The Western blot test for Lyme disease is usually used following a positive or indeterminate ELISA (or IFA) test which is more sensitive, but less specific, than the Western blot. This type of test is a serological assay for detecting antibodies to Borrelia bacteria that cause Lyme disease. Results from a Western blot test are usually reported in terms of titers which is a measure of the amount of dilution a sample needs before no antibodies remain detectable. A high titer means, therefore, a high level of antibodies and is likely to indicate a major response by the body to a current or recent infection. Western blot can be performed via venipuncture or by spinal tap (lumbar puncture) to test the cerebrospinal fluid. Whilst a positive CSF result is considered more accurate than a blood test result, it may be more difficult to isolate the antibodies in the CSF making false-negatives more likely with this method.
A Western blot may be performed to detect immunuglobulin M, the first and largest type of antibody produced in response to infections, immunglobulin G, the smallest but most prolific antibody, or both depending on the timing of the test. IgM antibodies are usually present at sufficient concentrations to be detectable two to four weeks after infection. IgG antibodies can take around two months to reach peak levels, but are usually detectable after four to six weeks. Early testing within just a week or so of a tick bite is unlikely to prove useful in establishing the presence of infection, with positive results likely indicating a previous infection rather than a current active case of Lyme disease. In later stages the IgM levels drop (usually after four to six months) to very low, if not undetectable rates. IgG levels also drop following treatment and infection eradication, but there will usually remain a detectable, but low, level of IgG antibodies in patients who have been previously infected at least for a short while. This is the body’s way of building up a resistance to infectious illness, with a quick response possible to recurrent infection due to the presence of such immune-system ‘memory’ cells. In the case of Lyme disease however, it does not appear that people develop immunity to the infection, despite repeated exposure to the Lyme disease bacteria.
Timing for Western Blot Test
Western blot IgM and IgG testing occur for patients with samples drawn within four weeks of disease onset, with IgG testing alone for samples taken after four weeks. This is due to the significant rate of false-positives in later-stage Western blot IgM testing. According to the Centers for Disease Control guidelines, a Western blot test cannot be intermediate, it must be either positive or negative. Misinterpretation of a Western blot result is one of the major factors in the over-diagnosis of Lyme disease, along with incorrect identification of erythema migrans (the Lyme disease rash), and the attribution of non-specific symptoms to Lyme disease.
LYMErix and Western Blot Test
The Western blot test is particularly at risk of misinterpretation for those patients who were vaccinated with the Lyme disease vaccine, LYMErix during its brief period on the market. This is because the outer surface protein A band is likely to show up as positive on a Western blot in such patients due to the vaccine containing a recombinant form of OspA lipoprotein of Borrelia burgdorferi. Physicians are usually advised to ignore the OspA band in patients previously vaccinated with LYMErix.