American Board of Internal Medicine (ABIM) Certification Practice Exam

If a positive Western blot IgM result is found in a patient with Lyme disease symptoms ongoing for over 30 days, how should this result be interpreted?

1. True positive result for acute Lyme disease

2. False positive result

3. Indicates need for further testing

4. Signifies effective immune response

The correct answer is: False positive result

A positive Western blot IgM result in the context of Lyme disease symptoms lasting more than 30 days should be interpreted with caution. IgM antibodies typically develop soon after infection and are most reliable in the early stages of Lyme disease, typically within 30 days of onset. If symptoms have been present for more than 30 days, the presence of IgM antibodies is less indicative of acute infection. In cases of chronic Lyme disease or prolonged symptoms, it is more common for IgG antibodies to be present, which signify past exposure. Therefore, a positive IgM result in this specific context raises the suspicion of a false positive because it does not align with the expected immunological response timeframe to Lyme disease. This interpretation highlights the importance of correlating laboratory results with clinical findings and symptoms, reinforcing that acute infections should primarily show IgM reactivity within the early phase of the illness.