Varicella – Serology

Testing Indications

Serology for Varicella Zoster (IgG) may be indicated to determine immune status following a natural infection or post-vaccination and assess infants suspected of having congenital varicella syndrome.

Unknown Immunization Status: Immunization vs. Serology
If a patient’s immunization records are unavailable, immunization with a Varicella-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 immunization documentation or serologic proof of immunity regardless of their birth year.

When Varicella Zoster (Chicken Pox/Shingles) infection is suspected – contact your local public health unit (PHU) before collecting specimens. Refer to the STAT and Critical Specimens Testing section for instructions on sending priority specimens to PHO’s laboratory.

Diagnosis of symptomatic patients requires additional samples for testing by PCR. Refer to the Varicella-Zoster Virus Detection 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 the rash.

Convalescent Specimen: Collect the convalescent sample a minimum of 7 and up to 30 days after the acute sample collection. Collection after 10 days from the acute specimen 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)

Varicella serology to detect Varicella 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 Varicella diagnosis of symptomatic patients or persons exposed to a confirmed case. Requests for priority testing must be reviewed and approved by a PHO 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 Varicella IgG test (request for Post-vaccination screening, Pre-employment screening (e.g. Healthcare workers) 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 qualitatively detecting IgG antibodies against Varicella in human serum.

The Varicella IgM test is performed using chemiluminescence immunoassay (CLIA) technology to qualitatively determine specific IgM antibodies to the Varicella virus in human serum. The test is performed on Diasorin’s Liaison® XL Analyzer platform.

Interpretation

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.

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