Serology is the study of blood serum and other bodily fluids that can help in the diagnose of different conditions. Serological tests can detect specific proteins in a patient’s sample, providing evidence that a person may have been exposed to a virus or other pathogens, potentially even if they did not have symptoms. Although serological tests are valuable, they alone may not be able to differentiate between an active infection and antibodies from a vaccine. Keep reading to learn more about serology antibody/antigen tests.
What is a serological test?
Serological tests are performed on the clear fluid that is separated after the blood clots, known as blood serum, as well as other bodily fluids. These tests examine the response of the immune system to microorganisms or other foreign substances, making them highly valuable in diagnosing infections and autoimmune diseases in patients.
Specifically, serological blood tests are used to detect antibodies against one or more microorganisms in a person’s blood. They are also known as serology antibody tests and can be utilised to determine if the person has been exposed to a particular microorganism earlier or currently has an infection.
Antibodies are proteins produced by the immune system in response to a foreign object or microbe. These proteins help in determining a person’s exposure to a microbe. In some cases, such as autoimmune conditions, the body generates antibodies against its own tissues. Serology lab tests can also determine a person’s blood type.
Although serological tests can detect the presence of antibodies, they cannot effectively differentiate between an active infection and a recent sensitization to the infection-causing microbe or the vaccine. Thus, their results may not be 100% reliable in confirming a diagnosis, and further testing may be required.
Generally, serological tests are conducted on blood samples (other bodily fluids may also be used in certain cases). A healthcare professional will collect the blood sample from the patient and send it to the laboratory for testing. The sample collection procedure for adults involves disinfecting the skin with a cotton ball dipped in clinical spirit, drawing blood using a fresh needle, and storing the sample away after naming it for processing later. Some pathology labs offer home collection for samples, which is extremely useful for patients who are unable to travel. The pathology laboratory then processes the sample and sends the report to the patient and sometimes to the concerned doctor in a few hours or days, depending on the type of test ordered.
Why are serological tests recommended?
If a pathogen or microorganism (such as bacteria, fungi, viruses, and parasites) enters the body through the mouth, nasal passage, or broken skin, the immune system kicks in and creates antibodies against it. The antibodies attach to the pathogen, deactivating them and rendering them useless. Serological lab tests can detect the presence of antibodies and certain antigens.
If a person has an autoimmune condition, the body attacks itself, creating health problems. Serological tests are recommended in such cases to arrive at a diagnosis and start treatment or palliative care.
Serology lab tests can diagnose multiple illnesses, including the following:
- HIV
- Brucellosis
- Measles
- Rubella
- Syphilis
- Fungal infections
- Amebiasis
Serological tests are often used for the following purposes:
- Identifying antibodies/antigens in a patient’s sample
- Investigating inefficiencies with a person’s immune system
- Checking compatibility for organ, tissue, and fluid transplantation
Types of serological tests
There are mainly two types of serological tests—primary and secondary. Primary serological blood tests detect the presence of antibodies in the serum of a patient’s blood sample, and secondary serological tests are used to confirm their presence and identify their type.
Primary serological tests serve both diagnostic and screening purposes, and they can provide a rapid diagnosis; secondary serological tests are generally used for diagnostic purposes only and usually take more time. Depending on the situation, the doctor will recommend a suitable test for the patient.
Antibodies are as diverse as antigens, and various serological tests are used to detect different antibodies. Here are some of the common serological tests:
Agglutination test
This type of serological test is used to identify antigens in a patient’s blood sample. The process involves adding a reagent containing antibodies specific to the suspected material to the sample. These antibodies in the reagent will bind to the antigens if the antigens being tested for are present in the sample and cause the particles to clump together.
Hemagglutination test
In this serological test, a specific amount of red blood cells is mixed with the patient’s sample and then incubated to check for the presence of antibodies. If the sample contains antibodies that are specific to the antigens, they will bind to the red blood cells and clump together.
Enzyme-linked immunosorbent assay (ELISA)
This type of serological test is used to evaluate the presence of specific antigens or antibodies in a patient’s sample. The ELISA test procedure involves the use of an enzyme-coated plate with specific antibodies that bind to the antigen or antibody in the testing sample. If the sample contains antigens or antibodies, the enzymes produce a coloured product.
Precipitin test
The precipitin test identifies antigens in a sample by mixing it with an antibody-containing reagent. If the antigen is present, it binds to the antibody and forms a visible precipitate under a microscope.
Western Blot test
This serological test is used to detect the presence of particular proteins in a complex mixture of proteins extracted from cells. This test involves different elements, including separation by size, transfer to a solid support, and marking up of target proteins using antibodies. A western blot test can also evaluate the amount of target proteins in a sample. It is often used to confirm an HIV diagnosis in a patient who has tested positive for ELISA. The accuracy of both of these tests combined is about 99.9% in diagnosing HIV.(8)(9)
Immunofluorescence Assay (IFA)
This test procedure involves the use of fluorescence dye to detect the presence of particular antibodies and measure their amount in a patient’s blood sample. This is usually helpful in diagnosing autoimmune conditions. The IFA is a standard virologic technique in serology that is used in HIV diagnosis; it may be used in combination with other tests to confirm a diagnosis.
Radioimmunoassay (RIA)
The RIA can measure the concentration of antigens by using antibodies. In the case of double-antibody RIA, the addition of a second antibody facilitates precipitation of the bound primary antibody. This serological test is sensitive and specific in nature, but it requires special handling and disposal of radioactive waste.
Additional information: Certain serological tests use a technology called flow cytometry to detect and measure particles in a sample. It can effectively detect the number of antibodies and measure the size and number of cells in a sample as well.
Interpreting test results
If the sample does not contain any antigen or antibody, it is highly indicative that the person does not have an infection, and the result is considered normal.
In the case of abnormal results, the test will show the presence of antibodies (indicating that either the patient has an infection or they were exposed to the pathogen earlier in life) and/or antigens. These are highly indicative of the patient having an infection. People who have an autoimmune disorder will also have abnormal test results. The presence of certain antibodies indicates that the person is immune to one or more antigens, and they will not fall ill if they are exposed to these pathogens in the future.
The next step after serological testing
If serological test results are inconclusive, the doctor may recommend additional tests or other diagnostic techniques to find the cause of a patient’s health problems. These can also help the doctor confirm a diagnosis. If the test results are conclusive, the patient is either discharged (in case of confirmed negative test results) or given a suitable care plan (in the case of confirmed positive test results).
The treatment after serological testing varies on the basis of the testing performed and the type of antibodies found. An individual’s immune response also influences their treatment. If the patient has an infection, they may be administered broad-spectrum antibiotics or other medication, depending on the diagnosis.
The importance of serological tests
Serological tests are extremely useful in diagnosing health conditions and monitoring infections during the recovery period. They also serve as an important tool for healthcare providers when observing a person with autoimmune conditions and certain allergies.
Serological tests can also detect if the person was previously exposed to a particular pathogen, which is extremely beneficial in managing and studying infectious diseases such as hepatitis, HIV, and tuberculosis. Healthcare providers can determine an appropriate course of action for a patient with a certain infection with the help of serological tests and monitor their progress over time.
Serological tests are also extremely useful in the treatment and management of health conditions like COVID-19. Rapid antigen serological tests can also help control the spread of infectious diseases.
