Lyme disease is becoming increasingly common in Canada, and Canadians with Lyme disease symptoms may seek diagnoses from laboratories in the United States, although many of the results will be false-positives, according to a commentary in CMAJ (Canadian Medical Association Journal).

"Patients with chronic subjective symptoms without a diagnosis can be vulnerable and desperate for an answer as to the cause of their illness," writes Dr. Dan Gregson, divisions of Medical Microbiology and Infectious Diseases, departments of Pathology and Laboratory Medicine, and Medicine, University of Calgary, Calgary, Alberta, with coauthors. "Giving them a false diagnosis based on flawed testing is misleading."

Canadians with nonspecific symptoms such as joint pain, fatigue and mental fog may turn to commercial US laboratories because they suspect they may have Lyme disease. Many of these laboratories use only a single test that relies on nonevidence-based interpretation, such as the Western blot test. A positive test result that relies solely on Western blot testing is most likely a false-positive.

Recent research has found false-positive results in people without Lyme disease at 3 of 4 commercial US laboratories (ranging from 2.5% to 25%), with a rate of more than 50% of false positives at one lab.

Tests at the National Microbiology Laboratory of Canada, which uses guidelines from the US Centers for Disease Control and Prevention (CDC), are as sensitive as those used in speciality US laboratories.

"Patients and physicians should be cautious in choosing a referral laboratory in the US when seeking 'second opinion' serology after receiving a negative test result in Canada," write the authors. "Laboratories that use the standard CDC two-tier testing algorithms should be preferred over those that report results based on unproven, unvalidated, in-house criteria."

Patients who are experiencing Lyme disease-like symptoms should receive a complete evaluation to determine the cause of their symptoms.