Re: Diagnostic testing for Lyme disease

Issue: BCMJ, vol. 58, No. 1, January February 2016, Page 9 Letters
By: Gerald J.M. Tevaarwerk, MD, FRCPC
Related To:

Diagnostic testing for Lyme disease: Beware of false positives

Thank you to Drs Kling, Galanis, Morshed, and Patrick of the BC Centre for Disease Control for providing some badly needed evidence to counter patients’ claims of better tests for Lyme disease, in their November article “Diagnostic testing for Lyme disease: Beware of false positives” [BCMJ 2015;57:396,399]. Not everyone understands that the accuracy of a test, investigation and also symptoms and signs, is made up of two characteristics: its sensitivity to diagnose disease (true positives) and its specificity to rule out disease (true negatives). These two characteristics move in opposite directions—as sensitivity goes up specificity goes down and vice versa.

Happily, they can be combined into a single number that expresses the discriminating power (or diagnostic power) of a test. Correcting sensitivity for its lack of specificity, and specificity for its lack of sensitivity, and adding the two numbers together provides a single, unitless number on a scale of 200. For the data shown in the article, the reference two-step testing yields a discriminating power of 94.6 (or 47.3%), while the alternative laboratory testing yields a discriminating power of only 2.7 (or 1.35%). Armed with these simple figures it should be easier to convince patients of the futility of looking elsewhere for better tests.
—Gerald Tevaarwerk, MD
Victoria

Additional reading
Tevaarwerk GJM. Measuring the efficacy and cost-effectiveness of laboratory tests. Ann R Coll Phys Surg Can 1995;28:217-220.

Gerald J.M. Tevaarwerk, MD, FRCPC. Re: Diagnostic testing for Lyme disease. BCMJ, Vol. 58, No. 1, January, February, 2016, Page(s) 9 - Letters.


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