Immunoassays are one of the major bioanalytical methods that detect the presence of a specific molecule, ranging from small molecules to macromolecules in a biological sample through the use of either antibody or antigen. It is a highly effective biochemical test that measures the presence or concentration of analytes. It remains an important tool for the diagnosis and management of infectious diseases. These assays employ antibodies as analytical reagents. Immunoassays were first introduced by Berson and Yalow in 1959 and their exemplary work on the radioimmunoassay technique awarded them with the Nobel Prize in Medicine in 1977. Since then, Immunoassays have evolved considerably. They proved to be one of the breakthrough technological contributions to medicine and life-science research in the twentieth century.

What is Antibody Test?

An antibody test detects the presence of antibodies in the blood or serum to see if an individual has been exposed to an infection. They do not determine whether or not that individual has the infection, but may tell you that the individual had a past infection meaning he/she has been infected with the virus at some time in the past. An antibody test is kind of a detection mechanism for antibodies in your bloodstream. However, CDC recommends that all individuals with confirmed positive test results must be considered infected and capable of transmitting the infection to others. The basic antibody test is an Enzyme-linked immunosorbent assay (ELISA) which is the most widely used immunoassay configuration. The ELISA test is used for the detection and quantification of antigens, antibodies, and other molecules. Besides its applications in clinical diagnostics, ELISA has been used extensively for many research purposes. The antibody test, however, doesn’t say anything about the likelihood of future infection.

What is Antigen Test?

An antigen test is a rapid diagnostic test that detects the presence of a particular viral antigen, which implies that the person is currently infected with a pathogen. Antigen detection methods are used for rapid diagnosis in the clinics, emergency departments, or the central laboratory. Antigen tests offer a cheaper and faster method for diagnosing infectious diseases, such as the diagnosis of COVID-19. In fact, antigen tests have long been used to detect infectious agents that are difficult, slow, or hazardous to culture. The basic fundamental for antigen detection assays is the specific binding of an antigen (protein or glycoprotein) to an antibody. Antigen assays are relatively more economical than molecular techniques but often less sensitive than other methods. Because of their low sensitivity, they often produce false negative results.

Difference between Antibody and Antigen Test

Function

 – Antibodies, also referred to as immunoglobulin, are a part of the body’s defense system that protects the body from foreign particles such as microorganisms and viruses. These are special protein molecules produced by the immune system in response to the foreign particles called antigens. To fight an infection or virus, the immune system custom makes an antibody that attaches onto the antigen. The function of antibodies is to recognize bad agents called antigens and then trigger further chemical reactions in the body to fight the antigens. Antigens are foreign particles that are capable of triggering an immune response.

Testing

 – Antibody tests detect the presence of antibodies in the blood or serum to see if an individual has been exposed to an infection. They look for the presence of a host response to the virus. They do not determine whether or not that individual has the infection, but may tell you that he/she has been infected with the virus at some time in the past. An antigen test, on the other hand, is a rapid diagnostic test that detects the presence of a particular viral antigen, which implies that the person is currently infected with a pathogen.

Reliability 

– The basic antibody test is an Enzyme-linked immunosorbent assay (ELISA) which provides 95 percent specificity. The ELISA is used for the detection and quantification of antigens, antibodies, and other molecules. However, the problem with antibody tests is that they do not become positive until about two weeks after symptoms have started. Also, it says nothing about likelihood of infection in the future. Antigen tests, on the other hand, detect the presence of protein or glycoprotein in a virus, which implies current viral infection. Antigen tests offer a cheaper and faster alternative to diagnosing infectious diseases, providing test results rapidly. 

Antibody vs. Antigen Test: Comparison Chart

Summary of Antibody vs. Antigen Test

In a nutshell, antibody tests detect the presence of antibodies to a virus or pathogen and this does not determine whether or not the individual has the virus. The antibody test suggests that the individual had a past infection. However, the individual with confirmed positive test results must be considered infected and capable of transmitting the infection to others. A negative test result indicates that the individual was not infected with the virus or was infected but his/her body had not developed the antibodies yet. It does not say anything about the likelihood of infection in the future. Antigen tests are rapid diagnostic tests that detect the presence of a particular viral antigen, which implies current infection. However, antigen tests have low sensitivity and therefore they often yield false negative results.

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