ELISA antibody test. Hand holding a multi-well sample tray in front of a computer monitor during an enzyme-linked immunosorbent assay (ELISA) antibody test. The ELISA test checks for antibodies to viral diseases, in this case hepatitis C. First, the patient's blood plasma is washed over a surface covered in the antigens of the virus being tested for. If antibodies are present then they will bind to these antigens. Next, peroxidase is added which binds to the antibodies. Then a chemical is added that changes colour when it comes in contact with peroxidase.