Diagnosis
The veterinarian may be able to determine if a dog has Lyme disease after performing a blood test that will show exposure to the Borrelia burgdorferi bacteria.  In previously vaccinated dogs, a special type of blood test must be performed to differentiate between the vaccine and actual exposure to the organism.  Unfortunately, these tests do not provide a simple yes or no answer.  The veterinarian must evaluate the results along with the dog's symptoms, and take into account whether the dog has been in an area of the United States where Lyme disease is endemic, such as the northeastern states, the upper Mississippi region, California, and certain southern states.  A positive response to treatment is also important in making a final diagnosis.