How Antibody Titer Is Calculated

Get the Facts About Titer Calculations

Doctor wearing gloves handling vials of blood to perform an antibody titer test.
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The "titer" is the measurement of the amount of a substance in a solution. An antibody titer test is a blood test that doctors may use to help with diagnosis.

Titer often describes concentrations of biological molecules (i.e., bioproducts) such as antibodies and other proteins. The titer indicates the number of times a solution can be diluted and still contain detectable amounts of a particular molecule.

Serial Dilution

To calculate antibody titer, a blood serum sample containing antibody is diluted in serial ratios (1:2, 1:4, 1:8, 1:16...etc.). Using an appropriate detection method (e.g., colorimetric, chromatographic, etc.), each dilution is tested for the presence of detectable levels of antibody. The assigned titer value is indicative of the last dilution in which the antibody was detected.

Examples of Calculations

Say, for instance, that the antibody was detected in each of the tubes listed above but was not detected in a 1:32 ratio dilution. In this case, the titer is 16. However, if it is detected in the 1:2 and 1:4 dilutions, but not in others, the titer is said to be 4.

Why Doctors Test a Patient's Level of Antibody Titers

Doctors test patients' antibody levels to determine whether or not they have been exposed to an antigen or another foreign substance. When this happens, antibody levels rise, because the body enlists the aid of antibodies to attack and destroy the threatening foreign substance.

Physicians often decide to test patients' antibodies to determine if they've been exposed to common childhood infections like chickenpox. These tests will help determine if a patient needs a booster shot or if an already administered immunization has triggered a strong enough response. Titer tests can also indicate if the patient has had a recent infection.

According to the U.S. National Library of Medicine, antibody titer is also used to determine if the body has had an immune response to the body's own tissues, which happens to people afflicted with lupus.

What Are Normal Levels of Antibody Titers?

It's difficult to say what a normal level of antibody titers is, without knowing why they are being tested. However, if the level is negative, that outcome is helpful in excluding a diagnosis of certain medical conditions. And if the antibody titers are tested to determine if the body is attacking its own tissues, the normal level is zero.