Blood coagulation: tests, disease, what is it?

Definition: what is blood clotting?

The ability of the body to making blood clot allows humans to survive in the event of injury since it causes it to close in order to limit blood loss. The platelets sometimes clump together on the small cut vessel to form a “soft plug”. Gradually, several blood proteins, such as prothrombin for example, react with each other to form a more solid plug: the clot which stops the bleeding. In the absence of injury, blood flows through blood vessels without forming clots thanks to a system of enzymes, proteins, and factors that prevent blood from clotting. This system ensures the normal fluidity of the blood thanks to its anticoagulant function. If an enzyme, protein, or factor is missing or functions abnormally, the blood clots more easily and increases the risk of thrombosis, particularly in the deep veins (phlebitis) or pulmonary arteries (pulmonary embolism). The three main tests which make it possible to assess blood coagulation are prothrombin time (PT or former “Prothrombin time”), activated partial thromboplastin time (ACT or sometimes TCK) and bleeding time (TS).

Why test blood clotting?

The doctor may order a blood coagulation test in people who have unexplained bleeding or those suffering from thrombosis, that is, producing clots in the veins or arteries. It can also be used to measure the effectiveness of anticoagulant drugs. “To lead to the formation of a blood clot, the coagulation process uses two pathways, called “intrinsic” and “extrinsic”, which each provide essential coagulation factors. In the event of a deficiency in one or more of these factors, or if these factors have abnormal function, clot formation is not possible and bleeding and/or thrombosis may occur.

What tests can check blood clotting?

Three tests are prescribed to analyze the proteins of different coagulation pathways:

► Prothrombin level (PT). PT measurement explores the extrinsic pathway of coagulation, including factors VII, V, X, II and fibrinogen (protein present in blood plasma essential for blood clotting). This is a test expressed in seconds or as a percentage with a rate normally between 70% and 100%. If it drops, it means that the blood is more fluid and has difficulty clotting. As part of the monitoring of treatments with vitamin K antagonists, the results are given in the form of INR (international normalized ratio). “Its normal value is equal to 1 and your doctor indicates the target you should have, for example between 2 and 3, and adjusts the treatment to reach it. indicates Dr François Blanchecotte, biologist.

► Activated partial thromboplastin time (aPTT). This exam measures the functionality of the intrinsic pathway by measuring the time it takes for plasma to clot in a test tube after the addition of cephalin and an activator. When the sample takes longer than normal to clot, the TCA is said to be “elongated”. This examination can assess the insufficiency or not of blood clotting factors (factor VII, factor IX and factor VIII and factor XII). Its normal value is between 24 seconds to 41 seconds.

► Bleeding time (BT). It measures the time observed between the creation of a wound and the stopping of bleeding. This measurement is no longer used in recent years because it is poorly reproducible and can be modified by aspirin and often leaves small, thin scars on the forearms. Other exams like platelet count and fibrinogen dosage may be necessary to interpret the coagulation assessment.

What are the diseases linked to blood clotting disorder?

A PT or INR and/or TCA abnormality means that the blood takes longer to clot than normal, and therefore that a problem of thrombosis is possible, or on the contrary that it is more “fluid” and that there is a risk of hemorrhage. In the event of anticoagulant treatment, an adjustment must then be made by the doctor. Without treatment, additional tests are necessary to determine the cause of these abnormalities: thrombosis (formation of a blood clot) thrombocytopenia (reduction in the number of platelets in the blood), thrombopathy (abnormal functioning of platelets), factor deficit VIII or IX (hemophilia A and hemophilia B, Willebrand disease, anemia, pregnancy, liver failure (cirrhosis, hepatitis) or taking aspirin For example. “It is possible to explore the path of TP to measure liver failure“ notes the biologist.

Blood clotting and diet

Vitamin K is an essential vitamin for clotting. This fat-soluble (fat-soluble) vitamin exists naturally in green vegetables, oils (rapeseed, soya, etc.), liver, fish) and in fermented cheeses. Deficiencies of this vitamin are rare and mainly concern people suffering from a fat absorption disorder such as cystic fibrosis, pancreatic and liver diseases (hepatitis and cirrhosis), intestinal parasitoses, major resections of the intestinal, or chronic diarrhea. Finally, people taking oral anticoagulants such as vitamin K antagonists are also exposed to a significant risk of deficiencies. It is therefore recommended that they limit foods rich in vitamin K to one serving per day (especially plants such as sauerkraut, cabbage, spinach, broccoli, avocados) and to vary their diet as much as possible, every day. Green tea, rich in vitamin K, can be consumed regularly but in small doses and only once a day.

What are anticoagulant medications?

Anticoagulant medications have the effect to prevent blood clots (thrombosis) from forming in the veins, arteries or heart. Their action consists of slowing down coagulation. These are mainly:

• THE vitamin K antagonists (AVK) which come in the form of a tablet to swallow and which prevent the formation of several clotting factors dependent on vitamin K.
• THE heparins which come in injectable form (administered by subcutaneous injections or by infusion) which increases the natural activation of the inhibitor of several coagulation factors.
• Aspirin can also be prescribed to modify coagulation but it is not an anticoagulant medication, it is an antiaggregant medication which does not act on coagulation but on platelets.

Standards differ depending on laboratory practices. Please be aware that blood test results alone do not constitute a diagnosis. It is therefore particularly important to consult a doctor in order to plan additional examinations or possible treatment.

Thanks to Dr François Blanchecotte, biologist, for his validation.