Infectious Disease Advisor: When is testing warranted and what test or tests do you recommend first?

Dr Waddell: Diagnosing LD can be difficult because symptoms of LD vary from one person to another and can resemble symptoms of other infectious diseases spread by ticks. The earlier a diagnosis is made, the greater the chances of successful treatment. Individuals should consult a healthcare professional as early as possible if they experience symptoms or feel unwell in the weeks following a tick bite.

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The PHAC’s National Microbiology Laboratory (NML) assists clinicians in diagnosing LD using a 2-tiered testing method that includes an enzyme-linked immunosorbent assay (ELISA) screening test (a blood test that detects antibodies to Borrelia burgdorferi) followed by a confirmatory Western blot test. The Western blot test also identifies LD antibodies and can confirm the results of an ELISA test. The NML uses the 2-tiered testing method because it offers more accurate results.

Infectious Disease Advisor: If someone has the characteristic EM rash associated with LD, is laboratory testing necessary to confirm LD?

Dr Hinckley: During the localized (early) stage of illness, LD may be diagnosed clinically in patients who present with an EM rash. As with serologic tests for other infectious diseases, the accuracy of the test depends upon the stage of disease. During the first few weeks of infection, such as when a patient has an EM rash, test results are expected to be negative. However, several weeks after infection, available ELISA, enzyme immune assay, indirect fluorescent antibody assay, and 2-tier testing have very good sensitivity.

It is possible for someone who was infected with LD to test negative because:

  • Some people who receive antibiotics (eg, doxycycline) within the first few weeks after the tick bite may not develop antibodies or may develop them at levels too low to be detected by the test.
  • Antibodies against LD bacteria usually take a few weeks to develop, so results of tests performed before that time may be negative even if the person is infected. In that case, if the person is retested a few weeks later, results should be positive if they have LD. It is not until 4 to 6 weeks after an individual receives the tick bite that test results are likely to be positive. This does not mean that the test is bad — only that it needs to be used correctly.

It’s important to note that a rash like that indicative of LD has been described in humans following bites of the lone star tick, Amblyomma americanum. The rash associated with a bite from the lone star tick may be accompanied by fatigue, fever, headache, and muscle and joint pain. This condition has been named Southern tick-associated rash illness (STARI). The cause of STARI is not known.

Infectious Disease Advisor: Is 2-tier testing always advised when testing is required to confirm LD?

Dr Waddell: Like other infectious diseases in which 2-tiered testing is used (for example, infections with human immunodeficiency virus), screening tests followed by confirmatory tests provide the greatest level of true positives while minimizing the number of false negatives. Simply put, a 2-tiered approach provides the most accurate information about an infection compared with either test conducted alone.

However, all laboratory tests have a margin of error, which is why PHAC recommends that LD be diagnosed first and foremost on the basis of a doctor’s assessment of symptoms, especially in the early stages of infection. Laboratory tests, including those for LD, are routinely subject to internal and external quality control processes to ensure results are accurate.

Although the accuracy of blood tests increases as the infection progresses, it is recognized that a small percentage of patients with later-stage LD may test negative for the disease. The stage of infection and the possible effect of treatment on the outcomes of blood testing should be taken into consideration during diagnosis.

Infectious Disease Advisor: What should clinicians do when results of initial tests are inconclusive? Should they retest?

Dr Waddell: Further testing may be recommended when a patient:

  • Was exposed to black-legged ticks outside of an endemic area
  • Has symptoms consistent with LD
  • Had initial samples collected during early stages of the disease
  • Has not received treatment 

We suggest that a second sample be submitted 3 to 4 weeks after the original submission. This approach is most helpful in acute cases of LD, in which the first blood sample may have been drawn too early for antibodies to be present or if the symptoms are not absolutely consistent with LD. In either case, a follow-up sample will help rule LD in or out of the diagnosis.

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Infectious Disease Advisor: Both PHAC and the CDC recommend a 2-tiered testing process in a patient with suspected LD. What if results of the immunoassay are negative?

Dr Hinckley: If the test has been used appropriately and the results are negative, the health care provider should consider an alternative diagnosis.

Image Courtesy of the Center for Disease Control and Prevention.9

Infectious Disease Advisor: Are there other tests used for LD that the CDC has concerns about or emerging tests that you consider promising?

Dr Hinckley: CDC recommends the use of tests cleared by the US Food and Drug Administration (FDA) for LD. Laboratory tests that are not recommended are found here.10


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