Serologic testing for Lyme disease is complex. Rational ordering and interpretation of these test results requires some understanding of the basic underlying principles and performance characteristics of the tests. The test results do not rule in or rule out Lyme disease; however, the results make a clinical diagnosis of Lyme disease more (or less) likely.
The most frequently used test is the enzyme immunoassay (EIA) or enzyme-linked immunosorbent assay (ELISA). Much less often used for this purpose is the immunofluorescent assay (IFA).
The principal limitation of these serologic tests has been the high frequency of both false-negative results and false-positive results. False-negative results occur during the acute phase of Lyme disease, when patients have not yet developed a sufficient antibody response to give a positive serologic test. Seroconversion can take as long as 6-8 weeks after a tick bite. The false-negative rate for ELISA is 32% in early disease.
A variety of diseases, including Rocky Mountain spotted fever, syphilis, systemic lupus erythematosus, and rheumatoid arthritis, can cause false-positive ELISA results. Also, a small percentage of the healthy population has positive test results with ELISA testing. For these reasons, confirmatory Western blot testing is recommended.
Patients with early Lyme disease who are treated with antibiotics may never develop positive titers. Of patients with early disseminated disease, 90% have a positive titer. Some patients with late disease are seronegative, but significant controversy exists regarding the frequency of late seronegativity. Most authorities suggest that this phenomenon is rare.
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