Assessing heparin neutralization following cardiac surgery: sensitivity of thrombin time-based assays versus protamine titration methods

Adequate assessment of heparin neutralization following cardiac surgery is critical in reducing the patient’s exposure to protamine. Both excessive protamine and residual heparin have been associated with postoperative bleeding and poor patient recovery. The activated clotting time (ACT) is the preferred intraoperative heparin monitor, while both protamine titration (i.e. a protamine-containing ACT) and thrombin time methods have been used to detect circulating residual heparin after protamine administration.

Following initial protamine dosing using the protamine response test (PRT), postoperative monitoring was employed in the operating room prior to transport of the patient to intensive care. Two point-of-care assays, the thrombin time (TT) and the protamine dose assay (PDA), were evaluated to determine their relative heparin sensitivity and their usefulness to quantitate protamine dose.

The PDA, which is based on the ACT, was shown in laboratory and clinical studies to detect residual heparin above 0.25 units/ml and to quantify additional minidoses of protamine (as low as 25 mg) required to obtain complete heparin neutralization. Differential evaluation of the TT and heparin neutralized thrombin time (HNTT) was shown in laboratory studies to be more sensitive to small amounts of residual heparin than the ACT. Clinical evaluations confirmed that additional protamine is required in approximately 12% of cardiac surgical cases managed using the PRT system. Both the PDA and TT/HNTT provided useful postoperative assessment of the adequacy of heparin neutralization. The TT/HNTT had slightly improved heparin sensitivity even in the presence of significant fibrinogen loss. These point-of-care assays provide the opportunity to optimize heparin and protamine management in the cardiac surgery patient.