There is a great deal of misunderstanding, among patients and doctors, about what laboratory test for the diagnosis of Lyme disease actually measure and what constitutes a positive test result.
The most common, widely used tests simply measure antibodies against the Lyme disease bacterium, Borrelia burgdorferi. Since antibodies are produced by the body’s immune system to fight infection, detecting the presence of antibodies against bacteria or a virus is a good way to determine if someone has — or had– an infection. Some of the confusion about Lyme disease testing is due to the fact that different types of antibodies are produced at various stages of the infection. The type of antibody produced changes as the immune response to infection matures. Immunoglobulin M (IgM) develops first, during the first 7 to 10 days of infection; it is followed by the development of Immunoglobulin G (IgG), one to two weeks later. IgM is a less specific antibody that is quite a bit stickier than IgG. The stickiness of IgM makes tests that measure IgM less reliable and more likely to be falsely positive.
Why do we use IgM assays at all? It is because IgM is produced first. IgM can be found in people very early during infection, well before IgG antibody is produced. Within 1-2 weeks following the onset of infection, IgM antibodies to B. burgdorferi can be detected in the vast majority of infected individuals. However, after one month, IgG antibody responses predominate and there is no longer a need to depend on an unreliable assay based on the detection of IgM antibody. Thus, because of the limitations of IgM assays and the high rate of false positives, their use should be limited to the first month of infection; however, that frequently is not the case. A positive IgM test along with a negative IgG test after the first month of infection almost always results in a false positive test.
There are a number of different ways to measure antibodies against B. burgdorferi. The two most common procedures are by ELISA and Western blot. An ELISA is carried out on a plastic plate and measures the amount of antibody that binds to one or more B. burgdorferi proteins (antigens). A Western blot is like a bar code. Different kinds of B. burgdorferi proteins are separated by size on a strip of special paper and the antibodies in the patient’s blood bind specifically to the proteins on the paper. The binding antibodies are then colored and the bar code is read. People who have been infected with B. burgdorferi have antibodies against certain specific proteins, thereby resulting in a positive bar code. What constitutes a positive pattern was established by the CDC in collaboration with many experienced physician scientists from major research centers based on thousands of comparative tests as well as an extensive analysis of antibodies known to be both specific and characteristic of various stages of B. burgdorferi infection.
There are some who claim that several important proteins (e.g., OspA, OspB, and other B. burgdorferi proteins or antigens) were not included in the positive pattern established by the CDC; however, these proteins were initially considered for inclusion, but were rejected because they did not contribute significantly to diagnosis. Although it is true that OspA and OspB antigens are indeed specific for B. burgdorferi, these antigens are produced only when the bacterium is grown on artificial laboratory media or in the midgut of Ixodes ticks. Since these antigens are not — or are only minimally produced — during the course of a human infection, they are of little or no diagnostic value for human disease; the presence of other antibodies recommended in the CDC standard criteria predominate and thus are of greater relevance, as demonstrated by the results of thousands of comparative laboratory tests.
In sharp contrast to the CDC standard criteria, some doctors and commercial laboratories (e.g., IGeneX) use or advocate non-standard criteria that have not been validated by rigorous comparative studies by the CDC and or FDA (http://igenex.com/Website) . Consequently, the results of their tests fall outside the range of standard practice and have a much greater rate of false positives than one would get using the CDC criteria.
The CDC is responsible for guiding physicians in the appropriate use of laboratory tests for the diagnosis of Lyme disease and other infectious diseases. The CDC has warned about nonstandard testing and the interpretation of laboratory test results using unvalidated criteria. This carries great weight among mainstream physicians, as well as scientists working at State public health laboratories. Thus, the CDC criteria remain the standard and other criteria are considered to be unvalidated and unacceptable.
As is the case for most serologic assays, Lyme disease serologic assays are not by themselves diagnostic. A diagnosis of Lyme disease can only be made in the presence of well defined objective clinical abnormalities associated with Lyme disease. Because the presence of fatigue or vague aches and pains are too nonspecific, a positive serology in such individuals would have a very low positive predictive value. Simply demonstrating that someone has an immune response against B. burgdorferi does not mean that person is actively infected, or that any general symptoms have anything to do with B. burgdorferi infection. It also is important to realize that the immune system has memory. That means that an individual who makes a mature antibody response against any infecting bacterium or virus continues to have detectable antibodies in their blood. This is true for all infections, including Lyme disease. Thus, a positive test after someone has been treated is in fact normal and does not indicate on going infection.
There is no indeterminate designation in the CDC criteria. Also there is no separate CDC surveillance criteria for serologic assays. The lack of a positive serology in a patient without the clear objective abnormalities known to be associated with Lyme disease has a very high negative predictive value, indicating that patient does not have Lyme disease.
The CDC has issued the following statement which summarizes its views on the diagnosis of Lyme disease (LD):
A two-test approach for active disease and for previous infection using a sensitive enzyme immunoassay (EIA) or immunofluorescent assay (IFA) followed by a Western immunoblot is the algorithm of choice. All specimens positive or equivocal by a sensitive EIA or IFA should be tested by a standardized Western immunoblot. Specimens negative by a sensitive EIA or IFA need not be tested further. When Western immunoblot is used during the first 4 weeks of disease onset (early LD), both immunoglobulin M (IgM) and immunoglobulin G (IgG) procedures should be performed. A positive IgM test result alone is not recommended for use in determining active disease in persons with illness greater than 1 month’s duration because the likelihood of a false-positive test result for a current infection is high for these persons. If a patient with suspected early LD has a negative serology, serologic evidence of infection is best obtained by the testing of paired acute- and convalescent-phase serum samples. Serum samples from persons with disseminated or late-stage LD almost always have a strong IgG response to Borrelia burgdorferi antigens.