Rubella Antibodies, IgM

Serum

0.5 mL

0.2 mL

Red-top tube or gel-barrier tube

For the in vitro detection of IgM antibodies specific for rubella. IgM antibodies are associated with acute viral infections. IgM detection is useful in the following situations: evidence of infection can be obtained from only one acute phase specimen if the IgM results are positive; the IgM test can also be used to differentiate between primary infection and re-exposure. Rubella-specific IgM is found in virtually all infected patients by three weeks postdevelopment of a rash. Rubella-specific IgM is also found in 80% of postvaccination patients by three weeks. Congenitally infected infants will show an IgM response at 2 to 12 weeks postnatally.

The absence of IgM at birth does not rule out congenital rubella since the frequency of IgM detection in cord blood decreases as the time between conception and fetal infection increases. Acquired rubella infection in infants is rare but must be considered if the blood is positive for IgM but was not obtained during the immediate postnatal period. Rubella-specific IgM may persist for months after an acute infection and, possibly, after vaccination as well. False-positive rubella IgM responses have been reported following mononucleosis, parvovirus B19 infections, and possibly other herpes-type viral infections. False-positive rubella IgM responses have also been reported in pregnant women. These reactions are usually accompanied by false-positive reactions to other viruses (eg, CMV and measles).

Chemiluminescent immunoassay (CLIA)

• Negative: <20.0 AU/mL

• Equivocal: 20.0−24.9 AU/mL

• Positive: >24.9 AU/mL