How is Lyme disease diagnosed
What diagnostic tests are available for Lyme disease?
In addition to the clinical diagnosis (tick bite in an endemic area followed by ECM), serologic antibody testing is commonly used. Detection of diagnostic levels of antibodies to B. burgdorferi is performed by a two-tiered approach of EIA/IFA for screening, followed by Western blot testing for confirmation. It is recommended for these tests to be performed in a reference laboratory and interpreted according to Centers for Disease Control criteria
Within 2 to 4 weeks following infection, 70% to 80% of patients will be IgM-positive by screening EIA or IFA tests. IgM antibodies peak at 6 to 8 weeks. IgG antibodies by screening EIA/IFA are positive after 4 weeks and peak at 4 to 6 months after infection. It is important to note that serologic testing by EIA and IFA is unreliable within the 1st month of disease onset (risk of false-negative results). Therefore, in a patient with acute disease (less than 1 month duration), IgM and IgG antibody responses should be measured in both acute and convalescent sera, and patients with obvious ECM should be treated empirically at onset regardless of the results from the initial sample. Both IgM and especially IgG antibodies can remain positive for years after successful therapy with antibiotics. Therefore, persistent IgM antibodies following adequate antibiotic therapy do not indicate active or inadequately treated infection.
The Western blot analysis should be ordered in patients with a positive screening test by EIA or IFA. This test can be used to detect antibodies against specific Borrelia proteins in the patient’s serum and may discriminate between cross-reacting antibodies from other sources and those directed toward Borrelia proteins (see Question 13). Western blot analysis is more specific but less sensitive than enzyme-linked immunosorbent assay (ELISA) for diagnosing Lyme disease. After the 1st month of infection, only the IgG Western blot should be used to support the diagnosis ( Fig. 39.3 ). If after the 1st month of infection, only the IgM Western blot is positive and not the IgG blot, then the IgM blot is likely to be a false-positive result. Western blot positivity is interpreted as follows:
- • The IgM Western blot is positive if at least two of the following three bands are present (1st month of infection): 23 (OspC), 39 (BmpA) or 41 (Fla) kDa
- • The IgG Western blot is positive if 5 of the following 10 bands are positive (after the 1st month of infection): 18, 21, 28, 30, 39, 41, 45, 58, 66, or 93 kDa.
The sensitivity of two-tiered testing is 20% to 50% during the acute phase and 70% to 80% during the convalescent phase. Virtually 100% of untreated patients will have positive IgG responses after 4 to 8 weeks. A positive result in the right clinical setting is strong evidence for infection, whereas a negative result is strong evidence against an infection. In July of 2019 the FDA approved the use of concurrent or sequential EIA testing as a method of diagnosing Lyme disease (as an alternative to a two-test methodology utilizing Western blot technique for the 2nd assay).
Polymerase chain reaction (PCR) analysis has a limited role in the diagnosis of Lyme disease. PCR can detect B. burgdorferi DNA in the skin, spinal and synovial fluid, and synovial tissue, but the sensitivities are low and/or uncertain in each of these settings. A positive PCR result from any bodily fluid should be considered a false-positive in a patient with negative serologic testing (EIA/IFA and Western blot). Finally, because Borrelia DNA can persist after the organisms have been killed, a positive PCR result does not necessarily indicate active infection.