5. Point of Care Tests (and ROTEM) Flashcards
Blood Glucose
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
Haemoglobin
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.
Arterial Blood Gases
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
Electrolytes and Others
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)
Thromboelastography (TEG
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.
Principle of measurement.
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 TEG trace
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
Pathways
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.
Abnormal patterns.
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
Limitations.
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
Focus of TEG
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
Key pts
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
Viscoelastic test interpretation
Clot formation
Clot strength
Clot formation
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
