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Editorials
TG much easier to perform even in less specialized labo- ratories. In the early 1990s TG was further modified to include a slow-acting chromogenic substrate that allowed thrombin to be monitored continuously, avoiding the subsampling procedures. More recently, the chromogenic substrate has been replaced by a fluorogenic one, which obviates plasma defibrination and makes TG applicable to platelet-poor or platelet-rich plasma. Currently, TG is designed to operate on a microtiter plate-based fluorime- ter, which, in combination with the computer software, displays TG curves and calculates the parameters illus- trated in Figure 1: the lag-time, peak thrombin concentra- tion, time to reach peak concentration and the endoge- nous thrombin potential (ETP). The lag-time is defined as the time needed for the first amounts of thrombin to be generated; it can be regarded as the conventional plasma clotting time and is expected to decrease or increase in conditions associated with hyper- or hypo-coagulability, respectively. The peak-thrombin defines the highest thrombin concentration that can be obtained under the experimental conditions and is expected to increase or decrease in conditions associated with hyper- or hypo- coagulability, respectively. The time to reach the peak defines the velocity of TG and should be prolonged or shortened in conditions associated with hypo- or hyper- coagulability, respectively. The area under the curve, known as the ETP, represents the net amount of thrombin that the test plasma can generate under the experimental conditions. ETP is considered to be the resultant of the two opposing drivers operating in plasma that favor TG (procoagulants) and thrombin decay (anticoagulants). Accordingly, when performed under standardized condi- tions, TG can be considered a laboratory tool suitable for investigating hypo- and hyper-coagulability much better than the conventional global tests of coagulation, such as the time-honored prothrombin time (PT) and activated partial thromboplastin time (APTT). PT and APTT are in fact static tests in which plasma clots soon after tiny amounts of thrombin (5%) are produced, leaving the
Figure 1. Typical thrombin generation curve with related parameters that can be obtained after in vitro activation of coagulation in plasma. See text for more explanation. ETP: endogenous thrombin potential.
remaining part undetected.6 Furthermore, PT and APTT are responsive to procoagulant factors (which is why they are used to diagnose hemophilia and allied disorders), but they are much less responsive to naturally occurring anti- coagulants (which is why they are normal rather than shortened in patients with congenital deficiency of pro- tein C or antithrombin).
It is therefore unsurprising that over the last two decades, TG attracted the attention of many researchers. Currently, there are 7,446 reports under the term “throm- bin generation” published in PubMed and most deal with the application of TG to investigate hypo- or hyper-coag- ulability. A scrutiny of the reported manuscripts reveals that TG has been applied to five categories: (i) to help elu- cidate the mechanisms of thrombogenesis in clinical con- ditions for which precise knowledge is still poor; (ii) lab- oratory diagnosis of disorders of hemostasis; (iii) moni- toring treatment with pro-hemostatic agents; (iv) moni- toring treatment with antithrombotic drugs; and (v) pre- dicting the risk of recurrence of venous thromboem- bolism. The following paragraphs summarize the current state-of-the-art concerning the application of TG testing.
Elucidation of mechanisms of thrombogenesis
This application has been the most productive in terms of results achieved. For example, the TG procedure when performed as such or modified by the addition of throm- bomodulin has been instrumental in challenging the old dogma of liver cirrhosis as the prototype of acquired hemorrhagic coagulopathies.7,8 TG has also been useful for understanding the mechanisms of thrombogenesis in a variety of clinical conditions associated with increased risk of thrombosis (especially venous thrombosis) such as obesity,9 diabetes,10 and Cushing disease.11
Laboratory diagnosis of hemostatic disorders
The TG procedure has been useful for understanding the contribution of the plasma levels of the individual coagulation factors in determining the bleeding pheno-
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