Serologic Testing for Rubella and CRS in Low Prevalence Setting
IgM and IgG Detection
Although rubella was officially declared to be eliminated from the United States in 2004, ongoing rubella activity in many other countries can result in sporadic U.S. cases or outbreaks. Detection of specific IgM antibodies in a serum sample collected within the first few days after rash onset can provide presumptive evidence of a current or recent rubella virus infection. 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.
On the day of rash onset only about 50% of cases are IgM positive. Therefore, if serum collected less than five days after onset is negative, a second sample would be necessary to confirm/rule out rubella. The anti-rubella IgG status is determined for every serum received at CDC for rubella testing to aid in case classification. The interpretation of rubella laboratory results must always take into account relevant clinical and epidemiological data.
IgG Avidity Detection
Since no assay is 100% specific, serologic testing of non-rubella cases using any assay occasionally produces false positive IgM results. In the United States where endemic circulation of rubella has been eliminated and the chance of contracting the disease domestically is low, most suspected cases are not rubella. 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). The presence of rheumatoid factor can also result in a false positive IgM.
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 women. Although routine IgM screening of pregnant women is not recommended, providers sometimes inappropriately order IgM tests. The measurement of rubella IgG antibody avidity can be used to distinguish between recent and remote rubella infection. Antibody avidity (the overall strength of binding between the antigen and antibody) increases with time; this is known as maturation of the immune response. As the immune response matures, low avidity antibodies are replaced with high avidity antibodies. 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. In acute rubella virus infections, specific, low-avidity IgG lasts for up to three months after appearance of the IgG response. The presence of high avidity antibodies, which develop by about three months after infection, provides evidence of remote infection. The cut-off between low and high avidities has to be established by using standardized sera and a particular EIA kit.
For pregnant women, avidity testing is most useful in early pregnancy to help rule out a rubella infection in the first trimester, when the risk of congenital defects due to rubella is highest. It is not as useful in late pregnancy because avidity will be high by the third trimester if infection occurred in the first trimester.
Serological testing for congenital rubella syndrome (CRS)
Congenital Rubella Syndrome (CRS), which can occur when a woman is infected with rubella during a pregnancy, consists of a variety of possible birth defects including cataracts, hearing loss, heart defects, developmental disabilities, and low birthweight. CRS was eliminated from the United States in 2004, but cases can still be imported by pregnant women who contract rubella in an endemic country. In rare cases, CRS can occur in the United States when susceptible pregnant women are exposed to imported rubella cases.
CRS cases can be diagnosed in newborns and young infants using detection of rubella IgM. Suspected cases should be tested as close to birth as possible and again at 1 month of age if the initial IgM test is negative. At 3 months of age, approximately 50% of cases would still have detectable rubella IgM in their serum. Additionally, the presence of rubella IgG in an infant after the decline of maternal antibodies (9 months of age) and the absence of vaccination or exposure to rubella will confirm CRS.
For more information about blood (serum) samples, see Specimen Collection, Storage, & Shipment.