The diagnosis of Lyme disease, caused by the bacterium Borrelia burgdorferi, relies heavily on identifying the body’s immune response to the infection. Because directly culturing the slow-growing organism from a patient’s blood is not practical, clinicians depend on indirect blood tests that look for antibodies. The Western Blot (WB) is a specialized test in this process, but its accuracy is frequently debated among patients and medical professionals. Understanding the test’s function and its inherent limitations is key to interpreting its results in the context of a patient’s overall health.
The Role of the Western Blot in Diagnosis
The Western Blot (WB) functions as a confirmatory step within the Centers for Disease Control and Prevention’s (CDC) recommended two-tiered testing protocol. This system begins with a highly sensitive screening test, such as an Enzyme-Linked Immunosorbent Assay (ELISA) or Immunofluorescence Assay (IFA). If this initial screen is negative, no further testing is usually recommended.
If the first-tier test yields a positive or indeterminate result, the Western Blot is then performed on the same blood sample to confirm the findings. The purpose of this two-step process is to maximize diagnostic efficiency by using a sensitive initial screen followed by a highly specific confirmatory test. The WB is designed to verify that the antibodies detected in the first test are truly reactive to specific proteins of Borrelia burgdorferi.
The Western Blot assay works by separating the specific proteins, or antigens, of the Borrelia bacterium by size, measured in kilodaltons (kDa). The patient’s serum is then exposed to these separated antigens. If the patient has developed antibodies against the bacterium, these antibodies will bind to the corresponding antigens, creating visible bands on a test strip.
The test differentiates between two classes of antibodies: Immunoglobulin M (IgM), which is typically produced early in the course of an infection, and Immunoglobulin G (IgG), which appears later and generally persists for a longer duration. By identifying which specific antigens the patient’s IgM and IgG antibodies react to, the Western Blot provides a more detailed picture of the immune response.
Defining Test Accuracy
When evaluating the accuracy of any diagnostic test, two statistical measures are paramount: sensitivity and specificity. Sensitivity refers to the test’s ability to correctly identify individuals who actually have the disease, minimizing false negatives. Conversely, specificity is the test’s ability to correctly identify individuals who do not have the disease, minimizing false positives.
For the Western Blot, the test is engineered to be highly specific, particularly the IgG blot, which is considered its strength as a confirmatory tool. A high specificity is prioritized to prevent the misdiagnosis of Lyme disease in individuals who may have antibodies from other conditions, such as syphilis or certain autoimmune disorders, which can cross-react with Borrelia antigens. Studies suggest that when strict interpretive criteria are applied, the specificity of the IgG Western Blot can approach 100% in late-stage disease.
This focus on specificity comes at the expense of sensitivity, especially in the early stages of the infection. The overall sensitivity of the two-tiered protocol may be as low as 35% to 50% in patients with early localized Lyme disease. This means that a significant number of people who are genuinely infected may receive a false-negative result shortly after infection.
The sensitivity of the Western Blot improves as the infection progresses because the body has more time to mount a robust antibody response. For instance, in later stages, such as in patients with Lyme arthritis or neurological involvement, the sensitivity can rise to a range of 75% to 89%. The accuracy of the test is therefore highly dependent on the stage of the disease at the time the blood sample is collected.
Factors Influencing Test Reliability
The reliability of the Western Blot is significantly influenced by the timing of the test relative to the infection. Antibodies, which are what the test measures, do not appear immediately upon infection; the body requires a period of time to produce them, a process known as seroconversion. Testing too early in the course of the infection will frequently result in a false-negative due to a lack of detectable IgM or IgG antibodies.
Generally, IgM antibodies begin to appear in the bloodstream around two to four weeks after infection, while IgG antibodies follow later, typically four to six weeks or more post-exposure. The CDC criteria for a positive IgM result is therefore only applicable within the first 30 days of illness, as an IgM-positive result after this period may be considered a false positive or simply a non-specific reaction.
The interpretation of the test results is a major factor in reliability, as a positive result is not based on a simple presence of antibodies but on the detection of specific banding patterns. The CDC criteria define a positive IgM Western Blot as having at least two of the three specific bands (23, 39, and 41 kDa) present. For the IgG Western Blot, a positive result requires the presence of at least five of ten designated bands:
- 18 kDa
- 23 kDa
- 28 kDa
- 30 kDa
- 39 kDa
- 41 kDa
- 45 kDa
- 58 kDa
- 66 kDa
- 93 kDa
Variations in how laboratories interpret a faint band or deviations from these strict criteria can impact the final reported result, leading to inconsistent diagnoses. Furthermore, cross-reactivity with antibodies from other conditions, such as other spirochetal infections or viral illnesses, can cause non-specific binding to Borrelia antigens, potentially leading to a false-positive reading, particularly on the IgM blot. The Western Blot must always be interpreted in conjunction with a patient’s symptoms and history of tick exposure.