viscoelastic haemostatic assays (VHA)

 

I wouldn’t consider this a ‘full’ Haembase page, but I wanted to have something on viscoelastic haemostatic assays on the site that hopefully covers a few principles of the tests, so that the terminology is recognisable when you encounter it at work. Practical-Haemostasis and the 2018 BSH guideline are good sources for greater detail.

 

Intro

TEG and ROTEM are two commercially available viscoelastic haemostatic assays. Their intended function is to provide point-of-care information on global haemostasis, including clot formation / stability / lysis. The need for this has arisen out of the turnaround time associated with standard laboratory tests.

 

Methods

TEG – Thromboelastography – whole blood is added to a small moving cup. A stationary pin suspended into the cup will also start to move once a clot forms and sticks the two together. Torsion of the pin is used to produce a graphical result.

ROTEM – Rotational Thromboelastometry – whole blood is added to a small stationary cup. A pin suspended in the cup moves until a clot forms and sticks the two together. A light change is recorded and converted into a graphical result.

In addition, different reagents can be added to modify the tests, e.g. the HEPTEM cartridge for ROTEM contains heparinase to neutralise the effects of heparin.

 

TEG/ROTEM Traces

Results are displayed graphically. Different sections of the graph represent different stages in the haemostatic pathway and so abnormal graph patterns can help to direct use of blood components.

The terminology is different for TEG and ROTEM but the information provided is similar.

Using TEG as an example:

Reaction (R) Time – latency from start of test to first fibrin formation. If prolonged it may indicate the presence of anticoagulants or factor deficiencies

Kinetics (K) Time – time for a set amount of fibrin to form. If prolonged it may indicate fibrinogen deficiency.

Alpha Angle – reflects speed of clot formation. If flattened it may indicate fibrinogen deficiency.

Max Amplitude (MA) – reflects strength of the fibrin clot. If reduced it may indicate impairment platelet function / thrombocytopenia

VHA copy.jpg

A30/A60 – amplitude at 30 and 60 minutes. If reduced it may indicate increased fibrinolysis

VHA.jpg

 

Basic responses based on above


Prolonged R time / CT —> Low clotting factors —> FFP

Reduced MA / MCF with normal lab FGN —> Low platelets —> platelet transfusions

Reduced MA / MCF with low lab FGN —> Cryoprecipitate

Reduced A30/A60 —> Increased fibrinolysis —> Tranexamic Acid


example interpretation in major haemorrhage

From BSH MH guideline, itself based on iTACTIC trial design


Evidence Base 

These statements reflect local opinions, based on available evidence. The technology was developed in 1948 and despite there still being no firm indication for its use, the point-of-care nature of the test continues to generate interest in finding a role for it in clinical practice.

 

Where viscoelastic testing may have a role:

  • ?Cardiac surgery – Possibly, although 2017 meta-analysis did not show improvement in clinical outcomes

  • ?Post-Partum Haemorrhage – Probably, mainly due to testing logistics (2014 review)

  • ?Liver Transplant surgery - May decrease blood loss and transfusion requirements

  • ?Trauma – iTACTIC Trial Jan 2021 - 400 patients randomised. No difference in no. patients alive / free of massive transfusion at 24 hours in those managed with viscoelastic assays versus conventional lab coag tests.

 

(Opinion will vary!) Viscoelastic testing probably does not have a role in:

  • Predicting pre-op bleeding risk

  • Predicting post-op thrombosis

  • General Surgery intra-op

  • The ITU ‘Sick, Oozy’ patient