ELISA (enzyme linked immunosorbent assay) screening test is the initial test to diagnose AIDS or HIV. The test works by detecting antibodies/substances or proteins which are produced in the blood when virus is present. Alkaline phosphatases and peroxidases are commonly used enzymes as key reagent to perform the ELISA test. These enzymes are used to provide antibody-antigen reaction/ complex in a specialised ELISA plate or tray