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Serologic testing: purposes of use
Last reviewed: 05.07.2025
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All serological reactions are based on the interaction of antigens and antibodies. Serological reactions are used in two directions.
The first direction. Detection of antibodies in the blood serum of the subject for diagnostic purposes. In this case, of the two components of the reaction (antibodies, antigens), the unknown components are the blood serum components, since the reaction is carried out with known antigens. A positive reaction result indicates the presence of antibodies in the blood homologous to the antigen used; a negative result indicates their absence. Reliable results are obtained by studying paired blood sera of the patient, taken at the onset of the disease (3-7th day) and after 10-12 days. In this case, it is possible to observe the dynamics of the increase in antibodies. In viral infections, only a fourfold or greater increase in the antibody titer in the second serum has diagnostic value.
With the introduction of the ELISA method into laboratory practice, it became possible to determine antibodies in the blood of patients belonging to different Ig classes (IgM and IgG), which significantly increased the information content of serological diagnostic methods. During the primary immune response, when the human immune system interacts with an infectious agent for the first time, predominantly antibodies belonging to IgM are synthesized. Only later, on the 8th-12th day after the penetration of antigens into the body, IgG antibodies begin to accumulate in the blood. During the immune response to infectious agents, IgA antibodies are also synthesized, which play an important role in protecting the skin and mucous membranes from infectious agents.
The second direction. Establishing the genus and species of the microorganism. In this case, the unknown component of the reaction is the antigen. Such a study requires setting up a reaction with known immune serums.
Serological studies do not have 100% sensitivity and specificity in relation to the diagnosis of infectious diseases, and may produce cross-reactions with antibodies directed to antigens of other pathogens. In this regard, the results of serological studies must be assessed with great caution and taking into account the clinical picture of the disease. This is the reason for the use of multiple tests for the diagnosis of one infection, as well as the use of the Western-blot method to confirm the results of screening methods.
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