It's All in the Timing For Lyme Disease Test
By Laura Johannes
You got bitten by a deer tick—now what? A Lyme disease blood test is used to look for evidence of infection with bacteria that cause the disease, according to laboratories that offer the test. Physicians say the test has poor accuracy until at least three weeks after the bite, but can be used if a patient has late-stage symptoms.
According to the Centers for Disease Control and Prevention, 27,444 cases of Lyme disease were reported in 2007. The disease is caused by the Borrelia burgdorferi bacterium and is often treated with antibiotics. It is spread by black-legged ticks, also known as deer ticks, and is most common in Connecticut, Massachusetts, Maryland, Minnesota, New Hampshire, New Jersey, Delaware, Pennsylvania, New York and Wisconsin, according to the CDC.
The most common test involves two tests from the same sample. It typically costs about $25 to $70 and is often covered by insurance. Based on CDC guidelines, the blood is usually analyzed first with an antibody test called ELISA. If it is positive or unclear, then a confirmatory test called the Western blot is done.
Allen C. Steere, a Harvard University scientist who led the team that discovered Lyme disease in 1975, says the two-tier test is "quite accurate" as long as you use it properly—which means for the most part, not too soon after the suspected infection. In a study published last year, Dr. Steere and his colleagues found that of 76 patients exhibiting a bulls-eye-shaped rash characteristic of early Lyme disease, only a third tested positive for the disease. The bulls-eye rash typically appears within a week or two of infection, according to the CDC. But three to four weeks later, two-thirds of the group tested positive.
Given the tests' low sensitivity in the early stages of Lyme disease, physicians recommend treating with antibiotics when the patient develops the rash or other clear symptoms.
Still, individuals bitten by a tick in an area where Lyme disease is prevalent often think they should be tested immediately. Many people go to their doctors and say "I got bit by a tick. Test me!" says Wisconsin researcher Edward A. Belongia, director of the Epidemiology Research Center at the Marshfield Clinic Research Foundation. Dr. Belongia is author of a 2004 study that found only 20% of 356 Lyme tests the scientists reviewed were clearly appropriate, based on the patients' symptoms. Doctors say it is also wrong to get a test after being treated for the disease, since the antibodies can linger for years in your blood.
Critics of the test, such as Raphael B. Stricker, a past president of International Lyme and Associated Diseases Society, a Bethesda, Md., group of doctors specializing in treating chronic Lyme disease, say the tests miss many patients with late-stage Lyme disease. These patients can often test negative, Dr. Stricker says.
Dr. Steere says that in his experience, the test is very effective in patients with late-stage manifestations of the disease, such as arthritis or nerve damage. He does his testing in a research lab at Massachusetts General Hospital. Accuracy may vary from laboratory to laboratory, Dr. Steere adds.
Some labs specializing in Lyme disease test for additional proteins they say are a sign of infection, and as a result some doctors believe they are more accurate. But Dr. Steere says the tests haven't been proven scientifically. The CDC Web site warns consumers to beware of labs offering nonstandard Lyme disease tests. The CDC encourages patients to "ask their physicians whether their testing was performed using validated methods."
You can get the tick tested, but doctors say that generally isn't necessary. Even if the tick has Lyme bacteria, the disease won't be transmitted to you unless the tick fed on you for 24 to 72 hours.
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