When to have blood tests to diagnose Lyme disease

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. However, current blood tests cannot tell the difference between an active infection and a past infection that was treated and cured. Even at this later stage, a positive test does not always mean that you have active Lyme disease.