Many times a person has been vaccinated against a particular disease but it is uncertain whether they still have sufficient immunity to fight off the disease. In that case, antibody titers can be performed to determine their level of immunity. These can also be done if the person claims to have had the disease.
Titers are often performed by first serially diluting the patient's serum and then testing the level of antibodies in each dilution. The image to the right is a reminder of how serial dilutions are performed. The example is of a 10-fold serial dilution.
One example of a test used to detect antibodies is an ELISA test—Enzyme-Linked Immunosorbent Assay. The principle of the ELISA test is summarized as follows:
- An antigen substrate specific to the antibody of interest is bound to a solid surface such as wells of a microtiter plate.
- When the patient's diluted serum incubates with this antigen, the antibodies will attach; unbound antibodies are removed during a wash step.
- Then an enzyme-labeled antibody to the patient's antibody is incubated with the antigen-antibody complex, after which a washing step is conducted to remove excess labeled antibody, and a substrate to the enzyme label is added.
- The enzymatic reaction will occur in the same proportion as the quantity of antibodies in the patient's serum.
- A person's titer is typically the reciprocal of the highest dilution where a reaction is detected; these are sometimes converted into international units and reported as such.
When antibodies are detected in higher dilutions, this usually indicates that the person's immunity is greater than that of someone whose antibodies reacted only in the lower dilution. However, each pathogen and each type of test will have different interpretations, which will be specified in the manufacturer's instructions for the test kit used.
