PHYSICAL METHODS TO PREVENT BLOOD CLOTTING
Coagulation of blood is
postponed or prevented by the following physical methods:
COLD
Reducing the temperature
to about 5°C postpones the coagulation of blood.
COLLECTING BLOOD IN A
CONTAINER WITH SMOOTH SURFACE
Collecting the blood
in a container with smooth surface like a silicon-coated container
prevents clotting. The smooth surface inhibits the activation of factor XII and
platelets. So, the formation of prothrombin activator is prevented.
PROCOAGULANTS
Procoagulants or hemostatic
agents are the substances which accelerate the process of blood coagulation. Procoagulants
are:
THROMBIN
Thrombin is sprayed
upon the bleeding surface to arrest bleeding by hastening blood clotting.
SNAKE VENOM
Venom of some snakes
(vipers, cobras and rattle snakes) contains proteolytic enzymes which enhance
blood clotting by
activating the clotting factors.
EXTRACTS OF LUNGS AND
THYMUS
Extract obtained from
the lungs and thymus has thromboplastin, which causes rapid blood coagulation.
SODIUM OR CALCIUM
ALGINATE
Sosium or calcium alginate
substances enhance blood clotting process by activating the Hageman factor.
OXIDIZED CELLULOSE
Oxidized cellulose
causes clotting of blood by activating the Hageman factor.
TESTS FOR BLOOD CLOTTING
Blood clotting tests
are used to diagnose blood disorders. Some tests are also used to monitor the
patients treated with anticoagulant drugs such as heparin and warfarin.
1. Bleeding time
2. Clotting time
3. Prothrombin time
4. Partial prothrombin
time
5. International
normalized ratio
6. Thrombin time.
BLEEDING TIME
Bleeding time (BT) is
the time interval from oozing of blood after a cut or injury till arrest of
bleeding. Usually, it is determined by Duke method using blotting paper or filter
paper method. Its normal duration is 3 to 6 minutes. It is prolonged in
purpura.
CLOTTING TIME
Clotting time (CT) is
the time interval from oozing of blood after a cut or injury till the formation
of clot. It is
usually determined by
capillary tube method. Its normal duration is 3 to 8 minutes. It is prolonged
in hemophilia.
PROTHROMBIN TIME
Prothrombin time (PT)
is the time taken by blood to clot after adding tissue thromboplastin to it.
Blood is
collected and oxalated
so that, the calcium is precipitated and prothrombin is not converted into
thrombin. Thus, the blood clotting is prevented. Then a large quantity of
tissue thromboplastin with calcium is added to this blood. Calcium nullifies the
effect of oxalate. The tissue thromboplastin activates prothrombin and blood clotting
occurs. During this procedure, the time taken by blood to clot after adding
tissue thromboplastin is determined. Prothrombin time indicates the total
quantity of prothrombin present in the blood. Normal duration of prothrombin
time is 10 to 12 seconds. It is prolonged in deficiency of prothrombin and other
factors like factors I, V, VII and X. However, it is normal in hemophilia.
PARTIAL PROTHROMBIN
TIME OR ACTIVATED PROTHROMBIN TIME
Partial prothrombin
time (PPT) is the time taken for the blood to clot after adding an activator
such as phospholipid, along with calcium to it. It is also called activated partial
prothrombin time (APTT). This test is useful in monitoring the patients taking
anticoagulant drugs. It is carried out by observing clotting time after adding
phospholipid, a surface
activator and calcium to a patient’s plasma. Phospholipid
serves as platelet substitute. Commonly
used surface activator is kaolin.
Normal duration of
partial prothrombin time is 30 to 45 seconds. It is prolonged in heparin or warfarin
therapy (since heparin and warfarin inhibit clotting) and deficiency or
inhibition of factors II, V, VIII, IX, X, XI and XII.
INTERNATIONAL
NORMALIZED RATIO
International normalized
ratio (INR) is the rating of a patient’s prothrombin time when compared to an
average. It measures
extrinsic clotting pathway system. INR is useful in monitoring impact of
anticoagulant drugs such as warfarin and to adjust the dosage of anticoagulants.
Patients with atrial fibrillation are usually treated with warfarin to protect
against blood clot, which may cause strokes. These patients should have regular
blood tests to know their INR in order to adjust warfarin
dosage. Blood takes
longer time to clot if INR is higher. Normal INR is about 1. In patients taking
anticoagulant therapy for atrial fibrillation, INR should be between 2 and 3.
For patients with heart valve disorders, INR should be between 3 and 4. But,
INR greater than 4 indicates that blood is clotting too slowly and there is a risk
of uncontrolled blood clotting.
THROMBIN TIME
Thrombin time (TT) is the
time taken for the blood to clot after adding thrombin to it. It is done to
investigate the presence of heparin in plasma or to detect fibrinogen
abnormalities. This test involves observation of clotting time after adding
thrombin to patient’s plasma. Normal duration of thrombin time is 12 to 20 seconds.
It is prolonged in heparin therapy and during dysfibrinogenimia (abnormal
function of fibrinogen with normal fibrinogen level).
Types and causes of purpura
Purpura is classified into
three types depending upon the causes:
i. Thrombocytopenic
purpura
Thrombocytopenic purpura
is due to the deficiency of platelets (thrombocytopenia). In bone marrow
disease, platelet production is affected leading to the deficiency of
platelets.
ii. Idiopathic
thrombocytopenic purpura
Purpura due to some
unknown cause is called idiopathic thrombocytopenic purpura. It is believed
that platelet count decreases due to the development of antibodies against
platelets, which occurs after blood transfusion.
iii. Thrombasthenic
purpura
Thrombasthenic purpura
is due to structural or functional abnormality of platelets. However, the
platelet count is normal. It is characterized by normal clotting time, normal or
prolonged bleeding time but defective clot retraction.
3. von Willebrand
Disease
von Willebrand disease
is a bleeding disorder, characterized by excess bleeding even with a mild
injury.
It is due to
deficiency of von Willebrand factor, which is a protein secreted by endothelium
of damaged blood vessels and platelets. This protein is responsible for adherence
of platelets to endothelium of blood vessels during hemostasis after an injury.
It is also responsible for the survival and maintenance of factor VIII in
plasma. Deficiency of von Willebrand factor suppresses platelet adhesion. It also
causes deficiency of factor VIII. This results in excess bleeding, which
resembles the bleeding that occurs during platelet dysfunction or hemophilia.
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