Characteristic transmission settings include the household, the workplace, and, in particular, where individuals live under crowded conditions. Infectivity is typically for 7 days before onset of rash until 7 days after onset of rash. Excretion of virus may, however, persist for a longer period of time, especially in the urine. The quarantine period, allowing for a margin of safety, has therefore been set at 3 weeks after onset of rash. Infants with CRS excrete virus in all secretions and are a potent source of infection for up to a year after birth.
Serologic testing is the mainstay of rubella diagnosis.
In those pregnant mothers who test IgM positive the avidity test distinguishes primary from secondary rubella. The importance of this test relates to the prognostic differences with respect to transmission to the foetus, as discussed above. The principle of the test is based on the increasing avidity that IgG antibodies display for their specific antigen with increasing time after infection. The difference in the final optical density reading, before and after urea washing, is expressed as an avidity index. Immunofluorescent antibody test for IgM and IgG antibodies are performed in only a few laboratories in the world.