Blood tests help a physician to diagnose and manage a disease. In addition to examining blood cells, there are many chemicals in the blood that give important information about the functioning of bodily systems. Important chemicals that may be measured include cholesterol, thyroid hormone, potassium, and numerous others. These various chemicals are dissolved in the plasma and circulate in the blood. For a chemical blood test, blood is drawn from a patient's vein and placed in an empty tube and usually allowed to clot; the fluid portion of the blood after clotting, called serum, is then used for the various chemical analyses.