An antibody titer is a type of blood test that determines the presence and level (titer) of antibodies in the blood. This test is carried out to investigate if there is an immune reaction triggered by foreign invaders (antigens) in the body.
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What is the purpose of an antibody titer?
Antibodies, or immunoglobulins, are specialized proteins produced by the immune system to identify and destroy foreign invaders, such as bacteria and viruses. An antibody titer blood test is done to determine the presence (qualitative) and amount (quantitative) of antibodies in the blood.
Also, the test is performed to determine if a person was infected by any pathogen in the past. In people with autoimmune disorders, such as systemic lupus erythematosus, the test is done to check the degree and extent of an immune reaction against the body’s own cells.
Regarding vaccination, the test gives an idea about the requirement of a booster dose. For people who have received vaccination against a specific disease, the test helps determine if the vaccine can provide full protection.
Determining the titer for neutralizing antibodies, which are specialized antibodies that bind pathogens and prevent them from spreading infection, is another important aspect to ensure protection against a particular disease.
Neutralizing antibody titers in the blood closely correlate with the protection provided by an effective vaccination. A long-term follow-up study on people receiving mumps vaccine has revealed that a neutralizing antibody titer of as low as 1:2 can ensure protection.
How an antibody titer is performed?
Antibody titer is a simple procedure that does not require any special preparation. While drawing the blood, a person may experience some level of discomfort ranging from stinging sensation to moderate pain. Slight bruising may also occur sometimes, which usually disappears easily. In some very rare cases, a person may experience excessive bleeding, fainting, hematoma formation, or infection, after the collection of a blood sample. Multiple punctures at the site of injection may also occur if the vein is difficult to locate.
For the test, a patient’s blood sample is diluted serially and incubated with known antigens to determine the presence of antibodies against these antigens. The number of dilutions to be prepared depends on the concentration of antibodies in the blood.
A higher number of dilutions are needed for a sample wherein the antibody concentration is very high. The test measures the highest dilution of the blood at which antibodies cannot be detected anymore.
How results of an antibody titer are interpreted?
The result of an antibody titer is generally presented as a ratio. For example, a ratio of 1:200 indicates that diluting one part of the blood sample to 200 parts of the diluent solution (saline) finally leads to an undetectable antibody level in the blood sample.
A successful antibody titer result depends on the type of antibody being detected. For example, immunoglobulin M (IgM) appears in the blood between 2 – 4 weeks post-infection, whereas immunoglobulin G (IgG) takes around 4 – 6 weeks to achieve a detectable blood level. Thus, an appropriate test timing is important for a successful antibody titer.
The normal values of an antibody titer depend on the type of antibody. If the testing is done to detect autoantibodies, the normal value should essentially be zero or negative. In the case of testing the efficacy of a vaccine, the normal test result depends on the definite value that is specific for that immunization.
An abnormal result usually depends on the type of antibody being tested. The most common causes of abnormal results include the presence of an autoimmune disease, weakened immune system, the inefficacy of vaccination, and infection caused by a pathogen.
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Is there any disadvantage of an antibody titer?
Certain things should be remembered while interpreting the test results. Having a high titer value does not always indicate an infection; similarly, having a low titer value is not necessarily associated with a low-grade infection or no infection.
An antibody titer can only detect free antibodies in the blood; the test does not allow detecting antibodies that are already bound to an antigen. Thus, a person with a severe infection may test negative or have a low titer because of higher numbers of antigen-antibody complexes and lower numbers of free antibodies in the blood.
In certain conditions, a low titer may essentially indicate the efficient removal of infectious pathogens by the immune system. In contrast, a high titer may simply be due to the presence of residual antibodies from a previous infection, or unsuccessful attempts to form antigen-antibody complexes.
References
Further Reading
Cardiovascular risks post-COVID-19 vaccination: Key findings