Investigations
1st investigations to order
salivary rubella IgM
Test
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.[16]
Result
positive
serology
Test
The most common diagnostic test is rubella-specific IgM serum antibody. The preferred test is capture ELISA. The optimum time-point for collection of serum is 5 days after the onset of symptoms (fever and rash), when >90% of cases will be IgM positive.[5] False positive IgM tests are possible, so all positive IgM tests should be confirmed by demonstrating a four-fold rise in rubella-specific IgG serum concentrations between acute and convalescent sera (drawn 2-3 weeks apart), or by measurement of IgG avidity (the overall strength of binding between the antigen and antibody, which increases with time as the immune response matures). Low avidity IgG antibodies can be detected for up to 4 months after infection and indicate recent infection, while the presence of high avidity IgG suggests a more distant exposure (which may be from either infection or vaccination). Rubella IgG can last a lifetime. Detection of rubella IgG should be used for assessing rubella immunity, including before, during, and after pregnancy. CDC: laboratory protocols - rubella Opens in new window CDC: laboratory support for surveillance of vaccine-preventable diseases Opens in new window
Rubella IgM can be used to diagnose congenital rubella syndrome cases. Suspected cases should be tested as close to birth as possible, and again at 1 month of age if the initial IgM test is negative. If paired sera are to be collected, the second sample should be collected 14 to 21 days after the acute specimen was collected. At 3 months of age, approximately 50% of cases would still have detectable rubella IgM in their serum. Additionally, the presence of rubella IgG in an infant after the decline of maternal antibodies (9 months of age) and the absence of vaccination or exposure to rubella will confirm congenital rubella syndrome.[5]
Result
IgM: positive in acute serum; IgG: seroconversion or 4-fold rise between acute and convalescent titres
FBC
Test
Occasionally thrombocytopenia may be present. This is thought to have an immunological basis.
Result
usually normal
Investigations to consider
viral culture
Test
Rubella virus can be isolated from the nasopharynx, throat, urine, blood, and CSF from about 1 week before to 2 weeks after the onset of rash. Viral cultures are not routinely obtained because they are labour-intensive and performed only in specialised reference laboratories. Viral isolation is important from an epidemiological perspective and should be attempted if rubella is strongly suspected. Specimens should be obtained as early in the course of the illness as possible. Information is available from the US Centers for Disease Control and Prevention laboratory protocols. CDC: laboratory protocols - rubella Opens in new window
Result
may be positive
reverse-transcriptase PCR (RT-PCR)
Test
Detection of rubella in direct clinical specimens or after incubation in tissue culture can confirm infection.
Available commercially and in some countries through government health authorities. Nasopharyngeal swabs are the preferred sample type. The utility of RT-PCR is limited because of the narrow window when the virus can be detected in clinical samples; in respiratory samples, rubella RNA is typically only detectable from 2 days before rash onset to 4 days after. Swabs should be collected as soon after symptom onset as possible, preferably 1 to 3 days after onset, but no later than 7 days post-onset.[5] RT-PCR assays on throat swabs, nasopharyngeal swabs, and urine specimens from a neonate can be used for confirmation of suspected congenital rubella syndrome cases. Samples should be collected prior to 3 months of age if possible, because by 3 months of age approximately 50% will no longer shed virus.[5]
Information is available from the US Centers for Disease Control and Prevention. CDC: laboratory protocols - rubella Opens in new window CDC: laboratory support for surveillance of vaccine-preventable diseases Opens in new window
Result
may be positive
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