Serology tests uses various methods to measure and characterise antibodies, antigens, and other immunological substances in blood and other body fluids.
Antibodies play an important role in the innate and acquired immune responses. Levels of antibody production are increased in infective or inflammatory conditions, and we can utilise this phenomenon to diagnose disease and sometimes also to stage disease progression. The detection and characterisation of antibodies can indicate infection with specific pathogens (bacteria, viruses or fungi) which are difficult to culture or expensive to detect with other methods such as molecular tests. It can also be used to diagnose inflammatory conditions including autoimmune diseases, where antibodies are produced that may damage tissues. Antibodies (also called immunoglobulins) are mostly very specific. For instance, if a patient is infected with the influenza virus, the antibodies will be identifiable as against influenza, and can be distinguished from the type of antibody produced in response to another type of infection (e.g. measles).
There are different classes of these immunoglobulins e.g. IgM, IgG, IgA and IgE, and identifying this various classes of antibodies can help us to determine the stage or progression of disease.
When a patient is for instance infected by a microorganism for the first time, antibodies of the class IgM are initially produced. IgM antibodies are thus usually an indication of an acute infection. As time goes by a class switch takes place and IgG antibodies will be produced. During this phase one could find both IgM and IgG antibodies present in the blood of an infected patient. The IgM response tends to decline within approximately 1 to 2 months, although low levels can persist for a year or more in certain infections. IgG antibodies are much more long-lasting, and can persist for the life span of the patient. Serology tests can therefore determine whether a person is currently infected, or was perhaps previously infected by a certain organisms, or is already immune to these organisms. Serology can also be used to determine the blood type of a person by detecting certain proteins (antigens) on the blood cells of the person, and can help to determine whether a patient has an immune deficiency (when there will be a lack of antibodies).
According to this model, the following is usually true
|Organism-specific IgM||Organism-specific IgG||Status|
|Absent||Absent||Susceptible to organism|
|Present||Absent or present||Current or very recent infection|
|Absent||Present||Past infection *|
* This often, but not always, indicates immunity to subsequent infection
Many pathogen specific factors will also influence the interpretation of serology results, for instance the incubation period of the infection, as antibodies may still be absent at the time that a patient first presents with an infection. In contrast, in infections with long incubation periods (a month or longer), both IgM and IgG might be present at the onset of symptoms.
In reinfection and reactivation serologic diagnosis is even more complicated than initial infections. It is therefore important to recognise the difficulties associated with some serological tests, and to consult us for advise in the use and interpretation of these tests, as required.