Why Titre Testing doesn't give you the full picture

Borrowed (with permission)

Thank you to Paul Seamons for this post

"Titre testing and immunity
I will try and explain Chris. When you do a titre test, the antibodies for the 3 basic core diseases are checked (Distemper, Hepatitis (Canine Adenovirus) & Parvovirus).
The level of circulating antibodies are then scored, with Vaccicheck these range from 0-6, with 6 being the strongest level of antibodies and 0 being the least.
Officially, provided a dog has a score of 1-6 for each disease, they have a protective level of antibodies recorded, but it's not that simple.
Dogs (and all other animals) have 2 types of immune system - the humoral system, which is what is being checked when a titre test is performed and the mediated immune system, which is basically the part of the immune system that has long term memory of diseases and can quickly instruct the humoral immune system to produce antibodies when a threat is detected.
The titre test only checks antibodies produced by the humoral immune system, so to have a 'score', there has to be antibodies circulating at a high enough level to be detected.
In an older dog (ie not a puppy), it is quite possible that a dog may be protected against a disease, but because the immune system has not recently been 'threatened' by a disease, there are no circulating antibodies in the system. A titre test may therefore record a score of '0' indicating that a 'booster' is required, but in fact the dog has perfectly adequate protection against that disease, but it is currently dormant, due to no present threat. It is unfortunately not possible to check the mediated immune system with a titre test, so this test will never be as accurate as people mistakenly think it is. Think of the humoral immune system as being a fire engine - when there is a fire, the fire engine is despatched from base to deal with the fire, it is not constantly driving around the streets hoping to find a fire in progress. The immune system works in a similar way, it is dormant until a threat is detected, then antibodies are rapidly produced to deal with the intruders.
When a titre test is done, it is very common that the Distemper level is recorded as '0', as there is almost no Distemper in the environment nowadays, and therefore the dogs immune system is not circulating antibodies against the disease. This does not equate to zero immunity!
The best use of a titre test is within a couple of months of a puppy being given its vaccinations. A strong immune response would be expected to confirm a protective level of circulating antibodies."