Measles, Mumps, and Rubella - Centers for Disease Control and ...

44 MMWR May 22, 1998The clinical diagnosis of acute rubella should be confirmed by laboratory testing(230 ). The demonstration of rubella-specific IgM antibody is the most commonly usedmethod to obtain serologic confirmation of acute rubella infection. Rubella-specificIgM antibody usually becomes detectable shortly after rash onset. The IgM antibodypeaks approximately 7 days after rash onset and remains detectable for 4–12 weeks,although it is more likely to be detectable if the serum specimen is obtained within4–5 weeks after rash onset. Occasionally, rubella-specific IgM antibody can be detectedup to 1 year after acute infection.To test for IgM, one serum specimen can be obtained as early as 1–2 days after rashonset. If IgM is not detectable in this first specimen, a second serum specimen shouldbe collected 5 days after the onset of rash or as soon as possible thereafter. Falsenegativerubella IgM antibody test results may sometimes occur even if the specimenis appropriately drawn. False-positive IgM test results may occur among persons withcertain viral infections (e.g., acute infectious mononucleosis, cytomegalovirus, or parvovirus)and among persons who are rheumatoid factor positive.For IgG assays, the criteria for a significant rise in rubella antibody level varyby type of assay and by laboratory. For HI assays, a fourfold rise in the titer of antibodyindicates recent infection. The acute-phase serum specimen should be obtainedas soon after rash onset as possible, preferably within 7 days. The convalescentphaseserum specimen should be drawn at least 10 days after the acute-phase serumspecimen. The acute- and convalescent-phase serum specimens should be tested simultaneouslyin the same laboratory. If the acute-phase serum specimen is drawn>7 days (and occasionally even if obtained within 7 days) after rash onset, a significantrise in antibody titer may not be detected by most commonly used IgG assays.In the absence of rash illness, the diagnosis of subclinical cases of rubella can befacilitated by obtaining the acute-phase serum specimen as soon as possible afterexposure. The convalescent-phase specimen should be drawn at least 28 days afterexposure. If acute- and convalescent-phase paired sera provide inconclusive results,rubella-specific IgM antibody testing can be performed. Expert consultation may benecessary to interpret the data.Among pregnant women of unknown immune status who experience a rash illnessor who are exposed to rubella, laboratory confirmation of rubella infection may bedifficult. A serum specimen should be obtained as soon as possible. Unfortunately,serologic results are often nonconfirmatory. Such situations can be avoided by performingroutine prenatal serologic screening of women who do not have acceptableevidence of rubella immunity (see Documentation of Immunity and Women of ChildbearingAge). In addition, health-care providers should request that laboratoriesperforming prenatal serologic screening retain such specimens until delivery, in caseretesting is necessary.Congenital RubellaSuspected cases of CRS should be managed with contact isolation (228 ). Whilediagnostic confirmation is pending, children with suspected CRS should be cared foronly by personnel known to be immune to rubella. Confirmation of diagnosis by virusisolation can be done by culturing nasopharyngeal and urine specimens. Serologicconfirmation can be obtained by testing cord blood for the presence of rubella-specificIgM antibodies. An alternative method for infants aged ≥3 months is to document