Principles of Diagnostic Testing Series

Part 2 – Test Characteristics: Infectious Disease Antigen and Antibody Testing

Some of the most common diagnostic tests for infectious diseases detect either antigens or antibodies. An antigen is a foreign substance that elicits an immune response. Bacteria and viruses have antigens on their surfaces. Antigens cause an immune reaction resulting in the production of antibodies. Antibodies are produced by the host’s immune system to combat disease and infection.

Diagnostic tests that identify antigens are useful for detecting the presence of an infectious organism within the body of the host. Antigen tests can be used to determine which individuals are infected with or shedding a pathogen. For example, during the ongoing COVID pandemic, a polymerase chain reaction (PCR) test has been used to detect the virus within patients.

Tests for antibodies detect the body’s immune response to a pathogen. Hosts exposed to a pathogen initially form immunoglobulin M (IgM) antibodies for short term protection before transitioning to immunoglobulin G (IgG) antibodies for longer-term protection. Tests that detect IgG are useful for detecting individuals who were exposed to an agent at some point in time while IgM tests are useful as evidence of recent exposure. The COVID RAPID tests detect IgG and IgM to the COVID pathogen and return results quickly. This test can inform people if they have been exposed to COVID and approximately how recent it was. Because antibodies take time to be produced following infection, individuals with a very recent infection may not have enough circulating antibodies to result in a test positive.