Overview

If you were exposed to ticks and you get an expanding, circular rash (erythema migrans), your doctor may treat you for Lyme disease without doing a blood test. Blood tests done in the first few weeks may not show Lyme disease even when you have the illness. This is because the body's immune system responds slowly to the bacteria that cause Lyme disease. Blood tests may not detect antibodies to the disease even though the infection may be present. So a negative test result at this stage does not rule out a diagnosis of Lyme disease.

Blood tests may be used if you have symptoms of later Lyme disease, such as problems involving the joints, heart, or nervous system. But commonly done blood tests may not be able to tell the difference between an active infection and a past infection that was treated and cured. A polymerase chain reaction (PCR) test can tell whether you have an active Lyme disease infection, but it is not available in many places.