Blood tests are very useful diagnostic tool.
Blood is made up of several different kinds of cells and other compounds, including various salts and certain proteins. The liquid portion of the blood is called plasma. When blood clots outside the body, the blood cells and some of the proteins become solid. The remaining liquid is called serum, which can be used in chemical tests and in tests to find out how the immune system fights diseases. Doctors can take blood samples and grow the infectious organisms that cause an illness to see exactly what they are through a microscope.