In human biology, serum is the watery component of blood that separates from the red-blood cells, white-blood cells and clotting proteins during the clotting process. Put another way, it is blood plasma with the blood cells and clotting proteins removed but all other proteins and electrolytes remaining. In order to obtain serum, you need to allow the blood to clot for some time, then rim the tube with a sterile applicator stick to dislodge the clotted blood that has adhered to the test tube, then centrifuge the tube. The clear substance in this procedure will be serum. (To obtain plasma, you must add an anticoagulant such as heparin, then centrifuge to separate the red blood cells, white blood cells, and plasma.)
Normal human serum looks clear and watery to the naked eye, but in fact includes a wide variety of important substances. Serum contains electrolytes, such as bicarbonate, sodium, potassium, calcium, chloride, phosphorus and magnesium, enzymes, such as alkaline phosphatase, amylase, lipase, creatine kinase and a number of liver enzymes, and other important substances, including cholesterol, glucose, bilirubin, creatinine, uric acid and triglycerides. Each of these normally exists within a given range, so doctors can detect the presence of certain disease states by assaying these substances -- for example, serum glucose may be grossly elevated in a person with uncontrolled diabetes mellitus.
