Blood tests can help doctors check for certain diseases. Blood tests also help check the function of organs and can show how well treatment is working. A routine blood test usually consists of three main tests, a complete blood count, a metabolic panel and a lipid panel, each for a different thing. A complete blood count focuses on three types of blood cells, white blood cells, red blood cells and platelets. By measuring the volume of blood cells, doctors can assess an individual’s overall health and check for underlying conditions such as leukemia and anemia. White blood cells are a major component of the body’s immune system, and a high white blood cell count can indicate the presence of certain infections, while a low count can point to a variety of conditions, including diseases such as HIV and systemic lupus erythematosus. Platelet counts can reflect whether an individual has anemia, cancer, or an infection. There is usually no special preparation required before having a blood test, but for some individuals, it may be necessary to fast 8-12 hours before the test.