Serum testing is also known as serological testing, because serum is a yellowish transparent liquid that is precipitated from the surface of the body after the blood plasma has clotted. However, serum contains all the antigens and antibodies in plasma, so serology is the main test done. In clinical practice, most antigenic and antibody diseases can be clearly diagnosed by the patient’s serum. In clinical practice, serum tests can be done for hepatitis B, hepatitis C, syphilis, HIV, mycoplasma, and tumor markers. For example, a patient’s serum can tell whether a person is positive for surface antigen and also whether a person is positive for HIV antibodies. If a person’s serum is positive for AIDS antibodies, it means that he or she is infected with HIV, and if the tumor markers in the body are found to be significantly higher through serological tests, it is often indicative of a malignant tumor in an organ or a system.