Thrombocytopenia is a deficiency of platelets (thrombocytes).
The blood usually contains about 140,000 to 440,000 platelets per microliter. When the platelet count falls below about 50,000 platelets per microliter of blood, bleeding can occur even after relatively minor injury . The most serious risk of bleeding, however, generally does not occur until the platelet count falls below 10,000 to 20,000 platelets per microliter. At these very low levels, bleeding may occur without any injury.
Many disorders can cause thrombocytopenia. Thrombocytopenia can occur when the bone marrow does not produce enough platelets, as happens in leukemia and some anemias. Infection with the human immunodeficiency virus (HIV), the virus that causes AIDS, often results in thrombocytopenia. Platelets can become entrapped in an enlarged spleen, as happens in myelofibrosis and Gaucher disease, reducing the number of platelets in the bloodstream. Massive blood transfusions can dilute the concentration of platelets in the blood. Finally, the body may use or destroy too many platelets, as occurs in many disorders, three of the most notable being immune thrombocytopenia, thrombotic thrombocytopenic purpura, and hemolytic-uremic syndrome.
Immune thrombocytopenia (ITP):
ITP is a disorder in which antibodies form and destroy the body's platelets. Why the antibodies form is not known. Although the bone marrow increases platelet production to compensate for the destruction, the supply cannot keep up with the demand.
Thrombotic thrombocytopenic purpura (TTP) and hemolytic-uremic syndrome (HUS):
TTP and HUS are rare, related disorders in which small blood clots form suddenly throughout the body. These blood clots block small blood vessels throughout the body, particularly those in the brain, heart, and kidneys. The blood clots mean that an abnormally high number of platelets are being used, which leads to a sharp decrease in the number of platelets in the bloodstream.
The main differences between TTP and HUS are that TTP is more common in adults and does not often cause kidney failure, whereas HUS is more common in children and typically causes kidney failure.
The cause of TTP is often unknown, but some people develop it after taking certain drugs (including quinine, cyclosporine, and mitomycin C) or during pregnancy. HUS usually occurs following an intestinal infection with Escherichia coli O157:H7 or some strains of Shigella dysenteriae.
Symptoms and Complications
Bleeding in the skin may be the first sign of a low platelet count. Many tiny red dots (petechiae) often appear in the skin on the lower legs, and minor injuries may cause small scattered bruises. The gums may bleed, and blood may appear in the stool or urine. Menstrual periods may be unusually heavy. Bleeding may be hard to stop.
Bleeding worsens as the number of platelets decreases. People who have very few platelets may lose large amounts of blood into the digestive tract or may develop life-threatening bleeding in the brain even though they have not been injured.
The rate at which symptoms develop can vary depending on the cause of thrombocytopenia. For example, in TTP and HUS, symptoms develop suddenly. In ITP, symptoms may develop suddenly or gradually and subtly.
Symptoms in TTP and HUS are quite distinct from symptoms of most other forms of thrombocytopenia. In TTP, the small blood clots that develop (using up platelets) cause a wide range of symptoms and complications, some of which can be life threatening. Symptoms that result from clots in the brain may include headache, confusion, seizures, and coma. Symptoms that result from clots elsewhere in the body include abnormal heart rhythms, blood in the urine that accompanies kidney damage, and abdominal pain. The predominant symptoms and complications of HUS are related to blood clots that develop in the kidneys, causing damage that is usually severe and may progress to kidney failure.
Doctors suspect thrombocytopenia in people who have abnormal bruising and bleeding. They often check the number of platelets routinely in people who have disorders that cause thrombocytopenia. Sometimes they discover thrombocytopenia when blood tests are performed for other reasons in people who have no bruising or bleeding.
Determining the cause of thrombocytopenia is critical to treating the condition. Certain symptoms may help determine the cause. For example, people usually have a fever when thrombocytopenia results from an infection. In contrast, they usually do not have a fever when the cause is ITP, TTP, or HUS. An enlarged spleen, which a doctor may be able to feel during a physical examination, suggests that the spleen is trapping platelets and that thrombocytopenia results from a disorder that causes the spleen to enlarge. HUS is diagnosed when poor kidney function is identified by blood tests that show high levels of urea nitrogen and creatinine.
A sample of blood may be examined under a microscope, or the platelet count may be measured with an automated counter to determine the severity of thrombocytopenia and provide clues to its cause. A sample of bone marrow removed and examined under a microscope (bone marrow aspiration and biopsy—see see Bone Marrow Examination) may be needed to provide information about platelet production.
People who have a very low platelet count are often treated in a hospital or advised to stay in bed to avoid accidental injury. When bleeding is severe, platelets may be transfused.
Treating the cause can often treat the thrombocytopenia. Thrombocytopenia caused by a drug usually is corrected by stopping the drug.
In ITP, the antibodies that destroy platelets can be blocked temporarily with a corticosteroid (for example, prednisone) or intravenous immune globulin, allowing the number of platelets to increase. Children usually recover following this treatment within several weeks to months. Some adults recover during the first year, but many do not. Adults who do not respond adequately to corticosteroids may require additional drugs that suppress the immune system, including cyclophosphamide, rituximab, and sometimes azathioprine. Many adults (but not children) with ITP eventually require surgical removal of the spleen (splenectomy) to increase the number of platelets. Newer drugs, called thrombopoietin-receptor agonists (romiplostim, eltrombopag), increase the rate of platelet production and may be effective for years. These drugs are especially useful for people who are not able to undergo (or are unwilling to undergo) splenectomy.
With TTP, people are often treated with corticosteroids and plasma exchange (plasma transfusions along with plasmapheresis—see see Testing Donated Blood for Infections).
With HUS, about half of children temporarily require kidney dialysis. Most often, the kidneys recover, but some children have permanent kidney damage. Plasma exchange is not used.
Last full review/revision December 2012 by David J. Kuter, MD, DPhil