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A Practical Guide to Haemostasis


Global Thrombosis Test [GTT]

Introduction

The Global Thrombosis Test [GTT] is a Point-of-Care test of Platelet function, Thrombin generation and Thrombolytic activity. This in vitro test evaluates the formation of a shear-induced thrombus and its subsequent lysis [Thrombolysis] under physiological conditions using non-anticoagulated blood samples.

For an illustration of the test - see Global Thrombosis Test Home Page.

The GTT evaluates:
  a. The time required to form an occlusive thrombus - termed the Occlusion Time [OT]. 
  b. The time required to lyse the Thrombus - termed the Lysis Time [LT]


Principles & Methodology

The GTT uses non-anticoagulated whole blood and assess the time to create an occlusive thrombus - the Occlusion Time [OT] and then measures the time taken to Lyse the Thrombus. [LT].

a. Whole blood is added to the GTT device and it flows into a conical tube containing 2 ceramic ball bearings.  The conical tube contains a flat segment along the inner wall of the tube and this prevents complete occlusion of the tube by the ball bearings and creates 4 channels that as the platelets pass through, exposes them to high shear stress leading to their activation.
b. In the space between the upper and lower ball bearings, platelet aggregates form and Thrombin [IIa] is generated leading to the formation of occlusive thrombi.  This leads to a reduction and ultimately cessation of the blood flow from the device and the time taken for the cessation of blood flow is measured - generally this is taken as a time period of >10s between drops of blood.  This is the Occlusion Time [OT]. 
3. The thrombus subsequently lyses and blood flow is restored. After a period of time - generally in the region of 300s - blood flow is restored and the time taken for the detection of the first drop of blood is measured and this is known as the Lysis Time [LT].

An optional graphic display of the measurements is possible [termed GTT2 and GTT3] - see Global Thrombosis Test Home Page. 

Reference Ranges

The reference ranges for healthy individuals are shown below together with data from patients on Warfarin or Dabigatran:

Cohort Occlusion Time [OT] Lysis Time
[LT]
Normal Individuals3 524.9s ± 17.0s 1409.8 ± 100.7s
Normal Individuals2 481s ± 24s -
Dabigatran3 784.5 ± 19.3s see below
Dabigatran2 748s ± 29s  
Warfarin3 581.7s ±26.3s see below

3See Reference 3: Patients in this study with Non-valvular Atrial Fibrillation [AF] when switched from Warfarin to Dabigatran showed no change in Lysis Times.
2See Reference 2

What Test Next?

The Global Thrombosis Test [GTT] is a useful test both to investigate platelet function, thrombin generation and thrombolysis in more detail.  Individuals who show an abnormal GTT should be investigated in detail to establish the underlying cause for the abnormalities assuming they are not on anti-platelet drugs or consuming foodstuffs that might potentially interfere with platelet function.