Assays

Different assays are used to help show different components of haemostasis.

INTEM – Contact activation
Result influenced by coagulation factors, platelets, fibronogen and heparin. LMWH activity detected at high concentrations.

EXTEM – Tissue factor activation plus heparinase
Screening test primarily for extrinsic haemostasis system.
Not affected by heparin as reagent contains heparinase, therefore the result is only affected by coagulation factors, platelets and fibrinogen.

HEPTEM – Contact activation plus heparinase
Essentially INTEM without effects of heparin.
Allows detection of coagulation deficiencies even whilst on heparin. The difference between the INTEM CT and the HEPTEM CT can confirm the presence and effect of heparin.

FIBTEM – Tissue factor activation plus platelet inhibition
Essentially EXTEM without effects of platelets. Eliminates activation of platelets through cytochasin D which inhibits the action of actin forming the platelet cytoskeleton into a stellate shape.
Allows detection of fibrinogen deficiencies, but also fibrin polymerisation deficiencies which may not be reliably detected with normal clotting tests.

APTEM – Tissue factor activation plus aprotinin/tranexamic acid
Fibrinolysis inhibited, therefore a significant improvement of clot stability compared with EXTEM suggests a need for antifibrinolytics. If there is no significant improvement this would suggest a requirement for other clotting products.

Method and Interpretation

Rotational Thromboelastometry

A good background from Life in the Fast Lane (mainly describing TEG)

Clinical relevance and short literature review from St. Emlyn’s


Viscoelastic method for measuring haemostasis of whole blood

Method

300 microlitres of citrated blood placed in disposable cuvette.
A disposable pin is placed in the blood, this is attached to a thin shaft and spring which oscillates slowly. The change in tension as the blood clots is detected by optical sensors.
Different reagents are used depending on the test required.

TEG (thromboelastography) similar, but it is the cup that rotates. ROTEM is less sensitive to mechanical shocks and movement than TEG which is an advantage.

Interpretation

ROTEM
CT – Clotting time
Latency from time of adding reagents to start of clot formation.
Prolonged CT may be a result of coagulation factor deficiencies or heparin. The contribution of heparin can be assessed by comparing the INTEM CT with the EXTEM CT.

CFT – Clot Formation Time and Alpha angle
CFT is the time until a clot firmness of 20mm is reached. The alpha angle is the tangent between the CT and CFT points.
These parameters denote the speed of solid clot formation – primarily influenced by platelet function but also by fibrinogen and coagulation factors.
Prolonged CFT: Low platelets or poor function, low fibrinogen or fibrin polymerisation disorders
Shortened CFT: Hypercoaguability

MCF – Maximum Clot Firmness
The greatest vertical amplitude of the trace.
Reflects the absolute strength of the fibrin and platelet clot.
Low MCF indicates decreased platelet count or function, decreased fibrinogen or disorders of fibrin polymerisation, or low activity of factor XIII.

A5, 10, 15 or 20
Clot amplitude after a certain number of minutes. Allows projection of likely MCF.

LI30 (Lysis Index after 30 minutes) and ML (Maximum Lysis)
LI30 is the percentage or remaining clot stability in relation to the MCF after 30 minutes. Can also be calculated to give LI45 or LI60.
ML describes the percentage of lost clot stability relative to MCF at any selected point in time or when the test is stopped.
A low LIx or high ML is indicative of hyperfibrinolysis.