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September 13, 2004
Tick Bitten Patient

A 55 year old woman with stage 2 hypertension came in for a regular check up. Her hypertension was well controlled with medications, but during the visit she mentioned that she likes to work in her garden and that she had already been bitten by two ticks during the summer - one last week and one two months ago. She was concerned about Lyme disease and asked if there was some kind of test that could be done to see if she was infected. The doctor was reluctant to order the test, as the patient was asymptomatic. (The serum test for Lyme consists of an ELISA test for IgG and IgM against Borrelia burgdorferi, followed by a Western blot to confirm.)

Question: How useful is this test in this patient? Under what circumstances should a patient be tested for Lyme disease?

According to this article (1), the false positive rate is very high, especially in low-risk patients. In fact, the diagnosis of Lyme disease should be a clinical one and the serum test should be done only to confirm what is suspected clinically. This article cited another one (2), a review of literature, which recommended testing for Lyme only if the pretest probability of having the disease (do these words sound familiar?) is between 0.20 and 0.80, because if the pretest probability is low (ie. the patient has few clinical signs of Lyme disease), and the serologic test is positive, it is most likely a false result. So the doctor was right in explaining to the patient that testing her for exposure to Lyme disease in the absence of symptoms was ineffective.

References:

1. A Review of Lyme Disease

Brown SL, Hansen SL, Langone JJ. Role of serology in the diagnosis of Lyme disease. JAMA. 1999 Jul 7;282(1):62-6.

2. Laboratory Evaluation in the Diagnosis of Lyme Disease

Tugwell P, Dennis DT, Weinstein A, et al. Laboratory evaluation in the diagnosis of Lyme disease. Ann Intern Med. 1997;127:1109-1123.

3. More About Lyme Disease

 

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