You might have heard your veterinarian say, “Let’s run a titer on him,” when referring to your horse and whether he’s protected against disease, or to figure out what might be causing particular clinical signs. What exactly does “titer” mean?
In immunological terms, titer refers to the concentration of specific antibodies in a blood serum sample. The titer is determined by serially (repeatedly) diluting the serum and assaying (quantitatively analyzing) each dilution for the activity (for example, how many virus-neutralizing antibodies are present). The last dilution of a sample that responds in the assay determines the titer.
The greater the concentration of the specific antibody you’re looking for in the serum sample, the higher the titer.
This information has several important uses. In clinical practice it can be used to determine if an individual has been exposed to an infectious agent. Prior to exposure the antibody titer would be very low or undetectable. Following exposure the immune system produces antibodies, resulting in an increase in the titer. Rising antibody titers, as determined using paired sera samples collected days or weeks apart, provide evidence for exposure to the infectious agent. This is particularly useful when it is not possible to isolate the agent for identification, as is the case for Potomac horse fever (you cannot isolate Neorickettsia risticii, the bacterium that causes Potomac horse fever).
Very high antibody titers are also useful in the diagnosis of purpura hemorrhagica (an immunologically mediated condition characterized by swelling of the limbs and widespread skin hemorrhages, varying in severity from a mild transient reaction to a severe fatal condition) and metastatic abscesses caused by Streptococcus equi, the bacterium that causes strangles infection.
Another use of titer is to determine vaccine efficacy. Protection from inf