An antibody titer measures the concentration of specific antibodies in a blood sample. Antibodies are proteins generated by the immune system in response to foreign invaders like viruses or bacteria. These proteins identify and neutralize antigens. The level of these antibodies reflects the body’s past exposure to an antigen or a vaccine.
How Antibody Titer Is Measured
Measuring an antibody titer begins with a blood draw. The plasma, which contains antibodies, is separated for laboratory analysis. Titer measurement involves serial dilution, where the plasma is repeatedly diluted.
For instance, a sample might be diluted 1:10, then 1:20, followed by 1:40, and so on. Each diluted sample is then tested for the specific antibody. The “titer” is the highest dilution factor at which the antibody is still detectable. A result of 1:200, for example, means the antibody was detectable when one part of the blood sample was diluted with 200 parts of a diluent solution.
Interpreting Titer Results
Titer results provide insight into the immune response. A high titer indicates a strong immune response, suggesting a recent or ongoing infection, or immunity from vaccination. Conversely, a low titer could mean no exposure to the pathogen, that a past infection occurred long ago and antibody levels have decreased, or that a vaccine was ineffective.
The type of antibody measured also offers clues about the timing of exposure. Immunoglobulin M (IgM) antibodies are among the first to appear, indicating a recent or current active infection. Immunoglobulin G (IgG) antibodies develop later and provide longer-term immunity, indicating a past infection or vaccine protection. A low titer does not always rule out a severe infection, as antibodies might be bound to antigens and not freely circulating for detection.
Clinical Use of Titer Testing
Antibody titer testing serves several practical purposes in healthcare, providing information for various medical decisions. One common application is assessing vaccine efficacy, particularly for diseases like measles, mumps, and rubella (MMR) or hepatitis. These tests confirm whether a vaccine has successfully prompted the body to produce protective antibodies, indicating established immunity. This can be particularly useful for individuals without complete vaccination records, such as those immigrating or needing proof for school or employment.
Titer tests also assist in diagnosing certain infections or confirming past exposure to pathogens like chickenpox. By measuring antibody levels, healthcare providers can determine if someone has immunity to a disease, which helps guide decisions about vaccination or disease management. In some instances, a test can help clarify if symptoms are due to a current infection, especially when other diagnostic methods are inconclusive.
Furthermore, titer testing plays a role in monitoring autoimmune diseases, such as lupus or rheumatoid arthritis. In these conditions, the immune system mistakenly produces antibodies against the body’s own tissues, and titer levels can help track disease activity or progression. The tests are also used in screening for organ donation or blood transfusions to ensure compatibility and to check for the presence of infectious diseases in donors or recipients.