False-Positive Results for Immunoglobulin M Serologic Results: Explanations and Examples.

Abstract

Scenario* A measles outbreak occurred last month among a cluster of unvaccinated children in a suburban area in your referral region. Two weeks ago, an adolescent who received two doses of measles-containing vaccine in childhood developed a febrile illness with rash. This patient tested positive for measles IgM in a commercial assay. Subsequent testing by the Centers for Disease Control and Prevention demonstrated seronegativity for measles IgM. (A low positive concentration of measles IgG was detected.) The initial IgM test result has been labeled as false positive. The family of the patient is upset about the disruption of life experienced due the false positive IgM result. The pediatrician who ordered the test has called to ask you how this result could have happened. What are potential explanations for this? The false-positive rate of any diagnostic test is a function of the specificity of the test and prevalence of the disease in the population represented by the patient. For serologic assays, the presence of antibodies that cross-react with microbial antigens used in the assay or interfering substances that interact with assay components can also lead to false-positive results. Technical performance issues such as over-reading of weakly reactive bands on immunoblots can also lead to false-positive serologic tests results for microbes for which these assays are used (eg, Borrelia burgdorferi )[ 1]. Thus, positive IgM assay results can require cautious interpretation—consideration of clinical course compatibility and epidemiological factors—and/or confirmation by other serological or molecular testing methods. The issue of reactivation of IgM production against herpes viruses is beyond the scope of this review.