The diagnostic test which is based on the principle of antigen-antibody reaction is called Enzyme Linked Immuno-Sorbent Assay (ELISA). When the body is infected by a pathogen, the particular pathogen can be detected in the body. The immune system synthesizes antibody against the pathogen, which can also be detected. ELISA is a test which is based on this principle.

Polymerase Chain Reaction (PCR) is a test which helps in detecting very low concentration of bacteria or virus in the body. It amplifies the pathogen using a chain reaction and helps in diagnosis, even the person is not suffering from any symptoms.