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Indiscriminate use of serologic testing for screening can lead to results that have very low positive predictive value (ie, the likelihood that a positive test indicates true presence of disease). Care must be taken to not rely excessively on serologic testing as the foundation of clinical diagnosis. If the pretest likelihood of Lyme disease (based on clinical evaluation) is estimated to be only 5%, a positive ELISA increases the likelihood to only 20%. If the pretest likelihood is considered to be lower yet (eg, 1%), a positive ELISA increases the likelihood to only 5%. In both of these scenarios, a false-positive is as likely as or more likely than a true positive. This is the case even though the specificity of serologic testing for disseminated Lyme disease is generally considered 80% or better. Patient selection has a dramatic effect on the posttest predictive value of positive serologic results.
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To optimize the predictive value of a positive test, serologic testing should be performed only in patients who are considered to be at risk epidemiologically and have clinical features truly suggestive of disseminated Lyme disease. If the pretest likelihood of Lyme disease is considered to be less than 20%, serologic testing is generally not indicated. The routine use of the two-step approach to serologic testing (ELISA followed by Western blot) increases the specificity of the tests; however, Western blot techniques have not been standardized, and this approach has not been quantitatively analyzed. Serologic testing is readily available in office and hospital laboratories and has been overused in recent years, at least in part because of patient demand for testing.
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In cases in which the pretest likelihood is low, a negative result virtually rules out the presence of Lyme disease. This information may be helpful, but, as mentioned, a positive result increases the likelihood of Lyme disease only marginally and may contribute more confusion than clarity. The presence of IgM antibody to specific B burgdorferi antigens is evidence of acute or recent infection. However, IgM reactivity to certain bands (25 kd and 41 kd) is relatively common in the general population, so even a positive Western blot by current criteria is not proof of recent-onset B burgdorferi infection. High titers of IgG antibody to specific B burgdorferi antigens offer persuasive evidence of immunoreactivity against the organism but do not indicate that current symptoms are attributable to Lyme disease. Cheap amoxicillin at canadian antibiotics online.
Negative serologic findings provide convincing evidence that B burgdorferi infection is not the cause of symptoms lasting longer than 4 weeks. B burgdorferi has been successfully cultured from blood, cerebrospinal fluid, and synovial fluid in symptomatic patients during this stage of illness. PCR has also been used to document B burgdorferi in various tissues and body fluids during disseminated Lyme disease. Neither of these two tools is available to practicing clinicians. Disseminated Lyme disease generally remits spontaneously even without therapy, but chronic inflammation may persist for months to years in a small portion of patients. Canadian viagra