Comparison of the aPTT With Alternative Tests for Monitoring Direct Thrombin Inhibitors in Patient Samples

OBJECTIVES: The activated partial thromboplastin time (aPTT) test has been used for years to monitor parenteral direct thrombin inhibitors (DTIs) and unfractionated heparin. Because the aPTT correlates poorly with unfractionated heparin levels, we hypothesized that the aPTT may not be the best test for monitoring parenteral DTIs. METHODS: Using 235 excess plasma specimens from 82 adult patients receiving treatment with DTIs (argatroban, bivalirudin, or dabigatran), we compared the aPTT with the ecarin chromogenic assay (ECA), the dilute thrombin time (dTT) test, and the prothrombinase-induced clotting time (PiCT) test. RESULTS: The aPTT correlated poorly with each of the other tests in both bivalirudin- and argatroban-containing samples (r(2) = 0.04-0.23). The ECA and dTT exhibited the best correlations (r(2) = 0.66-0.93). Intermediate correlations were seen when the results of the PiCT were plotted against the dTT or ECA (r(2) = 0.46-0.58). Nineteen specimens obtained from six patients receiving dabigatran showed a good correlation between the dTT and the ECA (r(2) = 0.92). CONCLUSIONS: The aPTT does not correlate well with other tests that might be used to monitor parental DTI administration. Further studies are needed to evaluate the clinical usefulness of alternative tests and their correlation with clinical outcomes.