Oh, sorry. It's a blood test to find out if a dog is still carrying immunity to whatever disease he was originally immunized against. For example, there's a parvo titre to see if your dog is still immune to parvo.
As the vet explained to me, most immunizations last in a dog's body anywhere from 2 years to 10 years, depending on the immunization and the individual dog. That's why it's not necessary to do annual immunizations (and can be deadly if the dog gets too many and his/her immune systems 'flips' and starts attacking healthy blood cells. I lost my female Newf a year and a half ago from that
). The results of a titre will give the percentage of protection against that disease. Last year, Cowboy was at about 75% if I remember correctly which is still considered protected.
The down side has always been that if you had to pay for the titre and pay for the immunization, it was double the cost. Obviously, now, though that's changed if in fact each titre is over $100!!