Blood tests are used to diagnose and monitor patients who have health conditions. Blood tests measure the levels of many substances in your blood, including chemicals, hormones, and waste, that your body uses to function. They help to detect many different diseases including cancer, hepatitis, and HIV. They are also used to monitor the level of drugs in the body. Blood tests can also determine how much fluid is building up in the body and check the levels of minerals and electrolytes. The test results of the blood are used to evaluate your health and health risks, detect and treat illness, and diagnose disease.