Venesection, or blood-letting, may be employed for two purposes. Most commonly, small quantities of blood may be required for analysis, as an aid to diagnosis or control of various diseases. For example, knowledge of the plasma glucose concentration is important in the diagnosis and management of DIABETES MELLITUS, or blood may be required in order to test for infections such as HIV or HEPATITIS. Blood may be obtained by pricking a fingertip, or inserting a needle into a vein, depending on the amount required. Controlled bleeding of larger amounts is used in the treatment of POLYCYTHAEMIA.