The Bugs in Lyme Tests

If you head to your doctor's office with a suspected case of Lyme disease, this is what you might expect:

The doctor would first listen to your history of tick exposure and symptoms, such as muscular pains and headache. You'd then be checked for visible signs of infection, such as the telltale bull's-eye rash often, but not always, present in Lyme cases. If you had this rash, you probably would proceed straight to your local pharmacist for antibiotics, says the National Institute of Allergy and Infectious Diseases, and skip the next step: laboratory testing for Lyme.

The main problem with blood tests for Lyme disease, according to NIAID, is that they're not ideal in either their sensitivity (ability to detect the presence of infection) or their specificity (ability to differentiate Lyme from another infection).

What's more, because the tests detect only the presence of Lyme antibodies -- rather than the actual Lyme bacterium -- a positive response is not proof of an active infection; antibodies can remain in the blood long after an infection has ended, NIAID says.

There's more: Lyme bacteria incubate for anywhere from three to 30 days before causing infection; if you are tested soon after a tick bite, you may test negative because antibodies haven't had time to form.

As a result, "a history of having had a deer-tick bite, followed by the characteristic 'bull's-eye' lesion . . . with flulike symptoms is considered to be the most reliable diagnostic indicator of Lyme disease," according to NIAID.

To confirm a Lyme diagnosis, the Centers for Disease Control and Prevention recommends two blood tests performed by reputable labs: The first is an ELISA (enzyme-linked immunosorbent assay) to measure antibody levels -- a test considered highly sensitive to late-stage infection. The second test, the Western immunoblot assay, identifies antibodies particular to Lyme.

If a patient with Lyme-like symptoms for more than three weeks has both blood tests performed and both come back negative, "it's negative," according to CDC epidemiologist Paul Mead -- meaning the patient doesn't have Lyme.

The CDC and the Food and Drug Administration have warned about the proliferation of unapproved Lyme tests -- including blood and urine tests, some used by commercial labs.

Megan S. Smith is a Washington area freelance writer who conducted molecular genetics research for her master's in biology. Comments:health@washpost.com.