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Saturday, May 17, 2008

Platelet transfusions

What are platelets?

Platelets are tiny cells that are found in the blood. Their function is to help it to clot. Platelets are made in large numbers by the bone marrow (the spongy material inside the bones). Each platelet develops in the bone marrow and is then released into the bloodstream where it circulates around the body. Platelets usually survive for 7–10 days before either being destroyed by the liver or spleen, or being used to clot the blood.

When a blood vessel is damaged (for example, by injury), platelets rush to stick to the damaged area and form a clump. They bind tightly and release chemicals. The chemicals make the blood vessels narrow to reduce blood loss and also attract more platelets to make the clump bigger. The platelets also stimulate the formation of a substance called fibrin, which binds them together to form a clot.

Why platelet transfusions are given

Sometimes the number of platelets in the blood is lower than normal. This may be because of illness (including cancer, leukaemia or certain blood disorders) or it can be a side effect of chemotherapy treatment. If your bone marrow is not working normally, the number of platelets in your blood (known as the 'platelet count') may drop. How low the number of platelets gets depends on how much the illness or the chemotherapy has affected the cells in the bone marrow.

The normal platelet count is between 150 and 400. A slightly lower platelet count will not usually cause any problems, since there are plenty to spare. A very low platelet count, however, can sometimes be serious. Signs of a low platelet count include nosebleeds, bleeding gums, heavy periods, bruising and tiny blood spots in the skin (known as petechiae), or rashes (known as purpura). Very rarely, more serious bleeding (for example, into the brain or the digestive system) can occur.

Serious bleeding is rare if the platelet count is above 10, although it may happen at higher levels (perhaps up to 30) if you also have a high temperature due to infection. Therefore, it is important that you contact the hospital if you know that your platelets are low and you develop a temperature above 38ÂșC.

In hospitals, platelet transfusions are often used to prevent the platelet count from going too low. Therefore serious bleeding due to low platelets is now very rare.

Your doctors will check your platelet count regularly and will be aware when this is falling. If you have a low platelet count it may be necessary for you to have a platelet transfusion. This is particularly important before certain procedures (such as a lumbar puncture or bone marrow aspiration) where there is a risk of bleeding. The platelet transfusion will reduce the risk of any bleeding during or after the procedure.

Having a platelet transfusion

The platelets are stored in small plastic bags which are kept at a low temperature. Each bag contains about 100mls of yellow fluid. When they are needed, the platelets are given by drip (infusion).

Each bag of platelets will be sent from the blood transfusion centre and will contain platelets collected from blood donated by 3–5 people (donors). Sometimes, the platelets are collected from a single person using a special machine that separates the platelets from the blood. The machine is called an apheresis machine and platelets collected in this way are referred to as 'apheresed platelets'.

In order to give you the platelets, a small tube (cannula) is put into a vein in your arm or hand (unless you already have a cannula). If you already have a central line or PICC line (a tube going into a vein near the heart) the platelets may be given through this.

The bag of platelets is then connected by a tube to the cannula, central or PICC line. Each bag is called a 'unit' and takes 15–30 minutes to give. Normally one unit is enough to prevent or stop bleeding. Occasionally two units are needed.

Despite having platelet transfusions, you may still notice problems with bleeding or bruising. It is important to let your doctors know about this, as other things can be done to help if this happens. As it only takes a short time to give the platelets, this treatment can be given in the day unit or outpatients department. Platelets can also be given before or after other treatments.

Possible side effects

Severe side effects from platelet transfusions are rare. The more common side effects include itching, rashes, a high temperature and shivering (sometimes referred to as having a 'reaction'). A nurse will monitor you during the transfusion. If you do have any side effects let the nurse or doctor know and they can give you medicines to help stop the reaction.

Having apheresed platelets reduces the risk of becoming sensitive to the platelets and having transfusion reactions.

If you are having platelets frequently, it is possible for them to become less effective – this is known as becoming 'refractory' to platelets. This can occur for two reasons: non-immune causes and immune causes. However, becoming refractory to platelets is rare.

Non-immune causes These include a high temperature. Platelets are used up faster if you have a high temperature. Some medicines, such as antifungal drugs, can lower the platelet count and if the spleen is enlarged platelets can collect there instead of circulating in the bloodstream.

Immune causes Platelets can become less effective when your immune system recognises the donated platelets as different from your own. Your body then produces antibodies (proteins produced by the body to fight a foreign substance). The antibodies attack and destroy the platelets quickly. This occurs most often in people who have had many blood and platelet transfusions.

If an immune reaction occurs, a blood test can be taken to check for antibodies, and the platelets you are given can be checked to make sure that they are similar to your own. This is known as cross-matching, and certain groups of proteins on the cell surface are looked at to make sure they are similar. Cross-matching should increase the effectiveness of the platelets that are given.

Having more than one transfusion

Many people need only a few transfusions during their illness, whilst others need many. Platelets can usually be given repeatedly without causing any problems.

Are platelet transfusions safe?

Some people worry that the platelets they are given may be infected by disease. People who donate blood and platelets are carefully screened to assess their risk of having an infection or virus, such as hepatitis, HIV or malaria. This means that the risk of being given infected platelets is extremely small. There have been no reports of CJD (Creutzfeldt-Jakob disease) infection due to blood or platelet transfusion. Precautions are taken to minimise the risk of infection. All donated blood is tested in the laboratory for infection. The platelets are not given to a patient until all tests from the blood are found to be negative.

Blood transfusions

Some people who have problems with low platelets also have a low number of red blood cells (anaemia). Anaemia can be caused by chemotherapy, radiotherapy or some types of cancer. Symptoms of anaemia include tiredness and breathlessness. Blood transfusions can quickly correct anaemia and the benefits can be noticed within 24 hours.

References

This section has been compiled using information from a number of reliable sources including:

  • National Blood Service. www.blood.co.uk
  • UK Blood and Transfusion and Tissue Transplantation Guidelines, chapter 13. www.transfusionguidelines.org.uk
  • Wintrobe's Clinical Haematology (10th edition).G Rlee et al. Williams and Wilkins, 1999.
  • Essential Haematology (4th edition) AV Hoffbrand, JE Pettit and PAH Moss. Blackwell Science, 2001.

For further references, please see the general bibliography.

Via: http://www.cancerbackup.org.uk

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