Blood clotting disorders are problems in the body’s ability to control how the blood clots. Normally, blood clots form during an injury to prevent bleeding. If you have a clotting disorder, your blood may not clot enough, which can lead to too much bleeding, or your blood may form clots even without an injury.

You can read about conditions that happen when your blood does not clot enough in our Bleeding Disorders health topic. This topic focuses on clotting disorders that happen when your blood clots more often than it should.

Blood clotting disorders are sometimes called coagulation disorders or thrombophilias. They are either inherited (meaning that you are born with the condition) or acquired (meaning you develop the condition as the result of another illness or injury). For example, antiphospholipid syndrome (APS) and disseminated intravascular coagulation (DIC) are types of acquired blood clotting disorders.

Blood clots can cause many health problems. Symptoms of blood clots depend on where in the body they form. Typically, they will form in the veins and appear in the legs or lungs. Blood clots in the legs can cause deep vein thrombosis. Blood clots in the lungs can cause a pulmonary embolism. It is rare for blood clots to form in the arteries. When they do, they can lead to heart attack or stroke.

If you think you may have a blood clotting disorder, your doctor will ask about your family and medical history. They may also run tests to be sure of the diagnosis. If you have a blood clotting disorder, you may need medicine to stop the blood from clotting. Your doctor may also talk to you about ways to prevent blood clots and to stay healthy.

When a blood vessel is injured, the damaged cells in the vessel wall send out chemical signals. These signals cause clots that slow or stop bleeding.

A blood clot forms through several steps:

1. The blood vessel narrows. First, chemical signals cause the injured vessels to narrow to prevent more blood from leaking out.

2. Platelets travel to the site of the injury. The chemical signals travel through your blood to the spleen, where many platelets are stored. The signals tell your spleen to release the platelets into your blood. Back at the injury site, the vessel walls become sticky and capture the platelets as they float past.

3. A platelet plug forms. The platelets change shape and become stickier. This allows them to attach to the vessel wall and clump together into a plug. 

4. The blood clot forms. Clotting factors in your blood are normally turned off so that you do not form abnormal blood clots. When there is an injury, platelets release molecules into the blood that help turn on clotting factors. One important clotting factor is fibrin, a long, thin, and sticky protein. When it is turned on, it forms a mesh to hold the platelet plug in place. This is called a fibrin clot. The mesh also traps red blood cells to form a blood clot. The platelets contract to pull the two sides of the damaged vessel closer together, so it is easier to repair.

Once the blood clot is formed, your body’s immune system repairs the injury. At this point in the process, factors in your blood start to break down the blood clot.

If you do not have enough platelets or clotting factors in your blood, your blood will not be able to clot as well. Read more in our Bleeding Disorders topic.

In other cases, your blood may clot too easily. Some conditions cause overactive clotting so that blood clots form in blood vessels throughout your body. Eventually, the platelets in your body are used up, which can then lead to bleeding. These conditions include: 

• Antiphospholipid syndrome 
• Disseminated intravascular coagulation (DIC) 
• Thrombotic thrombocytopenic purpura

Medicines and blood clots

Your doctor may give you medicine to control how your blood clots. You may also develop blood clotting problems because of medicines you take for another health condition.

Coagulation

Those more sophisticated and more durable repairs are collectively called coagulation, the formation of a blood clot. The process is sometimes characterized as a cascade, because one event prompts the next as in a multi-level waterfall. The result is the production of a gelatinous but robust clot made up of a mesh of fibrin—an insoluble filamentous protein derived from fibrinogen, the plasma protein introduced earlier—in which platelets and blood cells are trapped. Figure 18.14 summarizes the three steps of hemostasis.

Figure 18.14 Hemostasis (a) An injury to a blood vessel initiates the process of hemostasis. Blood clotting involves three steps. First, vascular spasm constricts the flow of blood. Next, a platelet plug forms to temporarily seal small openings in the vessel. Coagulation then enables the repair of the vessel wall once the leakage of blood has stopped. (b) The synthesis of fibrin in blood clots involves either an intrinsic pathway or an extrinsic pathway, both of which lead to a common pathway. (credit a: Kevin MacKenzie)

Clotting Factors Involved in Coagulation

In the coagulation cascade, chemicals called clotting factors (or coagulation factors) prompt reactions that activate still more coagulation factors. The process is complex, but is initiated along two basic pathways:

• The extrinsic pathway, which normally is triggered by trauma.
• The intrinsic pathway, which begins in the bloodstream and is triggered by internal damage to the wall of the vessel.

Both of these merge into a third pathway, referred to as the common pathway (see Figure 18.14b). All three pathways are dependent upon the 12 known clotting factors, including Ca²⁺ and vitamin K (Table 18.1). Clotting factors are secreted primarily by the liver and the platelets. The liver requires the fat-soluble vitamin K to produce many of them. Vitamin K (along with biotin and folate) is somewhat unusual among vitamins in that it is not only consumed in the diet but is also synthesized by bacteria residing in the large intestine. The calcium ion, considered factor IV, is derived from the diet and from the breakdown of bone. Some recent evidence indicates that activation of various clotting factors occurs on specific receptor sites on the surfaces of platelets.

The 12 clotting factors are numbered I through XIII according to the order of their discovery. Factor VI was once believed to be a distinct clotting factor, but is now thought to be identical to factor V. Rather than renumber the other factors, factor VI was allowed to remain as a placeholder and also a reminder that knowledge changes over time.

Clotting Factors

Table18.1 *Vitamin K required.

Extrinsic Pathway

The quicker responding and more direct extrinsic pathway (also known as the tissue factor pathway) begins when damage occurs to the surrounding tissues, such as in a traumatic injury. Upon contact with blood plasma, the damaged extravascular cells, which are extrinsic to the bloodstream, release factor III (thromboplastin). Sequentially, Ca2+ then factor VII (proconvertin), which is activated by factor III, are added, forming an enzyme complex. This enzyme complex leads to activation of factor X (Stuart–Prower factor), which activates the common pathway discussed below. The events in the extrinsic pathway are completed in a matter of seconds.

Intrinsic Pathway

The intrinsic pathway (also known as the contact activation pathway) is longer and more complex. In this case, the factors involved are intrinsic to (present within) the bloodstream. The pathway can be prompted by damage to the tissues, resulting from internal factors such as arterial disease; however, it is most often initiated when factor XII (Hageman factor) comes into contact with foreign materials, such as when a blood sample is put into a glass test tube. Within the body, factor XII is typically activated when it encounters negatively charged molecules, such as inorganic polymers and phosphate produced earlier in the series of intrinsic pathway reactions. Factor XII sets off a series of reactions that in turn activates factor XI (antihemolytic factor C or plasma thromboplastin antecedent) then factor IX (antihemolytic factor B or plasma thromboplasmin). In the meantime, chemicals released by the platelets increase the rate of these activation reactions. Finally, factor VIII (antihemolytic factor A) from the platelets and endothelial cells combines with factor IX (antihemolytic factor B or plasma thromboplasmin) to form an enzyme complex that activates factor X (Stuart–Prower factor or thrombokinase), leading to the common pathway. The events in the intrinsic pathway are completed in a few minutes.

Common Pathway

Both the intrinsic and extrinsic pathways lead to the common pathway, in which fibrin is produced to seal off the vessel. Once factor X has been activated by either the intrinsic or extrinsic pathway, the enzyme prothrombinase converts factor II, the inactive enzyme prothrombin, into the active enzyme thrombin. (Note that if the enzyme thrombin were not normally in an inactive form, clots would form spontaneously, a condition not consistent with life.) Then, thrombin converts factor I, the soluble fibrinogen, into the insoluble fibrin protein strands. Factor XIII then stabilizes the fibrin clot.

Blood clotting disorders can either be inherited or acquired.

• “Inherited” means that your parents passed the gene for the disease on to you. Mutations, or changes in certain Genes, can make your blood more likely to form clots. Some genetic changes are more common than others. The common genetic changes are not as likely to cause serous blood clots as the rarer genetic changes. 
• “Acquired” means that you weren’t born with the disease, but you developed it due to another disease or condition.

Just because you have a blood clotting disorder does not mean that you will develop blood clots. But it does increase your chance of having blood clots throughout your lifetime.

Inherited blood clotting disorders

Common inherited blood clotting disorders include:

• Factor V Leiden mutation, which occurs in 5% of people of European descent
• Prothrombin G20210A mutation (also called factor II mutation), which occurs in 2% of the population

Rare inherited blood clotting disorders include:

• Deficiencies in blood clotting proteins calledprotein C, protein S, and antithrombin
• Hyperhomocysteinemia
• Sticky platelet syndrome

Acquired blood clotting disorders

Examples of acquired blood clotting disorders include:

• Antiphospholipid syndrome (APS): This is the most common acquired clotting disorder. APS is an Autoimmune condition where the body makes antibodies that mistakenly attack cell molecules called phospholipids. Higher levels of APS antibodies in the blood raise the risk of blood clots.
• Disseminated intravascular coagulation (DIC): This conditionis caused by an infection (such as Sepsis) or an injury.

Blood clotting disorders occur when blood forms clots more often than it is supposed to. Your body maintains normal blood flow because of a balance of molecules called “procoagulant factors” and “anticoagulant factors.” Procoagulant factors help blood clots form, and anticoagulant factors prevent blood clots. Any imbalance of these factors can lead to a blood clotting disorder.

Many things can upset the balance of these factors.

Inherited blood disorders are caused by changes in the structures of your genes (called mutations) before you are born.

Causes of acquired blood clotting disorders include:

• Another condition, such as cancer, obesity, or an Autoimmune disorder, like lupus
• Not moving for long periods of time, such as after surgery or if you are put on bed rest during pregnancy
• Some medicines to treat cancer or bleeding disorders
• A vitamin deficiency in B6, B12, or folate that can cause high levels of an amino acid called homocysteine
• Infection, such as sepsis, HIV, or SARS-CoV-2, the virus that causes COVID-19

COVID-19 can trigger abnormal blood clots in some people.

Blood clotting disorders that cause your blood to clot more than normal can be very serious. You may experience different symptoms depending on which part of your body is affected by the blood clot. Your doctor will ask you about your symptoms and do tests to find out whether you have a blood clotting disorder.

Symptoms can include:

• Swollen and tender legs that are painful to the touch, if you have blood clots that block blood flow to your leg veins (called deep vein thrombosis or DVT)
• Shortness of breath and chest pain, if you have a blood clot that travels to the lungs (called a pulmonary embolism)

Visit your doctor if you have these symptoms. DVT is not life threatening, but it can lead to a life-threatening pulmonary embolism if not treated.

Less common, but just as serious, are blood clots that form in the arteries. These can lead to a heart attack or stroke.Learn the symptoms of a heart attack and warning signs of a stroke.

You may have other symptoms such as bruising easily or often or extreme tiredness if you have a bleeding disorder.

To find out whether you have a blood clotting disorder, your doctor may ask you about:

• Your medical history, including information about your symptoms, previous blood clots, Autoimmune disorders, or miscarriages
• Your family history, as blood clotting disorders often run in families

Your doctor may also do tests to find out whether you have a blood clotting disorder.

• Blood tests can help determine your blood’s clotting process and balance of clotting factors. Sometimes, certain medicines can affect blood test results. Tell your doctor about all the over-the-counter medicines you take.
• Genetic tests can tell you whether a relative has been diagnosed with a rare, inherited blood clotting disorder.

Your doctor may recommend that you visit a hematologist if you have frequent blood clots. A hematologist is a doctor who specializes in diagnosing and treating blood diseases and disorders.

Some people with blood clotting disorders may never get blood clots and may not need treatment. Your doctor will consider your previous history of blood clots as well as your current risk factors to decide a treatment plan.

How are blood clotting disorders treated?

If you have a history of blood clots, your doctor may prescribe blood thinners. You may take blood thinner medicine by mouth (such as warfarin or aspirin) or as a shot (such as heparin). Side effects of warfarin and heparin include heavy bleeding, severe headaches, and dizziness. Warfarin also can interact with over-the-counter medicines such as cold or allergy medicines or ibuprofen.

Blood thinning medicine is all about balance. Your doctor will test your blood often to make sure the dose of medicine is correct and your blood has the proper balance between bleeding and clotting.

A group of medicines called direct oral anticoagulants (DOACs)may be an alternative to warfarin or aspirin for some people, as they are safe and effective at preventing blood clots and do not require frequent blood testing. However, you may have to take more doses (twice daily compared to once a day for warfarin). DOACs are most often used in patients with low-risk, inherited blood clotting disorders (such as Factor V Leiden and Prothrombin G20210A). Talk to your doctor about whether DOACs may be right for you.

How do blood clotting disorders affect your health?

It is important to know the causes and risk factors that may increase your chances of developing dangerous blood clots. It is important to get regular checkups and look out for the symptoms of blood clots. If left untreated, blood clots can cause serious problems, including:

• Deep vein thrombosis, or a blood clot in the leg
• Pulmonary embolism, or a blood clot that travels to the lung
• Heart attack
• Stroke

Read more information about knowing the signs of and preventing heart attack and stroke.

You cannot prevent blood clotting disorders that are inherited, and you may not be able to prevent a clotting disorder that is acquired. However, even if you have a clotting disorder, you may not ever develop a blood clot.

Talk to your doctor about steps to help prevent blood clots if you are at risk.

Adopt healthy lifestyle changes

• Choose healthy foods like fruits, vegetables, and whole grains as a part of a heart-healthy eating plan.
• Be physically active to help your blood circulate and prevent the formation of blood clots.
• Quit smoking. Over time, smoking cigarettes can change the surface of the platelets in your blood and make them more likely to stick together and form blood clots.
• Manage stress to lower your chance of developing risk factors for blood clots, like high blood pressure.

Avoid certain medicines

Some medicines increase your risk of blood clots, including hormone replacement therapy for menopause and birth control pills with estrogen. Talk to your doctor about all the over-the-counter medicines you take.

Talk to your doctor before any planned surgeries

Blood clots can happen during surgery and while you are recovering from surgery. To prevent clotting, your doctor may give you blood thinners to take after the surgery or procedure.

Talk to your doctor if you are pregnant or thinking of becoming pregnant

Some blood clotting disorders can be harmful to you or your pregnancy. Your doctor will go over the risks with you and provide a treatment plan for your specific circumstances.