Thrombocytopenia refers to a low blood platelet count. Platelets are fragments of larger cells that circulate in the blood and play a role in preventing excessive bleeding. People with low blood platelets do not commonly display symptoms, but may be at risk for serious bleeding during and following trauma or surgery. Thrombocytopenia has many causes, including cancer, medications, alcohol, disease and infections.

Platelets are also known as thrombocytes. They are produced from large cells, called megakaryocytes, in the bone marrow. The main stimulus for platelet production is from a protein, called thrombopoietin, which is produced in the liver. Typically a platelet circulates for 10 days and then is destroyed in the spleen.

Hemostasis is a highly regulated process that prevents excessive bleeding. It requires the concerted actions of platelets, vascular factors, which stimulate vasoconstriction and clotting factors. In the process of hemostasis, an injury to a blood vessel activates the production of vascular factors, which reduce blood flow to the damaged area and attract platelets. The platelets aggregate at the site of injury and release other factors that stimulate clot formation.

Bone marrow production of platelets or those that increase platelet destruction in the blood and/or spleen can cause a low platelet count. Thrombocytopenia is a common manifestation of cancer, because sometimes the cancer and drugs used to treat the cancer affect platelet production in the bone marrow. Certain medications can cause platelets to decrease significantly, such as aspirin, non-steroidal anti-inflammatory medications, trimethoprim and quinine; bacterial and viral infections; autoimmune diseases and rare inherited disorders. Thrombocytopenia is prevalent in alcoholics because alcohol reduces platelet production and causes liver damage. The National Heart Lung and Blood Institute reports that 5 percent of pregnant women develop thrombocytopenia; however the cause is unknown.

A platelet count is conducted routinely during annual blood examinations. Normal platelet counts range between 150,000 and 450,000 platelets per microliter of blood. The National Heart Lung and Blood Institute reports that minor bleeding is associated with platelet counts below 50,000 per microliter of blood and a count less than 20,000 puts a person at risk for serious bleeding.

Because symptoms are often mild, thrombocytopenia is under-diagnosed and in patients that have been diagnosed either the underlying cause is treated or no treatment is required. People with low platelet counts because of immune destruction of platelets are often treated with corticosteroids or immunosuppressant drugs. In emergency situations, blood or platelet transfusions increase platelets. Rarely, the spleen is removed in patients with severe thrombocytopenia.