Common Test-Ordering Errors
Part 8: Overordered Microbiology Tests
Mumps, Measles and Varicella Serologies
When a patient has erythema migrans, laboratory tests are not generally recommended. In the context of tick exposure, the presence of erythema migrans is diagnostic in itself and patients do not require laboratory testing. Furthermore, since EIA screening tests are insufficiently sensitive in early stage disease (sensitivity is approximately 75% early in the disease), a false-negative result can occur, which would complicate the diagnostic process. Additionally, because Lyme serologies may also yield false-positive results (for example, IgM western blotting only requires 2 bands to be positive and 1 band can cross react with common urinary pathogens), the test should only be ordered in patients with symptoms and a high likelihood of a tick bite. Indiscriminate testing can result in a diagnostic dilemma and lots of confusion on the part of both physician and the patient. For additional information, I would refer you to MML's Hot Topic on Lyme disease.
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