# How Antibody Titer Is Calculated

## Get the Facts About Titer Calculations

The titer is the amount, or the concentration, of a substance in a solution. The term is often used to describe concentrations of biological molecules (i.e., bioproducts), such as antibodies and other proteins. The titer is an indication of the number of times a solution can be diluted and still contain detectable amounts of the molecule of interest. In fact, when calculating titer, the numerical value assigned to a titer is a direct indication of the dilution factor.

### 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... and so on). 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. If that is the case, then the titer is said to be 16.

However, if it is detected in the 1:2 and 1:4 dilutions, but no others, the titer is said to be 4. Therefore, the titer is the degree to which the antibody serum solution can be diluted and still contain detectable amounts of antibody.

### Why Doctors Test a Patient's Level of Antibody Titers

Doctors will test patients' antibody levels to determine whether or not they have been exposed to an antigen or another substance the body construes as foreign.

When this happens, antibody levels rise because the body enlists the aid of antibodies to attack, and destroy, the threatening foreign substance.

Often, physicians will decide to test patients' antibodies to see if they've been exposed to foreign substances in the past. This includes common childhood infections.

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 with lupus.

A patient's level of antibody titers may also be tested to see if the individual needs a booster shot or to see if a vaccine given in the past helped protect the patient from a certain disease. Additionally, doctors test the antibody levels of patients to see if they've had a recent infection or, an infection in the distant past.

### What Are Normal Levels of Antibody Titers?

It's very difficult to say what a normal level of antibody titers is without knowing why they are being tested. The definition of normal depends on which medical condition a patient is being tested for. However, if the level is negative, that outcome can be used to exclude a diagnosis of certain medical conditions, which is indeed helpful. Also, if the antibody titers are being tested to determine if the body is attacking its own tissues, the normal level is zero.

In the end, any discussion of results needs to include the medical practitioner involved because different laboratories may have different normal ranges, and that can skew everything.