5. Point of Care Tests (and ROTEM)

Electronic glucometers use the same initial oxidation reaction, but the strips also contain an electrode together with potassium ferricyanide which is converted to ferrocyanide by combination with gluconic acid

The commonest device in use in the UK is probably the Hemocue, for which a drop of blood is collected into a microcuvette. Within the cuvette there takes place a modified azide-methaemoglobin reaction Erythrocytes are haemolysed by sodium deoxycholate. This releases haemoglobin which is converted by sodium nitrite to methaemoglobin, which then combines with sodium azide to form azidemethaemoglobin. The absorbance of this compound is measured at 570 and 880 nm (which effectively corrects for any turbidity in the sample), and the device converts this to display as a haemoglobin concentration.

but arterial pH, for example, is measured by a pH electrode which compares the potential developed at the electrode tip with a reference potential, with the voltage being directly proportional to the hydrogen ion (H+) concentration

Devices such as the ‘i-Stat’ have different diagnostic cartridges which can measure sodium, potassium and calcium; urea and creatinine; and lactate. They can also assess the activated clotting and prothombin times (ACT and PT)

commonest and most sophisticated technology that is currently used for point of care coagulation testing. ROTEM system, which is probably the one with which most UK anaesthetists will be familiar.

A cylindrical pin is inserted into the cuvette which holds the blood sample, leaving a 1 mm gap between the side wall and the rotating pin, which rotates alternately clockwise and anti-clockwise. As long as the blood does not clot, this movement is unrestricted, but with increasing clot firmness the rotation of the pin begins to reduce. Pin rotation is inversely proportional to the integrity of the clot. It is detected optically and the decelerating rotations are processed by an integrated computer before generating the familiar ROTEM traces.

normal coagulation pattern, in which clot amplitude is plotted against time. ‘R’ for reaction time., which is the time from the beginning of the test to the start of fibrin formation amplitude at 10 minutes after CT and is an index of clot strength MA’ is maximum amplitude and ‘TMA’ is time to maximum amplitude This is the coagulation phase ‘K’ (kinetics) is the amplification phase of the cell-based model of coagulation and is the time taken to reach a specific level of clot strength, which is an amplitude of 20 mm Alpha, α, is the angle of the slope between ‘CT’ and ‘K’ and is used to assess the rate of clot formation.

The decrease in clot amplitude thereafter is the phase of fibrinolysis, w which can be prolonged

The INTEM tests the intrinsic pathway and provides similar information to the Activated Partial Thromboplastin Time (APTT). The EXTEM tests the extrinsic pathway and provides information similar to the Prothrombin time (PT). The FIBTEM uses an inhibitor of thrombocyte function and by removing the platelet contribution to coagulation allows an assessment of fibrinogen function.

In general, rather than specialist anaesthetic practice, the clinician will usually be using thromboelastography in the context of acute major haemorrhage: surgical, traumatic and obstetric TEG will identify the contribution to a coagulopathy of four main factors, alone or in combination. These are (1) low levels of clotting factors, (2) reduced platelet activity, (3) low concentrations of fibrinogen and (4) accelerated fibrinolysis. The main patterns of ROTEM abnormality are shown in

The ROTEM is not able to give information about every clotting derangement. It does not detect the effect on coagulation of low-molecular-weight heparins or warfarin, and it is also insensitive to the effect of platelet inhibitors such as clopidogrel and aspirin

The focus is not only on the initiation of coagulation, but also assessment of clot strength, dynamics and breakdown, reflecting the complexities of the now widely accepted cellbased model of coagulation

Point-of-care viscoelastic testing (VET) offers near-patient, rapid, global assessment of haemostasis, which is useful in the dynamic setting of acute bleeding.
Viscoelastic testing evaluates clot initiation, strength, dynamics and breakdown.
Newer, more user-friendly devices are leading to the more widespread use of VET.
Viscoelastic testing is of particular benefit in certain subspecialties
The tailored approach that VET facilitates can reduce transfusion requirements

Clot formation Clot strength

After activation of the coagulation system, there is generation of thrombin, which activates platelets and converts fibrinogen to fibrin. trace reaches 2 mm amplitude, the time variables are useful to assess the adequacy of clotting factors and presence of clotting factor inhibitors (e.g. heparin and warfarin). clot-formation kinetics can be gained from the k time (TEG speed of fibrin formation and how well it binds to platelets. Prolongation of the time or a decreased angle can be caused by both hypofibrinogenaemia and low/ dysfunctional platelets