeding, and the other 11 may have increased the risk of thrombosis.
Conclusion: There are significant statistical linear and log linear correlations between both clotting time tests and plasma heparin concentrations, with aPTT showing stronger correlation than ACT. However, decisions regarding heparin therapy based on ACT may increase a patient's risk of bleeding, whereas decisions based on aPTT may increase the risk of thrombus progression or rethrombosis.