Introduction:
- A serological test is a laboratory test that involves analysing a sample of blood or other bodily fluid to measure the presence and concentration of antibodies in the body. Antibodies are proteins produced by the immune system in response to the presence of foreign substances, such as viruses or bacteria.
- Serological tests are commonly used to detect and diagnose infections, such as HIV and COVID-19. They can also be used to determine whether an individual has been previously exposed to a particular infectious agent and has developed immunity to it.
- There are several different types of serological tests, including enzyme-linked immunosorbent assay (ELISA), Western blot, and immunofluorescence assay (IFA). These tests can be performed on a variety of samples, including blood, serum, and plasma, and are typically conducted in a laboratory setting by trained professionals.
- It’s important to note that serological tests are not always accurate, and a positive result does not necessarily mean that an individual is currently infected with a particular pathogen. Additional testing, such as a PCR test, may be needed to confirm a diagnosis.
Principle:
The principle of a serological test is based on the ability of the immune system to produce specific antibodies in response to the presence of foreign substances, such as viruses or bacteria. When the body is exposed to an infectious agent, the immune system recognizes the agent as being foreign and mounts an immune response by producing antibodies that are specific to the agent.
In a serological test, a sample of blood or other bodily fluid is analysed to detect the presence and concentration of these antibodies. The sample is typically incubated with an antigen, which is a substance that triggers the immune system to produce antibodies. If the sample contains the corresponding antibodies, they will bind to the antigen and can be detected using a variety of techniques, such as an enzyme-linked immunosorbent assay (ELISA) or a Western blot.
The results of a serological test can be used to determine whether an individual has been exposed to a particular infectious agent and has developed immunity to it. They can also be used to help diagnose infections, such as HIV and COVID-19. However, it’s important to note that serological tests are not always accurate, and a positive result does not necessarily mean that an individual is currently infected with a particular pathogen. Additional testing, such as a PCR test, may be needed to confirm a diagnosis.
Types:
There are several different types of serological tests that are commonly used to detect and diagnose infections:
Enzyme-linked immunosorbent assay (ELISA)
This is a common type of serological test that is used to detect the presence of specific antibodies in a sample. It works by attaching a specific antigen (a substance that triggers the immune system to produce antibodies) to a solid surface, such as a plate or a strip of paper. The sample is then added to the plate, and if the sample contains the corresponding antibodies, they will bind to the antigen. The presence of the antibodies is then detected using an enzyme that is specific to the antibodies.
Western blot
This is a more sensitive test that is used to confirm the presence of specific antibodies in a sample. It works by separating proteins in the sample using a technique called electrophoresis, which separates the proteins based on their size and charge. The separated proteins are then transferred to a membrane, where they are incubated with specific antibodies that are labelled with a detectable enzyme. If the sample contains the corresponding antibodies, they will bind to the labelled antibodies, and the presence of the antibodies is detected using the enzyme.
Fig: Types of various serological Tests
Immunofluorescence assay (IFA)
This test involves labelling antibodies with a fluorescent dye and adding them to a sample. If the sample contains the corresponding antigens, the labelled antibodies will bind to them and produce a fluorescent signal that can be detected under a microscope. This test is often used to detect the presence of specific antibodies in tissue samples.
Immunoblotting
Antibodies (or other particular ligands in similar techniques) are used in immunoblotting techniques to identify target proteins among a large number of unrelated protein species. They involve identifying protein targets through antigen-antibody (or protein-ligand) specific reactions.
Blotting is the process of transferring biological materials from a gel to a membrane and detecting them on the membrane’s surface. Towbin et al. introduced western blotting which is also known as immunoblotting since an antibody is employed to specifically detect its antigen in 1979. It is now a standard technique for protein analysis. The specificity of the antibody-antigen interaction allows a target protein to be detected in the midst of a complicated protein mixture. Western blotting can generate both qualitative and semi-quantitative data on the protein of interest.
In some infections, such as those caused by the human immunodeficiency virus (HIV), antibodies against specific virus components are more informative than other less-specific antibodies, and they can be detected using immunoblotting techniques.
Precipitation
This test is laboratory test that use the precipitation reaction to detect the presence of antibodies in a sample. These tests can be used to determine if a person has been infected with a particular virus or other pathogen in the past, as the body produces antibodies as part of the immune response to an infection.
In a precipitation serological test, a sample of the person’s blood is mixed with a solution containing an antigen, which is a substance that triggers the production of antibodies. If the person has antibodies to the antigen in the sample, they will react with the antigen and form a precipitate, or a visible solid substance. The presence of a precipitate indicates that the person has been infected with the pathogen in the past.
Agglutination
This test uses the agglutination reaction to detect the presence of antibodies in a sample. These tests can be used to determine if a person has been infected with a particular virus or other pathogen in the past, as the body produces antibodies as part of the immune response to an infection.
In an agglutination serological test, a sample of the person’s blood is mixed with a solution containing an antigen, which is a substance that triggers the production of antibodies. If the person has antibodies to the antigen in the sample, they will react with the antigen and cause it to clump together, or agglutinate. The presence of agglutination indicates that the person has been infected with the pathogen in the past.
Rapid diagnostic tests (RDT)
These are simple, point-of-care tests that can be used to quickly detect the presence of specific antibodies in a sample. They often use a strip of paper or a small plastic device that contains an antigen and is coated with a specific antibody. The sample is added to the strip or device, and if the sample contains the corresponding antibodies, they will bind to the antigen and produce a visible reaction, such as a change in colour.
Applications:
Serological tests are commonly used for a variety of applications, including:
Infectious disease diagnosis: Serological tests can be used to detect and diagnose infections, such as HIV, hepatitis, and COVID-19. These tests can help determine whether an individual has been exposed to a particular infectious agent and has developed immunity to it.
Fig: COVID-19 Serologic Diagnostic Test through Antibody Detection
Blood typing: Serological tests can be used to determine an individual’s blood type, which is important for blood transfusions and organ transplantation.
Allergy testing: Serological tests can be used to detect allergies by measuring the presence of specific antibodies in the body.
Autoimmune disease diagnosis: Serological tests can be used to detect the presence of autoantibodies, which are antibodies that the body produces against its own tissues. The presence of autoantibodies can help diagnose autoimmune diseases, such as lupus and rheumatoid arthritis.
Pregnancy testing: Serological tests can be used to detect the presence of human chorionic gonadotropin (hCG), a hormone produced during pregnancy, in a woman’s urine or blood.
Drug testing: Serological tests can be used to detect the presence of drugs or other substances in the body.
Research: Serological tests can be used in research to study the immune response to specific pathogens or vaccines.
Limitations:
There are several limitations of serological tests that should be considered:
Timing: Serological tests can only detect the presence of antibodies, which may not appear until several days or weeks for the body to produce enough antibodies to be detected by the test. This means that the test may not be accurate in the early stages of infection.
False negatives: Serological tests can produce false negatives, particularly in the early stages of infection when antibody levels are low. This means that the test may not detect the presence of antibodies even if the person is infected.
False positives: Serological tests can also produce false positives, meaning that the test may detect the presence of antibodies even if the person is not infected. This can be due to cross-reactive antibodies, which are produced in response to other infections or vaccines.
Limited specificity: Some serological tests may not be specific to a particular pathogen and may detect the presence of antibodies to related pathogens. This can lead to incorrect test results.
Limited sensitivity: Some serological tests may not be sensitive enough to detect low levels of antibodies, which can lead to false negatives.
Limited durability: The level of antibodies in a person’s blood can vary over time and may decline after the infection has resolved. This can make it difficult to accurately determine the timing and severity of an infection based on serological test results.
Variability: The performance of serological tests can vary depending on the quality of the test kit and the laboratory performing the test. This can lead to inconsistent results.
Interpretation: Interpreting the results of serological tests can be challenging and may require expert interpretation.
False negatives: A false negative result occurs when the test fails to detect the presence of antibodies in an individual who is actually infected with a particular pathogen. This can occur if the individual’s immune system has not yet produced a sufficient number of antibodies to be detected by the test, or if the test itself is not sensitive enough to detect the antibodies.
False positives: A false positive result occurs when the test detects the presence of antibodies in an individual who is not actually infected with a particular pathogen. This can occur if the individual has been vaccinated against the pathogen and has developed immunity to it, or if the test is not specific enough to distinguish between different types of antibodies.
References:
- Burrell, C.J., Howard, C.R. and Murphy, F.A., 2016. Fenner and White’s medical virology. Academic Press.
- Vainionpää, R., Waris, M. and Leinikki, P., 2015. Diagnostic techniques: serological and molecular approaches. Reference module in biomedical sciences.
- https://www.britannica.com/