Haemostasis system. Vessel-platelets hemostasis.
Coagulative hemostasis.
Anticlotting mechanisms. Fibrinolysis.
Hemostasis is the physiologic system, which support the blood in the fluid condition and prevent bloodless. Hemostasis system vital necessary and functionally connect with the cardiovascular, breathing, endocrine and other systems.
Functional-structure components of hemostasis system. The components of hemostasis are wall of the vessels, blood cells – platelets, erythrocytes, leucocytes, enzymes and nonenzymes components of plasma – clotting and anticlotting substances, fibrinolysis components of hemostasis.
There are 2 kinds of hemostasis. They are vessel–platelets (primary) and coagulative (secondary) hemostasis. Primary hemostasis activity begin the first after the destroyed of vessels. Secondary hemostasis add after that in case the primary hemostasis do not stopped the bloodless.
Vessel–platelets hemostasis (or primary hemostasis include in clotting first of all after the destroyed the safe of vessel wall.)
Activation of platelets (To do their function platelets must to activate. In the case of activation the platelets form psevdopodias, change the form. There are 2 groups of activators – the first from platelets and second from another cells, plasma. The outside platelets factors, which are produce in plasma, other cell besides platelets – Villibrandt factor, ADP, epinephrine and norepinephrine. The platelets factors, which are produce by platelets serotonin, ADP, thromboxan A2.)
Properties and function of platelets (Quantity of platelets is 180-320 G/L. Diameter of platelets is 1-4 micrometers, thickness – 0,5-0,75 micrometers. They are the little peace of megacariocytes cytoplasm (from one megacariocytes may develop few hundred of platelets). Platelets circulated in blood from 5 to 11 days and than destroyed in liver, lungs, spleen by the cells of macrophagal system. Function of platelets are:
1. hemostatic function – platelets produce substances, which are secure the hemostasis. Its produce 12 platelets factors
1 – proaccelerin,
2- factor, which are increase the speed of development the fibronogen in fibrin,
3 – platelets thromboplastin,
4 – antiheparinic factor,
5 – factor which promote aggregation of platelets,
6 – thrompostenin,
7 – antifibrinolizin,
8 – serotonin,
9 – fibrinstabilising factor,
10 – factor which activate profibrinolisin,
11 – inhibitor of thromboplastin,
12 – antilighting factor.
Other classiffication of platelets factors. The platelets have been shown to release seven factors that help in blood clotting.
Platelet factor 1 – It has been found to be the same as factor V.
Platelet factor 2 – It is the thromboplastic substance.
Platelet factor 3 – It is a phospholipoprotein, which behaves as thromboplastin.
Platelet factor 4 – It has heparieutralizing properties.
Platelet factor 5 – It acts as fibrinogen.
Platelet factor 6 –It acts as anti-fibrinolysin.
Platelet factor 7- It is the platelet co-thromboplaslin.
2. Angiotrophic function – provide trophic of endotheliocytes of vessel wall, support structure and functions of microvessels. These function is realize by adgesion of platelets to endotheliocytes and injection the enzymes into the endotheliocytes. For one day near 35 G/L platelets do this function.
3. Transport function – transfer the enzymes, ADP, serotonin and other.
4. Phagocytosis function – the contain of platelets help to kill viruses and antigens bodies.
5. Regeneratory function – platelets have the growth factor, which help to grow the endothelial and muscles cells which are present in the vessel wall.)
Stages of vessel–platelets hemostasis
1. Shorting spasm of the vessels – vascular spasm duration to 1 minute is caused by catecholamins and other enzymes. Diameter of vessels decrease on ½-⅓. Mechanism of it development determine by secretion of serotonin and thromboxan A2 from platelets and epinephrine from ending of sympathetic nerves.
2. Adgesion of platelets – activation of platelets and stick it to the place of defect in vessel wall.
3. Reverse aggregation of platelets – the thromb which are formed may make way for plasma.
4. Unreverse aggregation of platelets – the thromb which are formed caot may make way for plasma.
5. Retraction of platelets plug – decrease the size of plug, pack down the plug.
Investigation of vessel–platelets hemostasis
1. Calculation of the platelets quantity 180-320 G/L.
2. Determination of duration of capillary bleeding after Duke’s method – to 3 minute iorm.
3. Sample of fragility of capillars – to 10 petechias iorm in a round with diameter
STAGES IN PLATELET DEVELOPMENT
1. Megakaryoblast – It is the first cell which can be morphologically characterized and identified to form platelets. It arises, as other blood cells, from the non-specific pluripotent stem cell (CPU). It is 15 to 50 μm in diameter and contains a large oval or kidney-shaped nucleus with several nucleoli. The cytoplasm is scanty and intensely basophilic and has no granules. Mitosis may be seen.
2. Pro-megakaryocyte – It is 20 to 80 μm in diameter. The nucleus is oval or irregular in shape; cytoplasm is more abundant and contains fine bluish granules.
3. Megakaryocyte – This cell is so called because it possesses up to 64 N chromosomes instead of the normal 2 N chromosomes (46) of ordinary somatic cell. This poly-ploidy is brought about by a sequence of events termed as endoreduplication in which nuclear material replicates without cytoplasmic division. It has a diameter of 35 to 160 μm and shows two distinct stages. In the first in which the cell is termed as megakaryocyte without granular platelets, the nucleus is either indented or has multiple lobulations. The cytoplasm is finely and diffusely granular. In the second stage, the cell cytoplasm becomes still more increased in amount and the cell is termed as megakaryocyte with granular platelets or meta-megakaryocyte. The platelets differentiate at the periphery of the cell and when the cell dies, these break off from its cytoplasm to enter the blood stream.
The life span of the platelets is about 10 days in man. The spleen stores them as well as mainly sequestrates the damaged or effete (worn-out by age) platelets. Normally 80 % of the total platelets are in circulation and the remaining 20 % are in the spleen. If the spleen becomes enlarged, then it can store more platelets and this ratio may even be reversed. This may obviously result in a decreased blood platelet count, i.e. thrombocytopenia.
COAGULATION OR CLOTTING OF THE BLOOD
Blood has two remarkable properties; it remains fluid while in blood vessels and clots when it is shed. Both these properties are essential for normal life. The blood contains substances or factors, which favor coagulation (pro-coagulants); it also has substances, which are anti-coagulants. An optimum balance of these two opposing factors is essential for a normal life. The coagulation process actually is the property of plasma though it is commonly termed as clotting of blood. Although a complete understanding of the mode of action of the procoagulants is still not possible, but it can be said that clotting is produced by a complex series of reactions. Once initiated, the whole process proceeds like a chain reaction until clotting is complete. Three methods, which have been much employed for understanding the clotting mechanism are given below.
1. Appropriate techniques by which the clotting process can be stopped at any required stage followed by its re-start.
2. Studies on patients suffering from hemorrhagic diseases.
3. Experimental studies in animals; hemophilia occurs in dogs which have been used for research in this disease.
Blood Clotting Factors – The various factors, which are known to take part in the clotting process in various theories of blood coagulation are given below. These factors have been assigned numbers, which arc written in Roman pattern.
I. Fibrinogen
II. Prothrombin (Thrombin is factor II-a)
III. Thromboplaslin. This is the name given to a substance capable of converting prolhrombin to thrombin. It is present in tissues in an active form, the tissue thromboplastin, which is also called the tissue pro-coagulant material.
IV. Calcium ions.
V. Labile factor, Pro-accelerin, Accelerator or Ac globulin.
VI. It has been found to be the same as factor V; it is now obsolete.
VII. Stable factor, Pro-convertin, Auto-prothrombin-I.
VIII. Anti-hemophilic globulin (AHIG, Platelet cofactor-I. Anti-hemophilic factor A (AHF-A). This is the original compound called factor VIII. However, factor VIII has been found to have three subtypes. The original factor VIII (AHF-A) is now called factor VIII-C, C signifying coagulant action. The other two subtypes are factor VIII V.W. (also called Von-Willebrand protein) and factor VIII R.Ag (protein precipitated by specific rabbit anliserum).
IX. Christmas factor, Plasma thromboplastin component (PTC), Platelet co-factor-II, Auto-prolhrombin-II, Anti-hemophilic factor B.
X. Stuart-Prower factor.
XI. Plasma thromboplastin antecedent (PTA), Anti-hemophilic factor-C, Rosenthal factor.
XII. Hageman’s factor, Contact factor, Glass factor.
XIII. Fibrin stabilizing factor, Laki-Lorand factor, Transglutaminase, Pre-fibrinoligase.
In addition, the following factors are also associated with blood clotting process.
i) Von-Willebrand factor or the platelet adhesion factor. It is needed for platelet adhesion as well as for activity of factor VIII-C; it is called factor VIII V.W.
ii) Fitzgerald factor; it is the same as high mol. wt. kininogcn.
iii) Fletcher factor; it is pre-kallikrein.
Theories of Blood Coagulation
I. Classical theory of Morowitz (1905-1906) – Blood clotting was considered to take place in two stages.
(i) In the first stage prolhrombin is converted to thrombin by the enzyme prothrombinase, Ca2+ being necessary for this reaction.
(ii) In the second stage the thrombin acts as an enzyme on fibrinogen and converts it to fibrin.
II. Cascade or waterfall theory – For many decades, Morowitz’s theory was accepted. But great developments in this field resulted in several new theories, one of which is called cascade or waterfall theory because it involves a cascade of events; it is described below.
There are two systems of clotting, intrinsic and extrinsic, which converge upon what is called the final common pathway.
1. Intrinsic or the blood system – This system is called so, because all factors taking part in the process are derived from the blood itself and it can take place in pure blood (blood not contaminated with tissue juice) kept in a test tube. It is also called contact system because the process starts when blood comes in contact with a foreign surface, e.g. vascular sub-endothelial collagen or even glass. This process takes place in the following six stages. In the first five of these stages limited proteolysis converts an inactive factor to its active form. Each of these steps is regulated by plasma and cellular co-factors and Ca2+. The inactive and active blood clotting factors are distinguished by writing and a respectively after the factor.
Stage No. 1. Three plasma proteins, i.e. Hageman factor (XII), high mol. wt. kininogen and pre-kallikrein form a complex with vascular subendolhelial collagen. Factor XH-i becomes activated to Xll-a, which acceleates the conversion of pre-kallikrein to kallikrein which then accelerates the conversion of still more XII-i to XII-a.
Satge No. 2. Factor XII-a converts factor XI-i to XI-a.
Stage No. 3. Factor XI-a converts factor IX-i to IX-a.
Stage No. 4. Factor IX-a in the presence of factor VIII C, Ca2+ a platelet membrane lipoprotein (platelet factor 3) converts X-i to X-a.
Stage No. 5. Several factors take part in the conversion of prothrombin to thrombin. These include factor X-a, factor V-a, Ca2+ and phospholipids. Although the conversion of prothrombin to thrombin can take place on a phospholipid-rich surface, but it is accelerated several thousand-fold on the surface of activated platelets.
Stage No. 6. Conversion of fibrinogen to fibrin is brought about thrombin by the following mechanism. Fibrinogen is a symmetrical dimer; each half of its molecule has the following structure:
i) Alpha polypeptide joined to a short A-fibrinopeptide.
ii) Beta polypeptide joined to a short B-fibrinopeplide.
iii) Gamma polypeptide.
Fibrinogen can thus be represented by the structure, [Alpha(A), beta(B), gamma]2. Thrombin catalyzes the breakdown of fibrinogen in such a way that a part of the molecule separates leaving behind a fibrin monomer.
[alpha(A), beta(B), gamma]2 → [alpha, beta, gamma]2 (Fibrin monomer) + 2[fibrinopeptide A + B]
2. The extrinsic or the tissue system – This is called so because it needs the presence of tissue juice that contains tissue thromboplastin which is not present in blood. The tissue thromboplastin in the presence of factor VII and Ca2+ activates factor X-i to X-a. Subsequent reactions are the same as described under the intrinsic system and, being common to both the intrinsic and extrinsic systems, are designated as the final common pathway. Because the extrinsic system involves fewer steps than the intrinsic system, therefore it proceeds faster than the latter. For this reason, while the intrinsic system takes 2 to 6 minutes for clotting to take place, the extrinsic system takes as little as 15 seconds to do that.
III. Seeger’s hypothesis – This concept basically differs from the cascade theory in that prothrombin and factors VII, IX and X are considered to occur in a single molecular system and not separate from each other. This common molecule is believed to release all these clotting factors during clotting process. A common characteristic of all these clotting factors is that all of them require the presence of vitamin K for their biosynthesis. Factors VII, IX and X are designated by Seeger as autoprothrombin I, II and III respectively. The corresponding active forms of these factors arc called autoprothrombin A, B and C. There are serious objections to this hypothesis as various studies have shown that all these factors arc different and are quite distinct from each other.
Properties of Various Factors Participating in Blood Coagulation
Fibrinogen – It occurs in the plasma in a concentration of
Prothrombin – It is the proenzyme, the precursor of thrombin. It contains 2 to 10 % carbohydrate in its molecule and has a mol. wt. of 69,000. Its plasma concentration is 10 to 15 mg per 100 ml.
Thromboplastin – It implies an activity which converts prothrombin to thrombin. All body tissues have this activity and therefore it is termed as tissue or intrinsic thromboplaslin. The brain, lung, placenta and testes are especially rich in it. It is a complex of phospholipids, lipoproteins and cholesterol. Tissue extracts, if injected intravenously, can cause widespread clotting of blood. However, tissue thromboplaslin is not active as such but it needs Ca2+ and factor VII for its activation which normally arc present in blood. Russel viper venom has a strong thromboplaslin activity and is used for slopping bleeding from superficial areas by its local application in diseases like hemophilia.
Calcium – Ca in ionic form, Ca2+, is essential for clotting of blood and it acts at many stages. Ca ions serve to form complexes with lipids, which take part in blood clotting. In health or disease blood has always sufficient Ca2+ for this purpose. In other words, a Ca2+ deficiency is never a cause of a prolonged clotting time in man.
Factor V – It is activated by small amount of thrombin which in turn leads to a greater formation of thrombin. But an excess of thrombin destroys it and causes its disappearance from serum. It is unstable in the citrated plasma. Its congenital deficiency is the cause of parahemophilia, a mild bleeding disorder.
Factor VII – It is stable on storage. It acts as co-thromboplastin in the working of extrinsic system of blood cloning. Its congenital deficiency has been seen very rarely. It has up to 50 % carbohydrate in its molecule.
Factor VIII-C – It is also called platelet cofactor-I and anti-hemophilic globulin. Its deficiency causes the classical hemophilia (now called hemophilia A). Hemophilia is discussed later in detail. This factor is readily inactivated in vitro.
Factor IX – It is also called Christmas factor because its deficiency was first demonstrated in a patient with the surname Christmas whose bleeding disease was named Christmas disease. This disease is also called hemophilia B.
Factor X – It is an alpha globulin present both in scrum and plasma. I deficiency is seen in both sexes equally as a congenital defect.
Factor XI – Its deficiency causes hemophilia C, which is a mild bleeding disease.
Factor XII – It is activated by surface contact and according to the cascade theory, this process initiates the series of reactions leading to blood clotting. Blood deficient in this factor docs not clot in lest tube, i.e. in vitro. If blood taken from a vein (without letting it being mixed with tissue juice) is placed in a lest tube lined with silicone, it does not clot; this is because the silicone layer is smooth and unwettable and does not permit the activation of factor XII for the same reason. Blood also clots much more slowly when placed in polythene tubes as compared to glass tubes. The deficiency of this factor is seen in persons with Hageman’s trait, but they do not generally show bleeding tendency. Its additional roles arc the activation of fibrinolytic system and the plasma kinin syslem. It is activated by contact with glass, negatively charged surfaces, collagen fibers, unbroken skin, sebum, long chain fatty acids, uric acid, fibrin, elastin and homocysteine.
Factor XIII – It is the enzyme transglutaminase, whose function has already been discussed. Persons with congenital deficiency of this factor have bleeding tendencies and poor wound healing. Their blood clots all right, but the clot, unlike the normal clot, is unstable and can be solubilized in 5 molar urea or 1 % monochloracetic acid solution.
THE FIBRINOLYTIC SYSTEM
A proteolytic enzyme, fibrinolysin or plasmin, acts on fibrinogen and fibrin causing their breakdown or dissolution by converting them into smaller peptides which are soluble; these peptides are called fibrin/fibrinogen degradative products (FDP) as well as fibrin/fibrinogen split products (PSP). The FDP so produced have several important actions, which oppose hemostasis; in other words they promote bleeding.
1. They inhibit binding of fibrinogen to platelets and inhibit aggregation of platelets.
2. They inhibit thrombin formation and also destroy any thrombin formed.
3. They prevent fibrin polymerization.
Formation of plasmin – Normally plasmin occurs in blood plasma as its inactive precursor called pro-fibrinolysin or plasminogen. The conversion of plasminogen to plasmin involves cleavage of a single arginine-valinc bond and is catalyzed by an activator which itself occurs in an inactive form namely pro-activator. The pro-activator is changed to activator by enzymes called pre-kallikrein activators, which are derived from breakdown of active factor XII; in other words, the activators of pre-kallikrein are the fragments of active factor XII.
1. Fibrinolysokinase – It is present in plasma, many tissues and in many secretions, e.g. saliva, milk and tears.
2. Bacterial enzymes – These include staphylokinase and streptokinase which are produced by staphylococci and streptococci respectively. Streptokinase is used by intravascular infusion for the clearance of thrombo-embolism.
3. Urokinase – It is present in urine and is being used in therapeutics to accelerate fibrinolysis. It is formed in the kidney; its commercial source is the fetal kidney tissue culture. The urinary plasmin produced under its influence is believed to have a role in keeping the renal tubules patent by preventing the deposition of fibrin within them. Fibrin may be derived from the fibrinogen present in trace amount in the renal tubular fluid. In the same way, plasmin activity present in milk, tears and semen serves to keep the corresponding duct systems patent.
4. Cytokinase – It is present in tissue cells, WBCs and platelets.
5. Hormones – Growth hormone and thyroid stimulating hormone (TSH).
6. Miscellaneous – Exercise, adrenaline, hypoxia, histamine, bacterial pyrogens, ischemia, shock, tissue damage and chloroform.
Factors inhibiting plasmin formation – The plasmin activation is inhibited by the following factors.
1. Epsilon aminocaproic acid (Trasylol and tranexamic acid) – It is used in therapeutics to inhibit plasmin system.
2. ACTHl – (Adrcnocorlicotropic hormone).
Control of plasmin activity – A trace of plasmin activity is present even iormal plasma. When excessive amount of plasmin is formed, it brings about the following two types of effects:
(i) The plasmin gets bound to fibrin clots and breaks them to small fragments which arc cleared by macrophages. This is the physiological action.
(ii) A part of plasmin remains free in the plasma and enters circulation. However, its action is rapidly neutralised by the anti-plasmin. But if plasmin level of the plasma is too high, then it causes digestion of fibrinogen and also of factors V, VIII and XII. This is the pathological action. If this action is very marked, hypofibrinogenemia and deficiencies of factors V, VIII and XII result and lead to hemorrhage.
FACTORS PREVENTING BLOOD CLOTTING WITHIN BLOOD VESSELS
It has already been pointed out that blood must remain fluid within the blood vessels if life has to be normal. The factors, which help in preventing intravascular clotting (thrombosis), are given below.
1. Vascular endothelium – The normal endothelium of blood vessels, has been described to act as a non-wettable surface and docs not allow the activation of factor XII. Similarly the platelets also cannot adhere to normal endothelium. Both these factors prevent the initial steps in clotting. The endothelial cells arc also rich in fibrinolysin activators. In atherosclerosis the normal intima is replaced by a diseased one, and clotting is favored resulting in thrombosis. The endothelium can also be damaged by bacteria, injuries such i as fractures, pressure on veins, etc.
2. Chemical substances in blood – There arc certain substances in the blood, which normally antagonize any tendency towards clotting. These include the following:
(i) Heparin – It is produced by the mast cells of the connective tissue which occur especially in the lung and liver. Heparin is an acidic mucopolysaccharide composed of alternating sulfated D-glucosamine and D-glucuronic acid units. Up to 40 % of its molecular mass is H2SO4. This makes heparin the strongest organic acid in the body. Heparin is formed in small amounts continuously, but its release from the mast cells is greatly increased in conditions like peptone shock and anaphylactic shock. The mechanism of heparin action is manifold. It antagonizes all the stages of blood clotting, e.g. formation of active factor X, thrombin and fibrin. It also inhibits the agglutination of platelets thus decreasing the release of platelet factors. In combination with a proteiamely anti-thrombin, it inactivates thrombin. This is probably the most important action of heparin.
(ii) Antithrombins – These arc substances present in plasma which can inactivate large amounts of thrombin very quickly. Thrombin formed during the clotting of only 10 ml of blood can clot the whole body blood. But normally the excess of thrombin is destroyed by antithrombins, and thus the extension of the clotting process to other body regions is prevented; the fibrin clot also adsorb a lot of thrombin and this helps in keeping the clotting process localized. Heparin binds to the lysine residue on the antithrombin molecule, which results in an enhanced activity of the latter. Antithrombins are of great physiological importance.
(iii) Anlithromboplastin substances – Blood and body tissues contain substances, which neutralize any thromboplastin released by damage to tissues.
(iv) Protein C-protein-S system – Protein C is a vitamin-K dependent plasma protein. It first occurs in an inactive form. It is activated by thrombin. Its active form acts as a powerful protease; in the presence of a cofactor, thrombomodulin, present on the endothelial surface it digests factors V and VIII and thus clotting process is inhibited. Protein S has the role of accelerating the activation of protein C. These two proteins are believed to have a physiologic role in helping to keep the blood unclotted.
(v) Fibrinolysis – This process has an important role in keeping the blood in a fluid state under normal circumstances. It is believed that micro-thrombi are formed in the blood eveormally, but fibrinolysis liquefies these clots. Cellular phagocytosis of small particles of fibrin thrombi is also a protective mechanism.
(vi) Prostacydin – Its role has already been described under platelets.
3. Vigorous circulation of blood – Under normal circumstances this is an important factor in preventing intravascular clotting of blood. This is brought about by not allowing the reactants to accumulate at a certain site, which could initiate clotting. In conditions associated with stasis of blood (bed-ridden patients, congestive heart failure, polycylhemia, hyper-gammaglobulinemia), intravascular clotting is quite common. Use of oral contraceptive pills and pregnancy also produce venous stasis by causing venous dilatation
ROLE OF LIVER AND VITAMIN K IN BLOOD COAGULATION
Several factors needed for blood coagulation arc formed in the liver. The formation of four of these factors, i.e. prothrombin and factors VII, IX and X is dependent upon the presence of vitamin K. A deficiency of vitamin K leads to a deficiency of these factors, which may produce a tendency to bleed easily. Substances, which antagonize vitamin K, such as dicumarol, also produce a deficiency of these factors producing hemorrhagic tendency. If the liver is severely diseased, it cannot make these factors at a normal rate and even the administration of vitamin K will fail to exert a beneficial effect on the hemorrhagic tendency. Other factors synthesized in the liver but independent of vitamin K are fibrinogen, factor V and factor XIII. The sites of the formation of factors VIII, XI and XII are .not known, although factor VIII is believed to be produced in the reticuloendothelial cells.
SOME LABORATORY TESTS DONE FOR INVESTIGATING BLEEDING DISORDERS
(1) Clotting time – (Method of Lee and White). Without letting tissue juice to contaminate the blood, venous blood is withdrawn into a clean, dry, all glass syringe with a wide bore needle. A stopping watch is started the moment blood appears in the syringe and blood coagulation time is recorded from the moment. The needle is detached and 1 ml of blood is delivered into each of the four dry, chemically clean glass tubes with a size 10 by
After 3 minutes have elapsed, each of these tubes is lifted from the water bath and is tilled very gently. The clotting time is taken when the tube can be inverted without its contents spilling. The clotting time for each tube is recorded separately and the clotting time is reported as the average of the four tubes. The normal clotting time by this method is 4 to 10 minutes.
(2) Prothrombin time – (Quick’s one-stage method). Oxalated plasma of the patient is obtained and to it is added an excess of thromboplastin, which is usually an emulsion of rabbit’s brain. Calcium chloride solution is added. The time taken for clotting of the plasma to occur after the addition of calcium chloride is noted; it is termed as the prothrombin time and normally it is 12 to 14 seconds.
(3) Bleeding time – (Duke’s method). When injury to a small vessel produces only a small defect, the platelet plug, i.e. the white thrombus alone can cause hemostasis. This is the basis for the bleeding time that is employed clinically to distinguish hemostatic deficiency caused by abnormalities of blood capillary wall from those caused by coagulation defects. In doing the test a standardized cut is made usually into the skin of the car lobule. The blood, which flows, is absorbed by a filter paper every 15 seconds.
(4) Platelet count – This is normally 180,000 to 320,000 per cu mm of blood. Bleeding usually occurs when the count falls to 50,000 platelets per cu mm or below.
(5) Plasma fibrinogen level – This is found to be low in conditions associated with excessive fibrinolysis. In this condition the presence of fibrinogen degradative products (FDP) in the plasma and urine can be demonstrated.
(6) Assay of clotting factors – Factors V, VIII, IX, etc. can be quantitatively assessed in the blood.
(7) Capillary fragility test – A strong positive pressure (by inflating sphygmomanometercuff) or a negative pressure (by applying suction cups) is applied and the number of minute petechial hemorrhages (petechiac) is counted in the skin; their number is found to be much increased in conditions associated with an increased capillary fragility, e.g. purpuras.
