Tests
1st tests to order
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 ages 1 month 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 ages 3 months, 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 (ages 9 months) 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 titers
CBC
Test
Occasionally thrombocytopenia may be present. This is thought to have an immunologic basis.
Result
usually normal
Tests 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 labor-intensive and performed only in specialized reference laboratories. Viral isolation is important from an epidemiologic 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 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 through several state health departments. 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 ages 3 months if possible, because by ages 3 months approximately 50% will no longer shed virus.[5]
Information is available from the CDC. 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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