Laboratory testing can be an important aid in the diagnosis of Lyme disease. Proper use and interpretation of laboratory tests requires an understanding of the type of test, the stage of illness, and the underlying likelihood that the patient has the disease.
Like blood tests for many other infectious diseases, the test for Lyme disease measures antibodies made by white blood cells in response to infection. It can take several weeks after infection for the body to produce sufficient antibodies to be detected. Therefore, patients tested during the first few weeks of illness will often test negative. In contrast, patients who have had Lyme disease for longer than 4-6 weeks, especially those with later stages of illness involving the brain or the joints, will almost always test positive. A patient who has been ill for months or years and has a negative test almost certainly does not have Lyme disease as the cause of their symptoms.
Because all laboratory tests can sometimes give falsely positive results, it is important when faced with a positive result to consider the underlying likelihood that a patient has the disease. If a patient has not been in an area where Lyme disease is common or their symptoms are atypical, positive results are more likely to be false positives. Similarly, if a patient is tested numerous times and only rarely tests positive, it is likely that the positive result is a false positive.
Several laboratories offer "in-house" testing for Lyme disease using their own assays or testing criteria. Such in-house assays do not require evaluation or approval by the Food and Drug Administration. Because of the potential for misleading results, CDC and FDA recommend against using in-house assays whose accuracy and clinical usefulness have not been adequately validated and published in the peer-reviewed scientific literature. Ask your care provider about the validation of the tests being used.