Investigations

Learning Bite:

In the presence of erythema migrans no investigations are warranted, the rash in itself is diagnostic.

However, in the absence of a rash, serological tests are required to detect the antibodies to the Borrelia bacteria that cause the disease.

A two-tier approach to this serological testing is taken (figure 4): an ELISA test is recommended as the initial investigation given its sensitivity. A more specific confirmatory test is then performed (immunoblot) if the ELISA test is positive or equivocal. Treatment should be initiated whilst awaiting investigation results if there is high clinical suspicion of Lyme disease.

However, there are concerns about the sensitivity of the ELISA test, which can range from 5-100% depending of the timing of the test. It can be falsely negative if done too early or if the patient is immunocompromised. Nevertheless, the specificity of the immunoblot test is high.

The limitations of these tests should be explained to patients so that they are aware of the implications of both false positive and false negative results and how these can occur.

If there is a suspicion of Lyme disease it is best to discuss with your local Infectious Diseases specialist, especially if there is organ involvement. The diagnosis should not be ruled out if there is high clinical suspicion even if the serological tests are negative.

Figure 4: Lyme disease: laboratory investigations and diagnosis

Tests for Lyme disease should only be carried out at laboratories that:

• Are accredited by the UK accreditation service (UKAS) and
• Use validated tests and
• Participate in a formal external quality assurance programme