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Course of LABORATORY MEDICINE
Hemostasis and Thrombosis
Hemostasis (blood coagulation; blood clotting) is the physiological process that stops an hemorrage and forms the semi-solid blood clot, which is then reabsorbed in the course of several days, in parallel with tissue repair. Hemostasis is actually costituted by two independent and very different processes: platelet aggregation and fibrin precipitation. Platelet aggregation is faster but the platelet plug is short lived; fibrin precipitation is slower but yields the much longer lived fibrin clot. As any other phyiological process, blood coagulation may go wrong because of many reasons and several pathologies related to abnormal coagulation are known.
Platelet activation. Platelets (thrombocytes) are fragments of the cytoplasm of the megakaryocytes of the bone marrow. They number >140.000/mm3 in the blood of healthy individuals, and are subject to rapid turnover (average lifespan is approx. 7 days). If platelet enter in contact with the collagen or the von Willebrand factor (a protein produced by the endothelium) because of lesions of the endothelial wall, they become activated and acquire the abiility to adhere to the tissue or to each other, thus forming the platelet plug over the lesion(s). Platelets are also activated by thrombin. Activated platelets produce and secrete local hormones called thromboxanes (whose precursor is arachidonic acid), that in turn activate and recruit other platelets, thus amplifying the reaction. Several drugs interfere with platelet activation: e.g. aspirin is an inhibitor of the enzyme cyclooxygenase that starts the pathway converting arachidonic acid to thromboxane.
The process of platelet activation is relevant also to fibrin clotting: this is because activated platelets release coagulation factor V that participates to the activation of thrombin (coagulation factor V is present in the blood in two fractions: a circulating one and one contained in the platelets alpha-granules; it is a protein devoid of protease activity but required for the interaction between prothrombin and factor Xa).
Fibrin clotting. Blood plasma contains 200-400 mg/dL of fibrinogen, a large (MW 340 KDa) soluble protein prduced by the liver. Proteolytic digestion of two short segments (firbinopeptides) of its polypeptide chain at the carboxy- and amino-terminus converts fibrinogen into fibrin. Fibrin is essentially insoluble and readily aggregates forming a network that adheres to the platelet plug. The fibrin network is further stabilized by the formation of covalent isopeptide bonds catalyzed by coagulation factor XIII.
The conversion of fibrinogen into fibrin is due to the specific proteolytic enzyme thrombin. Thrombin is a peculiar protein whose surface glutamic acid residues are post-translationally carboxylated and converted to gamma-carboxy glutamates; it requires calcium to function. The enzyme that gamma-carboxylates glutamic acid uses vitamin K as a cofactor and in avitaminosis K (a rare condition, being this vitamin produced by the intestinal flora) thrombin is produced but is not post-translationally modified and is inactive. Hence deficit of vitamin K (observed during long term antibiotic treatments) or administration of its inhibitors (antivitamin K; e.g. warfarin) cause defective blood clotting.
Hemorragic disorders may be due to three groups of causes, usually quite clearly distinguishable on clinical grounds: (i) defects of the vascular wall (e.g. scurvy, allergic purpura, hereditary hemorragic teleangiectasia, etc.); these are not disorders of the coagulation and all laboratory tests of coagulation are normal. (ii) Platelets disorders, due either to a reduction in number or to functional impairment; the cardinal manifestations of these disorders are prolonged bleeding and purpura, i.e. formation of cutaneous (and internal) petechiae on minor traumas. (iii) Defects of fibrin clotting (most often due to hereditary genetic defects of coagulation factors); the cardinal manifestation is temporary hemostasis (because of the formation of the platelet plug) followed by multiple episodes of re-opening of the lesion and bleeding (because of the dissolution of the platelet plug before any fibrin clot is formed).
Fibrinolysis, the dissolution of the fibrin newtork is an important physiological event, and is due to selective proteolysis. The enzyme responsible for this process is called plasmin and is present in the blood as the inactive precursor plasminogen. Plasminogen is activated by thrombin, thus the coagulation process is auto-regulated.
Laboratory tests of coagulation. The most important test of coagulation is the prothrombin time. Blood is drawn an stored in a tube containing sodium citrate as an anticoagulant (it is important to fill the test tube to the exact volume required since in this test the blood:anticoagulant ratio must be standardized). Citrate prevents coagulation by chelating calcium ions. In the clinical laboratory the plasma is prepared and calcium chloride is added in order to saturate citrate and obtaining a standard concentration of the free calcium ion. Tissue factor is also added and the time required for the precipitation of fibrin is measured by turbidimetry. Typical prothrombin time in healthy adults is in the order of 12-14 sec.
Prothrombin time is increased in the following conditions: administration of warfarin or deficiency of vitamin K; Disseminated Intravascular Coagulation syndrome; liver failure (the liver produces all the coagulation factors); congenital afibrinogenemia; hereditary deficiency of coagulation factors V and X. Prothrombin time is normal in platelet diseases and in hereditary coagulation disorders affecting the intrinsic pathway.
The thromboplastin time is a clinical measurement similar to the prothrombin time, in which the intrinsic pathway is activated by addition of phospholipids and calcium to the patient's plasma. Values in the healthy adult range between 30 and 50 sec. The thromboplastin time is increased in the following conditions administration of warfarin or deficiency of vitamin K; Disseminated Intravascular Coagulation syndrome; liver failure (the liver produces all the coagulation factors); congenital afibrinogenemia; hereditary deficiency of coagulation factors VIII and IX (hemophylias A and B).
Thrombosis is the pathological process of intravascular coagulation. It is usually due to the activation of the extrinsic coagulation pathway in the proximity of a lesion of the vascular endothelium, e.g. over an atheromatous plaque in an artery or in an inflamed vein (thromboflebitis). Arterial thrombi rarely grow to form large structures because of the rapid blood flow; however they may cause infarctions or may become organized (i.e. they may be transformed into scars, rigid because of the deposition of collagen) and cause a permanent stenosis of the affected artery. Venous thrombi are large and fragile structures and may release emboli that may in turn cause arterial obstructions and infarctions.
A very severe condition occurs if the thrombus forms in the left (or more rarely in the right) atrium of the heart, often during chronic atrial fibrillation. A fragment of the thrombus may be expelled in the pulmonary artery and cause massive pulmonary embolism, a condition that is often fatal.
Whenever a risk condition is present, anticoagulant therapy is indicated. Unfortunately risk conditions may not be easy to detect (e.g. flebitis of internal veins).
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