Rubella laboratory findings

All patients with suspected rubella infection must be investigated further to confirm the diagnosis. Serological tests to look for the presence of rubella specific IgG antibodies and IgG avidity and RT-PCR should be done to confirm the diagnosis.

• 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. Therefore, if serum collected less than five days after onset is negative, a second sample would be necessary to confirm/rule out rubella.

• The measurement of rubella IgG antibody avidity can be used to distinguish between recent infection and remote rubella infection.^[1]
• 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.

• Real–time RT-PCR (rRT-PCR) and endpoint RT-PCR to detect rubella RNA. RT–PCR helps to confirm the diagnosis.
• The rRT-PCR, which is more sensitive than endpoint RT-PCR, is used for detection of rubella RNA, while endpoint assays are used to amplify the region of the rubella genome required to determine genotype.
• Pharyngeal samples, throat swabs, nasal swabs, or nasal aspirates, are the preferred specimens for detecting rubella RNA.
• Specimens should be collected as soon after symptom onset as possible, preferably one to three days after onset, but no later than seven days post-onset of symptoms.

• Molecular epidemiology of rubella viruses is an important component for confirming connections between cases or excluding cases during investigations.^[2]
• When combined with global surveillance information and epidemiological data, it can also identify the source of wild-type rubella strains.
• In addition, genotype determination from clinical samples or virus isolates can distinguish the vaccine strain of rubella virus from wild-type viruses.
• The World Health Organization (WHO) recommends the collection of rubella virus genotype data to support global control and elimination programs.

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