Thrombocytopenia (reduced platelet count)

What are platelets?

Platelets are tiny cells that circulate in the blood and whose function is to take part in the clotting process.

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?

Platelets are essential in the formation of blood clots to prevent haemorrhage - bleeding from a ruptured blood vessel.

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?

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.

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

Specific types of low platelet count include:

What causes a low platelet count?

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.