![]() The cut-off between low and high avidities has to be established by using standardized sera and a particular EIA kit. High avidity rubella IgG suggests a more distant rubella exposure, which may be from either via infection or vaccination. Low avidity rubella IgG suggests recent infection and can be detected for up to four months after infection. These avidity differences can be detected by using protein denaturants such as diethylamine (DEA) in the washing step of an enzyme-linked immunoassay (EIA) for rubella IgG. As the immune response matures, low avidity antibodies are replaced with high avidity antibodies. Antibody avidity (the overall strength of binding between the antigen and antibody) increases with time this is known as maturation of the immune response. The measurement of rubella IgG antibody avidity can be used to distinguish between recent exposure to rubella and more distant rubella exposure. It is important to distinguish IgM reactivity caused by primary infection from that caused by IgM persistence or cross-reactivity with other antigens, especially in pregnant people (see Testing Pregnant Patients section below). Due to the possibility of false positive results, IgM should not be included in routine testing to determine evidence of prior rubella immunity. The presence of rheumatoid factor can also result in a false positive IgM. ![]() Rash and fever illnesses are more likely due to a number of other rash–causing illnesses such as parvovirus B19, enteroviruses such as coxsackieviruses and echoviruses, or human herpesvirus–6 (roseola). In the United States where endemic circulation of wild-type rubella virus has been eliminated and the chance of contracting the disease domestically is low, most suspected cases are not rubella. Since no assay is 100% specific, serology testing of non-rubella cases occasionally produces false positive IgM results. The interpretation of rubella laboratory results must always take into account relevant clinical and epidemiological data. When CDC receives serum samples to test for clinically suspected rubella, CDC also determines the anti-rubella IgM or IgG status for each sample to aid in case classification. Therefore, if serum that collected less than five days after onset is negative, a second sample would be necessary to confirm or rule out rubella. ![]() On the day of rash onset only about 50% of cases are IgM positive. ![]() ![]() The optimum time-point for collection of serum is five days after the onset of symptoms (fever and rash) when >90% of cases will be IgM positive. Detection of specific IgM antibodies in a serum sample collected within the first few days after rash onset can provide presumptive evidence of a recent rubella virus infection. While rubella IgM persists for up to three months, rubella IgG can last a lifetime.Īlthough rubella was officially declared to be eliminated from the United States in 2004, ongoing rubella activity in many other countries can result in imported sporadic U.S. Rubella-virus specific IgG antibodies (rubella IgG) become detectable by day four after rash onset in rubella infections and reach peak levels in the following one to two weeks. Rubella-virus specific IgM antibodies (rubella IgM) develop shortly after primary infection or vaccination. ![]()
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