In hematology, thrombocytopenia is a condition characterized by abnormally low levels of platelets (also known as thrombocytes) in the blood. Low levels of platelets in turn may lead to prolonged or excessive bleeding. It is the most common coagulation disorder among intensive care patients and is seen in a fifth of medical patients and a third of surgical patients. A normal human platelet count ranges from 150,000 to 450,000 platelets/microliter (¦ÌL) of blood. Values outside this range do not necessarily indicate disease. One common definition of thrombocytopenia requiring emergency treatment is a platelet count below 50,000/¦ÌL. Thrombocytopenia can be contrasted with the conditions associated with an abnormally high level of platelets in the blood ¨C thrombocythemia (when the cause is unknown), and thrombocytosis (when the cause is known).

Reference
Wiki: Thrombocytopenia