Classic and dedicated NOAC unique testing methods, seen in Table 5, have been developed for laboratory use over recent years. These testing methods include:
- Dilute thrombin time (dTT) and ecarin-based assays correlate best with dabigatran's therapeutic range. In addition, certified plasma calibrators and controls with target-specific values for dabigatran are commercially available.
- Ecarin clotting time (ECT), a test based on direct prothrombin activation, provides high sensitivity and specificity for detecting dabigatran. It is not performed routinely, and the turnaround time may be problematic. The test results will be prolonged in the presence of direct thrombin inhibitors. In addition, the test will be abnormal if prothrombin levels are reduced due to inherited prothrombin deficiencies and if the fibrinogen levels are reduced, as in hypo- or dysfibrinogenemia.
- Ecarin chromogenic assay (ECA) is independent of any variability in fibrinogen levels and prothrombin times and is not influenced by heparin or vitamin K antagonists such as warfarin.
- The anti-Xa activity (Factor Xa) can be calibrated by apixaban, edoxaban, and rivaroxaban, the preferred assay. Factor Xa inhibitors are increasingly prescribed for outpatients, and patients transitioning to unfractionated heparin (UFH) for hospital admission are monitored by an anti-Xa assay. This assay is suitable for patients transitioning off apixaban or rivaroxaban.
| Assay | Description |
| Anti-factor IIa | The anti-FIIa is a chromogenic assay that demonstrates plasma's ability to inhibit thrombin (IIa). Suitable amounts of a patient’s plasma and excess amounts of thrombin are mixed. The residual thrombin is measured using a synthetic chromogenic substrate specific for thrombin. The smaller the residual thrombin, the greater the inhibitor concentration. Plasma calibrators for dabigatran are commercially available. |
| Anti-factor Xa (Anti-Xa) | This chromogenic assay demonstrates the ability of plasma to inhibit FXa. Suitable amounts of a patient’s plasma and excess amounts of FXa are mixed. The residual FXa for thrombin is measured using a synthetic chromogenic substrate. The smaller the residual FXa, the greater the inhibitor concentration. Plasma calibrators for apixaban, edoxaban, and rivaroxaban are commercially available to test for each named drug. |
| Dilute thrombin time (dTT) | The dTT has the same characteristics as its parent TT, but the patient’s plasma is diluted in pooled normal plasma in the former. This dilution is mainly aimed at decreasing the excessive responsiveness of the TT to thrombin inhibitors (especially dabigatran). The dTT can be easily set up in an ordinary coagulometer. It can be carried out even in emergency settings, with little expertise, and results can be expressed as ng/mL by interpolating the patient’s clotting time from a dose-response curve constructed by testing plasma calibrators at fixed concentrations of the inhibitor being tested. |
| Ecarin Clotting Time (ECT) | The conventional ECT involves adding small amounts of various concentrations of ecarin, a snake venom, to plasma and measuring the time until clot formation results from converting fibrinogen to fibrin. ecarin solution is a direct thrombin inhibitor. Meizothrombin, an intermediate of prothrombin activation, is generated by ecarin. When meizothrombin is inhibited, clot formation is prolonged. The ECT relies on the prothrombin and fibrinogen in a patient’s specimen. The test can convert prothrombin into thrombin, skipping the coagulation cascade upstream. |
| Ecarin Chromogenic Assay (ECA) | The ECA is similar to the ECT, but the ecarin solution contains human prothrombin, and the generation of meizothrombin and its subsequent inhibition by direct thrombin inhibitors is measured using a specific chromogenic substrate. Ecarin is used as the specific prothrombin-activating principle. The cleavage of a chromogenic substrate by meizothrombin is inhibited in a concentration-dependent fashion by direct thrombin inhibitors. ECA is more sensitive than the ECT and a thrombin-based chromogenic assay. |
