Health Centres - Thrombocytopenia (reduced platelet count)
What is thrombocytopenia?
Thrombocytopenia is the term for a reduced platelet (thrombocyte) count. It happens when platelets are lost from the circulation faster than they can be replaced from the bone marrow where they are made.
What are platelets?
Inside each platelet are many granules, containing compounds that enhance the ability of platelets to stick to each other and also to the surface of a damaged blood vessel wall.
The platelet count in the circulating blood is normally between 150 and 400 million per millilitre of blood. Newborn babies have a slightly lower level, but are normally within the adult range by three months of age.
Many factors can influence an individual's platelet count including exercise and racial origin. The average life span of a platelet in the blood is 10 days.
What do platelets do?
An adequate number of normally functioning platelets is also needed to prevent leakage of red blood cells from apparently uninjured vessels.
In the event of bleeding, muscles in the vessel wall contract and reduce blood flow. The platelets then stick to each other (aggregation) and hold on to the vessel wall (primary haemostasis). The coagulation factors are then activated, resulting in normally liquid blood becoming an insoluble clot or glue.
What are the risks of a low platelet count?
There is not a close relationship between the number of platelets and the severity of bleeding, but there is an increasing risk of haemorrhage if platelet numbers fall or if platelet function is impaired (for example by aspirin, which reduces the 'stickiness' of the platelets).
There is a particularly high risk of spontaneous bleeding once the platelet count drops below 10 million per ml. The bleeding is usually seen on the skin in the form of tiny pin-prick haemorrhages (purpura), or bruises (ecchymoses) following minor trauma.
Bleeding from the nose and the gums is also quite common. More serious haemorrhage can occur at the back of the eye (retina), sometimes threatening sight.
The most serious complication, which is potentially fatal, is spontaneous bleeding inside the head (intracranial) or from the lining of the gut (gastrointestinal).
Types of thrombocytopenia
-
idiopathic thrombocytopenic purpura
What causes a low platelet count?
- Clot in the sample.
Congenital thrombocytopenia
- Rare inherited disorders (eg May Hegglin anomaly, Bernard Soulier syndrome).
Defective platelet production
- Bone marrow aplasia (failure).
Diminished platelet survival
- Antibodies in response to drugs, blood transfusion or another disease, eg glandular fever, malaria.
Loss of platelets from the circulation
- Massive blood transfusion or exchange.
Artefactual (false) thrombocytopenia
These antibodies also bind to a chemical in blood that is tested in the lab, giving a falsely low platelet count. For this reason, it is helpful to repeat the sample in different tubes with different chemicals.
The platelet count can also be reduced if the blood sample is difficult to take and the blood clots - thus using up some of the platelets.
Platelets are tiny cells that circulate in the blood and whose function is to take part in the clotting process.
Platelets are essential in the formation of blood clots to prevent haemorrhage - bleeding from a ruptured blood vessel.
The main effect of a reduced platelet count is an increased risk of bleeding, but this rarely occurs until there are less than 80-100 million platelets per ml.
Specific types of low platelet count include:
-
thrombotic thrombocytopenic purpura
- haemolytic uraemic syndrome.
The many different causes of thrombocytopenia are detailed below. These causes are not mutually exclusive and more than one may be responsible for an abnormal platelet count.
- Platelets clumped.
- Metabolic disorders, eg kidney failure, alcohol.
- Abnormal platelet precursors: viral infections, inherited abnormalities.
- Bone marrow infiltration, eg leukaemia, lymphoma.
- Unknown cause (ITP).
- Clotting disorder (DIC).
- Blood disorder (TTP).
- Enlarged spleen.
Some people have platelets that stick together due to the presence of proteins in the blood (antibodies) that bind to the platelets.
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