Rubella – Serology

Testing Indications

Serology for Rubella Immunity ( IgG) may be indicated to determine immune status following a natural infection, post-vaccination or during prenatal evaluation of pregnant women.

Unknown Immunization Status: Immunization vs. Serology
If a patient’s immunization records are unavailable, immunization with a Rubella-containing vaccine is preferred, rather than ordering serology to determine immune status. This avoids the potential for false positive results, reduces the risk of missed opportunities for immunization and is consistent with advice from the Canadian Immunization Guide (CIG). This does not apply to specific occupational groups, such as healthcare workers, who require either documentation of immunization or serologic proof of immunity regardless of their birth year.

When Rubella infection is suspected – contact your local public health unit (PHU) prior to collecting specimens.

Diagnosis of symptomatic patients requires additional samples for testing by PCR. Refer to the Rubella-Diagnostic PCR test information sheet, for specimen collection and testing information.

Timing of Specimen Collection

Acute Specimen: Collect the acute sample within 7 days after the onset of rash.

Convalescent Specimen: Collect the convalescent sample a minimum of 7 and up to 21 days after the acute sample collection. Collection after 10 days from the acute is preferred to improve the chances of observing a change in the antibody levels.

Limitations

Hemolysed, icteric, lipemic or microbially contaminated serum is not recommended for testing.

Storage and Transport

Specimens should be stored at 2-8°C following collection and shipped to PHO’s laboratory on ice packs. Centrifuge if using serum separator tubes (SST). Place the specimen in a biohazard bag and seal.

All clinical specimens must be shipped in accordance with the Transportation of Dangerous Good Act.

Test Frequency and Turnaround Time (TAT)

Serology testing to detect Rubella IgG and IgM is performed daily Monday to Friday.

Turnaround time is up to 5 days from receipt at PHO’s laboratory.

Stat and Critical Specimens Testing

STAT testing is not available.

Priority testing is available for Rubella diagnosis of symptomatic patients or persons who have been exposed to a confirmed case. Requests for priority testing must be reviewed and approved by the PHOL Medical Microbiologist in conjunction with the appropriate Public Health practitioner. Priority specimens must be packaged separately from routine specimens using TDG-compliant packaging and labelling. To assist with laboratory workflow and identifying priority specimens, mark STAT on the outside of the package. Contact customer service at 416-235-6556 or 1-877-604-4567 prior to specimen transfer to provide PHO’s laboratory with the expected arrival time.

The Rubella IgG test is performed using the BioPlex 2200 MMRV IgG kit for use with the Bio-Rad Bioplex 2200 System. It is a Multiplex Flow Immunoassay test intended for the qualitative detection of IgG antibodies against Rubella in human serum.

Diagnostic Rubella IgG/IgM testing is performed using chemiluminescence immunoassay (CLIA) technology for the qualitative determination of specific IgG and IgM antibodies to Rubella virus in human serum. The test is performed on Diasorin’s Liaison® XL Analyzer platform.

Interpretation

Rubella IgG test results are reported qualitatively together with an interpretation of the patient’s immune status based on the level of Rubella IgG detected.

Following a primary infection, both IgG and IgM antibodies develop within 3-7 days after rash onset. Both antibodies then increase reaching a plateau 2-3 weeks later.

Antibody levels may be reported as:

• Non-reactive - no detectable antibody
• Indeterminate - level of antibody detected is considered borderline reactive or equivocal
• Reactive - antibody is detectable within the positive range of the assay

Non-Reactive and Indeterminate levels of both IgG and IgM antibodies may be observed in the acute stage of infection, rising to reactive levels in subsequent weeks. In the absence of acute infection, indeterminate levels of IgG may represent very low levels of antibody many years after initial infection or vaccination, whereas indeterminate levels of IgM may be due to declining levels of IgM several months post-infection or possibly due to assay non-specificity.

If the acute blood sample shows reactive, indeterminate or non-reactive IgG and/or IgM results then a convalescent sample should be collected. Recent infection is confirmed if a significant rise (greater than 2-fold) in antibody levels is observed between acute and convalescent sera.

Reporting

Results are reported to the physician, authorized health care provider (General O. Reg 45/22, s.18) or submitter as indicated on the requisition.

Specimens that are positive (reactive) for Rubella IgM are also reported to the Medical Officer of Health as per Health Protection and Promotion Act.