Case of the Month #45 Major Obstetric Haemorrhage - Part 2

• As blood clots and the clot strengthens, its viscosity and elasticity increases. 

• The main differences between ROTEM and TEG are the apparatus, chemical coagulation activators used and physical principles employed. 

• The same measurements are named differently for the two techniques. 

• However, as different reagents used, results are not directly comparable between the two. 

ROTEM and TEG are techniques implemented on different proprietary machines. In the U.K. commonly used machines for ROTEM are the ROTEM DeltaÒ and ROTEM SigmaÒ and for TEG, the TEG6SÒ and TEG5000Ò. Deaton’s review (Ref. 1) provides more detail. 

ROTEM (Rotational Thromboelastometry) 

A ROTEM instrument comprises four separate channels for different, simultaneous assays, much like TEG (see below). 

• The blood sample is inserted into a cuvette and a cylindrical pin is immersed into it.  

• The pin is rotated by a spring alternating to the left and the right. 

• As a clot forms and strengthens, it exerts increasing resistance on rotation of the pin, the degree of rotation of the pin is inversely proportional to clot strength. Degree of rotation is measured using a light-emitting diode shone through the sample onto a detector (optical detection) to produce a ROTEM curve as well as numerical parameters. 

• ROTEM is more resistant to mechanical shock, which may be an advantage in the clinical setting. 

The ROTEM instrument performs several tests simultaneously on citrated blood samples, using different activators or inhibitors. Standard parameters are measured in each of these tests. Tests of importance in MOH- 

• The EXTEM test- a screening test for the extrinsic coagulation pathway. Haemostasis is activated by Tissue Factor. The result is influenced by extrinsic coagulation factors, platelets and fibrinogen. It is used for therapeutic decisions regarding the administration of fresh frozen plasma, coagulation factors, fibrinogen or platelets. 

• The FIBTEM test eliminates the platelet contribution to clot formation by inhibiting them irreversibly with cytochalasin D. Coagulation is activated as in EXTEM. It allows the detection of fibrinogen deficiency as the resulting clot formed relies solely on fibrin formation and polymerisation. FIBTEM results correlate well, in many cases, with the Clauss fibrinogen assay. 

TEG (Specifically TEG6S, which is the most widely used TEG machine in the UK) 

Blood is inserted into a cartridge where it is drawn into four different channels containing the different required reagents.  

• Clot properties are analysed using the resonance method - blood in the sample is exposed to a fixed external vibration frequency (20-500 Hz).  

• The change in the harmonic vertical motion of the blood meniscus (increased resonance frequency as blood changes from liquid to a gel like state) is measured using optical detection. 

• The analyser displays these variations in resonant frequency as tracings of clot dynamics (clot strength vs time). 

Four tests are performed simultaneously using different reagents. The standard TEG parameters are measured in each of these tests:- 

• CK (Citrated Kaolin) - Kaolin is used as the standard activator of clot formation in this test. This is the standard TEG. 

• CKH (Citrated Kaolin with Heparinase) - assesses the effect of Heparin. Heparinase neutralises any heparin in the blood sample the allow assessment of the patients underlying haemostatic profile. 

• CRT (Citrated RapidTEG) - tissue factor (TF) is used as an activator in addition to Kaolin to provide a rapid assessment of clot strength. The assessment of clot initiation is omitted. 

• CFF (Citrated Functional Fibrinogen) - the Functional Fibrinogen reagent allows assessment of fibrinogen contribution to clot strength; a functional measurement of the patient’s fibrinogen level.