Laboratory diagnostics: Serology

Serology is the study of blood serum and other bodily fluids. Serological tests may be performed for diagnostic and surveillance purposes and to monitor vaccine responses. Antibodies are typically formed in response to an infection, against introduced antigens (for example, to a vaccine), or to an animal’s own proteins (during auto-immune disease).

Serological tests are in vitro tests designed to detect the presence of antibodies in serum (antibody detection tests), or the presence of antigen using antibodies as detection reagents (antigen detection tests). In some cases the mixing of antibody and antigen gives rise to a visible reaction, in other cases some other secondary mechanism is needed to detect whether a reaction has taken place. Serology techniques that can be used include agglutination, precipitation, complement-fixation, ELISA and fluorescent antibodies tests.

Although some serological tests are qualitative tests, most provide a quantitative measure of the antibody present in the serum sample. The level of antibody present is usually critical to the interpretation of the test because in most tests, the level of antibody must exceed a critical level before the test is regarded as a positive test.

When a new test is developed an essential part of the process is the establishment of standards for interpretation. It is important to determine the cut-off (threshold) levels for classifying test results as negative, suspicious and positive. However, it is also important that test results are not just mechanically classified as positive or negative, according to test titres. Careful consideration should be given to the epidemiological principles that are important for interpreting the results of a particular case and the reason for which the test is being done.

The following topics will be covered in this module: