A proposed model to monitor heparin therapy using the concentrated thrombin time which allows standardisation of reagents and improved estimation of heparin concentrations.

The concentrated thrombin time (CTT), a thrombin time performed with a high concentration of thrombin, was evaluated as an alternative to the activated partial thromboplastin time (APTT) for monitoring of heparin therapy. Forty-nine plasmas from patients receiving unfractionated heparin therapy were tested. It was first demonstrated that CTTs using three commercial reagents could be standardised against CTTs performed with a reference reagent, MRC reagent 66/305. For comparison, APTTs were performed on the plasmas. As a benchmark of the degree of heparinisation, the heparin concentration of the plasmas was determined by chromogenic anti-IIa heparin assays, therapeutic range being 0.2-0.4 units/ml. The optimal relationships of the CTTs and APTT with the heparin concentration were established. These were used to predict the heparin concentrations of the plasmas from the results of the APTT, CTT performed with the reference reagent, and transformed CTT performed with each of the three commercial reagents. In predicting the assayed plasma heparin concentrations, the accuracy of the APTT was only 53%, while the CTT was from 78 to 82%. The CTT can be standardised and, subject to results of clinical trials, could provide an improved method of monitoring heparin therapy.