Bleeding disorder

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How does blood clot?

When blood vessels are damaged during injury, platelets and clotting proteins collect at the site of the damage to form a clot, acting like a plug.

The injury can then start healing and blood can’t escape the blood vessel.

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When blood clots too much or can’t clot

While too much clotting can lead to conditions such as heart attacks and strokes, the inability to form clots can also be a problem due to excessive bleeding.

Bleeding can result from abnormal platelets or low platelet counts (e.g. thrombotic thrombocytopenic purpura), abnormal or low amounts of clotting proteins (e.g. haemophilia), or abnormal blood vessels.

Thrombotic thrombocytopenic purpura (TTP)

Thrombotic thrombocytopenic purpura is a condition which results in clotting, low platelet count and bleeding or bruising. Clots form in some blood vessels such as in the kidneys, or in the brain, which can lead to stroke. Another term for clot is ‘thrombosis’, which is where the name comes from.

Low platelet count (thrombocytopaenia) results in bleeding which mostly occurs into the skin, forming a type of bruising known as ‘purpura’. The usual treatment for thrombotic thrombocytopenic purpura is plasma exchange where a machine called an apherisis machine removes your plasma and replaces it with cryodepleted plasma.


Patients with haemophilia have low levels of a protein essential for blood clotting, which means they can’t make clots properly. This puts them at risk of bleeding. 

There are two main types of haemophilia: 

  • Haemophilia A is the most common type and is due to a deficiency of Factor VIII. 
  • Haemophilia B or Christmas disease is due to a deficiency of Factor IX.   

Both types of haemophilia can range from mild to severe depending on how much Factor VIII or IX is missing. Haemophilia is treated by replacing the missing factor protein. Both Factor VIII and IX are available in recombinant (genetically engineered) and plasma forms. 

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