When a body tissue is injured and begins to
bleed, it initiates a sequence of clotting factor activities -the
coagulation cascade- leading to the formation of a blood clot. This
cascade is comprised of three pathways: extrinsic, intrinsic, and
common.
Two laboratory tests are
used commonly to evaluate coagulation disorders: Prothrombin Time
(PT) which measures the integrity of the extrinsic system as well as
factors common to both systems and Partial Thromboplastin Time
(PTT), which measures the integrity of the intrinsic system and the common
components.
Blood Clot Formation
Clotting is a function of
plasma.It
depends upon the orderly interaction of a group of plasma
proteins (which are sequentially activated following vascular
injury) with some phospholipid (from either damaged tissue or
platelets) and some Ca++. The final stages include
the formation of thrombin, which then converts:
soluble
plasma protein fibrinogen ---------------->insoluble fibrin.
Another
factor converts the fibrin into a cross-linked polymer which
stabilizes the platelet plug and traps RBCs in the meshwork to form the actual blood clot.
Depending on the type of vascular damage or abnormality, clotting can be
initiated and proceed according to two different cascading pathways:
the
intrinsic (initiated by contact with and abnormal/foreign
surface) or the extrinsic (initiated by exposure to tissue
factors). Note that:
the two pathways
converge, so that the final steps are common to the two schemes
although clotting can
be initiated via either the more rapid (15-20 secs) extrinsic scheme
or the slower (2-6 mins) intrinsic scheme,
the division into two pathways is only an artifact ofin vitro testing: the two pathways interconnect
at several levels. In vivo,both
pathways must be activated for effective hemostasis.
both coagulation
pathways, by a series of feedback mechanisms, control their own
activity (e.g. traces of thrombin enhance the activity of earlier
factors in the scheme).
Note
also that, in addition to the coagulation-promoting factors, there
are also substances in blood which inhibit coagulation (e.g., an
anti-thrombin factor which inactivates thrombin).Whether or not blood coagulates depends on the balance that
exists between the two groups of factors (pro-coagulants and anti-coagulants).
Prothrombin time test
The PT test is used to monitor patients taking certain medications as well as to help diagnose clotting disorders.
A sample of the patient's
blood is obtained by venipuncture. The blood is decalcified (by
collecting it into a tube with oxalate or citrate ions) to prevent the
clotting process from starting before the test. The blood cells are
separated from the liquid part of blood (plasma) by centrifugation. The PT test is
performed by adding the patient's plasma to some source of Tissue Factor
(e.g.: a protein, thromboplastin, from homogenized brain tissue) that
converts prothrombin to thrombin. The mixture is then kept in a warm
water bath at 37°C for one to two minutes. Calcium chloride (excess
quantities of ionized calcium) is added to the mixture in order to
counteract the sodium citrate and allow clotting to start. The test is
timed from the addition of the calcium chloride until the plasma clots.
This time is called the Prothrombin Time.
The prothrombin test
specifically evaluates the presence of factors VII, V, and X,
prothrombin, and fibrinogen. A prothrombin time within the 11 -15 second
range (depends on the source of thromboplastin used) indicates that the
patient has normal amounts of the above clotting factors.
A prolonged prothrombin time
indicates a deficiency in any of factors VII, X, V, prothrombin, or
fibrinogen. It may mean that the patient has a vitamin K deficiency
(vitamin K is a co-factor in the synthesis of functional factors II
(prothrombin), VII, IX and X) or a liver disease (the liver is the
site of synthesis of the plasma protein factors). The prothrombin time
of patients receiving a vitamin K-competing coumarin drug such as
warfarin (anticoagulation therapy used in deep venous
thrombophlebitis) will also be prolonged, usually in the range of one
and one half to two times the normal PT time.
Activated Partial Thromboplastin
Time test
The activated partial thromboplastin
time (aPTT) is a test performed to investigate bleeding disorders and to
monitor patients taking an anticlotting drug such as heparin which
inhibits factors X and thrombin, while activating anti-thrombin.
The aPTT test uses blood
which is decalcified to prevent clotting before the test begins. The
plasma is separated by centrifugation. (Ionized) Calcium and activating
substances are added to the plasma to start the intrinsic pathway of the
coagulation cascade. The substances are: kaolin (hydrated aluminum
silicate) and cephalin. Kaolin serves to activate the contact-dependent
Factor XII, and cephalin substitutes for platelet phospholipids. The
partial thromboplastin time is the time it takes for a clot to form,
measured in seconds. Normally, the sample will clot in 35 seconds.
PTT measures the integrity
of the intrinsic system (Factors XII, XI, VIII, IX) and common clotting
pathways.
Increased levels in a person
with a bleeding disorder indicate a clotting factor may be missing or
defective. At this point, further investigation is needed and warrants
the use of sensitive assays for specific coagulation factors. Liver
disease decreases production of factors, increasing the PTT.
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