Practical-Haemostasis.com

A Practical Guide to Laboratory Haemostasis

 

Thromboelastography [TE] and Rotational Elastometry [ROTEM]



Introduction

Thromboelastography [TE] was first described by Hartert in 1948. Thromboelastography® (TEG®) and Thromboelastometry (ROTEM®) provide global information on the dynamics of clot development, stabilisation and dissolution that reflect in vivo haemostasis. Although TE has not been subjected to the same evaluation processes as conventional haemostatic tests, its use as a POCT monitor in complex major surgery has been shown to significantly reduce the use of blood component therapy and overall blood loss.

Principles

The figures below shows the traces obtained using the TEG and ROTEM whilst the table summarises the descriptive data reported with both the TEG and ROTEM.
1. Thromboelastogram [TEG®



2. Thromboelastometry (ROTEM®)

Variables measured by the TEG® and ROTEM®

Variable TEG® ROTEM®
Measurement period - Reaction Time [RT]
Time from start to when the waveform reaches 2mm above baseline R Clotting Time [CT]
The time from 2mm above baseline to 20mm above baseline K Clot Formation Time [CFT]
Alpha angle [°] α [slope between R and K] α [angle of tangent at 2mm amplitude]
Maximum angle - CRF
Maximum strength Maximal Amplitude [MA] Maximal Clot Firmness [MCF]
Time to Maximum strength - MCF-t
Amplitude at a specific time A30, A60 A5, A10...
Clot elasticity G MCE
Maximum lysis - CLF
Clot Lysis[CL] at a specific time [minutes] CL30, CL60 LY30, LY45, LY60
Time to lysis 2mm from MA CLT [10% difference from MCF]


Method

Whole blood is added to a heated cuvette at 37°C . Within the cup a pin is suspended connected to a detector system - this is a torsion wire in the case of TEG and an optical detector in the case of the ROTEM device. The cup and pin move relative to each other through an angle of 4°45'. The movement is initiated from either the cup (TEG) or the pin (ROTEM). As the blood clots, fibrin strands forms between the cup and pin and rotation of the cup is transmitted to pin in the case of the the TEG or impedes or rotation of the pin in the case of the ROTEM. This is detected and a trace generated - see above. Links to the manufacturers Websites are provided in the References section and these provide more information on the principles of the TEG and ROTEM.

Although originally the TEG involved fresh whole non-anticoagulated blood, both the TEG and the ROTEM commonly employ citrated whole blood that is re-calcified to initiate coagulation. It is also common to use an activator as this standardises the test and in addition speeds up the rate at which clotting takes place and hence the rate at which a result is generated. The TEG and ROTEM devices have a number of separate channels allowing a number of samples to be run simultaneously or sequentially.

Modifications of the TEG and ROTEM Devices

Modification Interpretation
Tissue Factor The use of an activator when undertaking thromboelastography is generally recommended to standardise the initiation of the clotting process.
Tissue Factor activation of the TEG enables the maximal amplitude (MA) to be established within 10 minutes but will result in significant shortening of the reaction time (R value) and as a result much of this latter information is lost.
R Time and Heparinase The R value in a native TEG is sensitive to trace amounts of heparin and endogenously released heparan sulphate.
The use of a heparinase-coated reaction cuvette for the TEG will demonstrate any heparin present in the sample or in the patient and enables assessment of haemostasis in patients who are fully anticoagulated with heparin e.g. on CPB.
Tissue factor/kaolin activated TEG and the ACT By incorporating both tissue factor and kaolin into the TEG cuvette, the TEG approximates to the Activated Clotting Time [ACT.]
Fibrinogen and platelet function The MA is primarily a reflection of clot strength and is affected by changes in fibrinogen, platelet count and function. The MA is established either from a native sample with no activator or from the combined Tissue Factor/kaolin activated TEG. There is a strong linear correlation between the log platelet count and MA.
Abciximab is a potent platelet GpIIb/IIIa inhibitor and an abciximab-modified TEG can help to discriminate between hypofibrinogenemia and platelet dysfunction as a cause of decreased MA.
Fibrinolysis The degree of fibrinolysis can be established from either a native sample with no activator, from the Tissue Factor activator or the combined Tissue Factor/kaolin activated TEG.
Hyperfibrinolysis is increasingly recognised as a cause of perioperative microvascular bleeding and is readily detected by analysing the clot lysis index on the TEG or ROTEM .
The ability to detect and determine the severity of fibrinolysis avoids empirical or inappropriate anti-fibrinolytic therapy.
Mathematical derivations of changes in elastic modulus derived from the amplitude have been used to quantify the extent of fibrinolysis in clinical and laboratory settings, as well as to guide antifibrinolytic therapy
Hypercoagulability The TEG may be helpful in screening for hypercoagulable states. TEG analysis of patients with a history of thromboembolic complications showed shorter R values and accelerated clot propagation compared to healthy reference subjects

 

Rotational Thromboelastometry (ROTEM®)

The ROTEM analyser provides a trace similar to that of the TEG with related parameters including clotting time (CT) and maximum clot firmness (MCF). Additional tests include:

Test Interpretation
INTEM Contains phospholipid and ellagic acid as activators and provides information similar to that of the APTT
EXTEM Contains Tissue Factor as an activator and provides information similar to that of the PT
HEPTEM Contains lyophilised heparinase for neutralising heparin
APTEM Contains aprotinin for inhibiting fibrinolysis
FIBTEM Utilises cytochalasin D, a platelet inhibitor which blocks the platelet contribution to clot formation, allowing qualitative analysis of the functional fibrinogen component.
ECATEM Contains Ecarin and so is similar to the Ecarin Clotting Time. This makes it very sensitive to presence of direct thrombin inhibitors.

First Derivative: The rate and amount of thrombin generation is considered to be predictive of the risk of thrombosis or haemorrhage. The TEG system provides the 'thrombus generation velocity curve' - the first derivative of the standard TEG waveform. Although the graph appears similar to that of a thrombin generation test, it provides different information. In addition to providing information on thrombus generation it can provide similar information on the rate of thrombus lysis.
Similar curves can be generated using the ROTEM device.

Interpretation

The websites for the ROTEM and TEG provide a comprehensive explanation of the waveforms generated in various disorders.

Reference Ranges

It is important to remember that the shape of the waveform is often as useful as the results of individual values. Reference ranges will also change depending upon whether a test uses an activator e.g. Tissue Factor or not i.e. a native blood sample.

 

What Test Next

On the basis of the TE traces, additional tests may be suggested. However, the TEG and ROTEM machines are often used to both investigate disorders of haemostasis and guide blood product replacement without any additional tests and they may, therefore, be performed in isolation.

 

Data Interpretation

Click HERE to go to the Data Interpretation Exercises.