Its a titer test.
From the article:
the practice of repeatly vaccinating an animal is neither a necessary nor a healthy one. The evidence is strong that immunity persists for years or for life from vaccines early in life, and the risk of chronic illness is significantly increased with vaccine repetition. So, if someone runs a titer test in place of vaccinating Spot, and Spot's titer is low, perhaps 6-8 years after his last vaccine, the recommendation is likely to be "Spot needs another round of vaccines to keep him safe." I'd like to show that this is a wrong line of thinking that will get a lot of animals unnecessarily vaccinated, and therefore, at greater risk for developing chronic disease.
Here is an article discussing pro and cons of titer.
Veterinary Vaccinations: The Fallacy of Titer Tests