Lyme disease is spread by ticks.
Enzyme-linked immunosorbent assay or ELISA is a common biochemical technique used to determine whether a specific kind of molecule is present in a sample. It's often used as an initial test for patients who may have Lyme disease; since it has a high false-positive rate, it's typically followed with a second test using a technique called Western blotting. False-negatives for Lyme disease with ELISA are rare.
ELISA Technique
Antibodies are proteins manufactured by the immune system to bind to foreign or unrecognized molecules and invaders. They are specific--each antibody is unique to a particular antigen or target and ordinarily will only bind to that target. In the ELISA procedure, an antigen exhibited by the bacteria that cause Lyme disease is fixed to a surface, then the blood sample is added to it. If the patient's blood contains antibodies against the bacteria, the antibodies will bind to the antigen on the surface. Next an antibody that targets the patient's antibodies is modified by attaching an enzyme to it, and the modified antibody is applied. If the patient's blood contained antibodies and they stuck to the antigen, the modified antibodies will bind to them in turn. The technician can now add a special chemical to the mix; if the antibody-linked enzyme is present, it will alter the chemical so that it becomes fluorescent or changes color and thereby make the results of the test visible. If the change in color or fluorescence is over a certain threshold, the test result is considered positive; if the result is too close to the threshold to be sure, however, the result is considered indeterminate and further testing is performed.
ELISA & Lyme disease
ELISA is the first lab test performed to diagnose Lyme disease. It's extremely sensitive, however, so some people who don't actually have Lyme disease will still test positive. Consequently, if ELISA returns a positive result, a second test called Western blotting is performed. According to the Centers for Disease Control, if the ELISA test returns a negative result, no further lab testing is considered necessary.
False Negative Rate
Diagnosing Lyme disease can be difficult because the symptoms resemble those of many other illnesses. The lab tests can sometimes return false positive or false negative results. Since the tests screen for the presence of antibodies in the blood, they're only effective once the patient's body is producing a measurable level of antibodies for the bacteria. Since the human antibody response to these bacteria only develops slowly, it may be a couple weeks before these levels are reached. During the early stage of the disease the test may have a sensitivity of only 50 to 75 percent, according to an article in the "American Family Physician," meaning that the test only detects the disease in 50 to 75 percent of those who truly have it. In later stages of the disease, however, false negative results are uncommon; the test sensitivity in late stages is over 95 percent.
Different Strains
According to a study published in the "Journal of Medical Microbiology," it's possible that false negative results might occasionally occur for another reason. The study found "a small but real risk" that tests conducted using antigens for certain strains of the bacteria might sometimes fail to detect antibodies against other strains.
Pretest Probability
While there are other tests for Lyme, ELISA/Western blot are preferred because the other available tests have higher false-positive/false-negative rates or have not been properly validated. ELISA/Western blot can also return false positive or false negative results as well, however, and there's still some controversy among doctors over the role lab testing should play in diagnosis. A 2005 article in "American Family Physician" recommends that doctors use "pretest probability" when deciding whether to order lab tests. The more closely the patient's symptoms match and the more endemic the disease is in the area, the higher the pretest probability becomes. If the probability is high enough, the doctor may even diagnose early Lyme disease without lab testing in some cases. Please note, however, that none of the above is intended as medical advice; if you have any questions about testing or treatment for Lyme disease, you may want to discuss this condition with your doctor.
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