Titers only tell us only that an animal has mounted an immune response and produced antibodies. They don't tell us whether the animal will be able to fight off disease.

Here is what the AVMA Council on Biologics and Therapeutics has to say in their most recent report (2002):

"COBTA's review did not focus on serologic testing [titers] per se, but the council did conclude that it is currently impossible to determine the immune status of an animal relative to all the infectious diseases of concern without conducting challenge testing [in other words, without trying to actually infect the animal with the disease]. Further, serologic results [titer tests] do not appear to be a sensitive indicator of immune response for some diseases or vaccines in cats and dogs. It was concluded that there are variations within and among laboratories, as well as a lack of validated sensitivity, specificity, and confidence intervals, leading to the conclusion that serologic testing is generally unreliable. It is noted that USDA licensing of serologic tests does not require validation of this type.

COBTA agrees that higher serologic titers are generally associated with greater resistance, but acknowledges it is possible for an animal with no titer for a specific organism to have solid resistance to challenge. Conversely, an animal with a titer that is generally regarded as protective for a specific organism may also become ill as the result of challenge, possibly because of overwhelming exposure or immune suppression."

Scientifically, a pet can have a zero titer and still resist infection. It can have a high titer and still become infected. So what does the "titer" number really mean? Titers are a measure of the number of antibodies per volume of blood, and that's all. They do not reflect immunity to disease.  (Remember back to the page on humoral and cell-mediated immunity. An animal can have lots of antibiodies but lack cell-mediated immunity and become infected. An animal can also have no antibodies but lots of cell-mediated immunity, and be resistant to infection. It depends on the organism and its particular weaknesses.)

Titers are a valuable tool in one circumstance - when we are trying to determine whether an animal has been vaccinated before (as in the case of a stray animal with an unknown vaccine history). I will also do titers in some cases in pets that have had appropriate vaccinations previously, but where we want to have some reassurance that the animal probably has mounted an adequate immune response. This might include cases where puppies were vaccinated at a very young age, or in a pet that might have been immunosuppressed at the time of vaccination. Sometimes we will titer test new patients when we want to change from an annual schedule that the previous vet had advocated to a three-year protocol, and the owner needs some reassurance that "protection" is there.

In most cases, though, titers give a false sense of security. There is absolutely no consensus as to when to revaccinate an animal based on titer tests. Do you revaccinate if the titer gets under 1:50? 1:20? 1:5? Zero? Since there is no way to correlate the number with actual protection, there is no "magic" titer number at which we vaccinate.

If it doesn't tell us whether the dog is protected, and doesn't tell us whether he needs a booster vaccination, a titer test is pretty much a waste of money. We get a result, but the result is of little practical or scientific value.