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Thrombocytosis and essential thrombocythaemia

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Thrombocytosis and essential thrombocythaemia

Written by Dr Claire N Harrison, Consultant Haematologist, St Thomas' Hospital, London and Professor Samuel J Machin, Department of Haematology, University College London Hospital

What is thrombocytosis?

Thrombocytosis is a medical term used to describe a high number of platelets in your blood.

Platelets (thrombocytes) are small fragments of cells that stick together to stop bleeding after a cut or injury. They stick to each other and the damaged wall of the blood vessel to form a blood clot.

Did you know? Platelets are smaller than red and white blood cells.

What causes a high platelet count?

A high platelet count can happen for many reasons, but there are two basic processes involved.

The cells within the bone marrow produce more platelets.

Less platelets are removed from the blood by the spleen.

In most cases thrombocytosis is caused by increasing platelet production. Reduced platelet removal only occurs following surgical removal of the spleen (splenectomy).

Why is it a problem if you have too many platelets?

Platelets are essential to stem bleeding, but if you have too many it can cause clots to form when you don't need them (thrombosis). If the platelet function is defective as well, this can cause bleeding (haemorrhage).

Term watch Thrombus: medical term for a blood clot.

Reactive (secondary) thrombocytosis

The causes of reactive (secondary) thrombocytosis are described below.

Infection Infections can cause a high or low platelet count. In children, an infection is often accompanied by a raised platelet count.

This is thought to be due to the hormones, called cytokines, that are produced as part of the body's normal defence against infection.

A high platelet count is less common in adults who have infections.

The thrombocytosis usually resolves as the infection recovers, although it may take longer to settle.

Inflammatory disorders

These conditions may cause a high platelet count in a similar way to infections.

Kawasaki's disease is a rare condition, mainly affecting children, in which there is widespread inflammation of the arteries. It is associated with a high platelet count that gradually resolves in the recovery phase.

In adults either a high or low platelet count may accompany other symptoms of autoimmune diseases such as rheumatoid arthritis.

Blood loss

In event of an injury, the response of the bone marrow to blood loss is to produce more red blood cells and more platelets.

Tissue damage from trauma or surgery

The platelet count will also increase when a relatively large amount of body tissue is damaged either intentionally following surgery or after an accident. This is part of your natural defence mechanism to ensure adequate clot formation and prevent fatal bleeding.

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