Platelets are small cell fragments found in your blood that are responsible for helping blood clot. They release chemicals that signal for blood to clot, as well as stick together to form a plug to stop bleeding. Normal platelet counts generally range from 150,000 to 400,000 per milliliter, or mL, of blood. Platelet counts below 150,000 per mL of blood are considered low. A low platelet count is called thrombocytopenia and most often occurs secondary to another disease or as a side effect of certain drugs.
Decreased Platelet Production
With certain diseases, platelets may not be made in adequate amounts. Platelets are produced in the bone marrow by cells called megakaryocytes. If a disease affects the bone marrow, platelet production can be decreased. For example, leukemia, lymphoma or a cancer that has spread to the bone marrow may interfere with normal bone marrow function by crowding out normal cells with cancer cells. Aplastic anemia prevents the bone marrow from making platelets as well as other blood cells because the marrow cells are decreased in number or not functioning properly. Other bone marrow conditions such as myelofibrosis can similarly interfere with the production of sufficient numbers of platelets. Chemotherapy used to treat cancer can damage the cells in the bone marrow that produce platelets. Alcohol can slow down the production of platelets as well.
Decreased Platelet Survival
The normal life span of a platelet in the blood is about a week. Many conditions can cause a decrease in platelet survival and result in thrombocytopenia. Autoimmune diseases and immune thrombocytopenia can cause a low platelet count. This can occur either during a viral infection when antibodies cross-react with platelets and target them for destruction or when your body makes antibodies against your own platelets for an unknown reason and targets them for destruction. Many medications -- such as quinine, heparin, certain antibiotics including penicillin and sulfonamides and nonsteroidal antiinflammatory drugs -- can cause the immune system to produce antibodies that combine with platelets and target them for destruction. In addition, other nonimmunological ways platelets can be destroyed include the passage of platelets through mechanical devices, such as those used for blood transfusions, bypass surgery or with man-made heart valves. Conditions that cause abnormal blood clotting, such as the severe bleeding disorder called DIC, can also increase platelet destruction.
Platelet Sequestration by the Spleen
Your spleen helps regulate the numbers of red blood cells and platelets in your blood as well as helping to regulate your immune system. The spleen typically stores 30 to 40 percent of your platelets. However, if your spleen becomes enlarged, it can retain up to 90 percent of platelets, leaving only a small number in your blood. Diseases such as blood cancers involving the spleen or liver disease can cause the spleen to become enlarged and sequester a larger number of platelets from circulation in the blood.
Dilution of Platelets
A less common cause of low platelet count is dilutional thrombocytopenia. The dilution of platelets in your blood may occur during a large blood transfusion in which about 1.5 times your blood volume is replaced. The platelets found in the units of blood that are used for blood transfusions typically lose their function after a few days of storage. Therefore, the transfused blood is replacing plasma and red blood cells but not necessarily the platelets that are being lost.