Investigations

Confirmation of acute rubella infection should be undertaken in all suspected cases. The diagnosis is usually made serologically (1).

Commercially available enzyme immunoassays are able to detect rubella specific IgM within 4 days of onset of the rash till about 4-24 weeks after.

If rubella specific IgM is detected (alone or together with IgG) serum should be collected again within 7-10 days and both samples should be tested simultaneously and sent to a reference facility in order to differentiate recent primary infection from re infection (1)

In the UK, oral fluid is the preferred sample for detection of anti-rubella IgM and should be taken from all patients with suspected rubella (2)

Child:

• saliva samples are appropriate for a child.

Adult:

• serological samples are essential in the case of a pregnant woman with suspected rubella infection (1)
• haemagglutination inhibition antibodies appear soon after the rash and reach peak titres in 6 - 12 days. A rapid rise in HIA titres in paired sera obtained 2 weeks apart in patients presenting within 2 weeks of exposure confirms rubella. Alternative methods of confirming the diagnosis include examination of serum for rubella-specific IgM or virus isolation

Baby:

• the foetus does not produce specific IgM until 23 weeks gestation and no methods able to detect these in cord blood are currently in use - PCR is being evaluated
• congenitally infected babies are identified by detecting rubella virus specific IgM in the infant's blood:
  • less than 3 months, 100% IgM positive
  • 3 to 6 months, 90% IgM positive
  • 6 to 12 months, 50% IgM positive

If IgG or IgM are both negative the individual should be considered as “susceptible individual” for rubella infections and if he/she is suspected to have been in contact recently a second sample should be obtained one month later (1).

References:

1. Centers for Disease Control and Prevention. Rubella (German measles, three-day measles). Dec 2020 [internet publication].
2. UK Health Security Agency.​ Rubella: the green book, chapter 28. Apr 2013 [internet publication].