Coronavirus Disease 2019 (COVID-19) – Serology

COVID-19 serology should not be used as a diagnostic test, except in very rare circumstances, due to the potential for false negative and false positive results. SARS-CoV-2 antibodies do not correlate with recovery or infectivity. Serology also cannot be used to assess whether a person is immune to COVID-19 or to determine their COVID-19 vaccination status.

This test provides qualitative detection of serum IgG antibodies to SARS-CoV-2, the causative agent for novel coronavirus disease (COVID-19).

Testing Indications

This test is intended for use as an aid in determining if a patient with an adaptive immune response has been previously exposed to SARS-CoV-2. Currently, it is not known whether the presence of SARS-CoV-2 IgG antibodies correlates with immunity. It can take at least 7-14 days from symptom onset to develop a measurable SARS-CoV-2 IgG response with some individuals never seroconverting. In evaluations at PHO, the highest sensitivity of the assay was seen at >14-21 days from symptom onset. Further, the duration of the IgG response is variable, with a reduction in IgG levels and seronegativity in as little as 2-3 months in some patients. Additional information about COVID-19 serology testing can be found here: What We Know So Far – COVID-19 and Serology Testing

In patients with a low pretest probability (e.g. no high-risk exposure or symptoms compatible with COVID-19), there is a risk that a positive COVID-19 serology result is a false positive, even with high test specificity. Conversely, there is an increased risk of a false negative result if serum is taken too early (i.e., <2-3 weeks after symptom onset) or too late (i.e. antibody waning), from a patient with a mild infection, or if the patient is immunocompromised. Given these caveats and significant gaps in our understanding of the immune response in COVID-19, serology testing has very limited clinical value for individual patients. Detection of viral RNA by molecular testing, such as PCR, is the gold standard for diagnosing COVID-19 in suspected patients. If the patient is symptomatic, a respiratory specimen should be tested by COVID-19 PCR Testing.

Currently, the limited clinical value for individual patient testing precludes the widespread use of COVID-19 serology as a clinical diagnostic tool. It may be considered for clinical use as an adjunct to COVID-19 PCR testing in:

  • Patients suspected to have multisystem inflammatory syndrome in children (MIS-C) or adults (MIS-A) with a negative, indeterminate, or inconclusive PCR test result or who were not tested

Other clinical scenarios of severe illness with negative PCR tests, where serology results may be helpful for clinical management and/or public health action, will be considered following consultation and approval by a PHO Microbiologist before specimen collection. Specimens submitted for testing for indications other than MIS-C/MIS-A or without prior approval will be rejected.

Serology should NOT be used for:

  • The diagnosis of acute infection, reinfection, or determining the infectivity of the patient
  • Determining immune status of the patient (i.e. protection against future infection)
  • Determining COVID-19 vaccination status of the patient or serological response to vaccination

The PHO Microbiologist can be contacted through the PHO Laboratory's Customer Service Centre at 416-235-6556 / 1-877-604-4567.

Specimen Collection and Handling

Specimen Requirements

Test Requested Required Requisition(s) Specimen Type Minimum Volume Collection Kit

COVID-19 IgG

Serum

1.0 ml

Blood, clotted - vacutainer tubes (SST)

Submission and Collection Notes

1

Specimens submitted for testing for indications other than MIS-C/MIS-A, as described above, or without prior approval by a PHO Microbiologist will be rejected.

2

Complete all fields of the dedicated COVID-19 Serology Test Requisition form (do not use the COVID-19 Virus Test Requisition that is used for PCR testing). Include the patient's full name, date of birth, Health Card Number (must match the specimen label), patient address, physician name and address, specimen type and date of collection, reason for test, patient setting, clinical symptoms (including onset date) and medical history (including COVID-19 vaccination status), COVID-19 PCR result(s) and date(s) of testing, travel, and other exposure history.

3

Centrifuge specimen if using SST. Place specimen in biohazard bag and seal. Specimens should be stored at 2-8°C following collection and shipped to PHO Laboratory on ice packs as soon as possible and within 7 days from collection.

4

Specimens may be stored on or off the clot or red blood cells for up to 1 day at 15-30°C or 7 days if refrigerated at 2-8°C. Stability past 7 days from the collection has not been evaluated at PHO Laboratory. If testing will be delayed beyond the recommended storage time, remove serum from the clot and store at -20°C or colder and ship frozen.

5

Specimens should not be heat-inactivated or pooled. Grossly haemolysed, icteric, lipemic or microbially contaminated sera and specimens with fungal growth will not be tested.

Timing of Specimen Collection

Collect serum at least 14-21 days from symptom onset. If collecting earlier, consider collecting paired sera with the second collection at least 14-21 days from symptom onset and within 4 weeks of the first collection. For patients suspected to have MIS-C/MIS-A, sera may be collected at the time of investigation.

Preparation Prior to Transport

Label the specimen container with the patient's full name, date of collection and one other unique identifier such as the patient's date of birth or Health Card Number. Failure to provide this information may result in rejection or testing delay.

Special Instructions

Complete all fields of the dedicated COVID-19 Serology Test Requisition form (do not use the COVID-19 Virus Test Requisition that is used for PCR testing). Include the patient's full name, date of birth, Health Card Number (must match the specimen label), patient address, physician name and address, specimen type and date of collection, reason for test, patient setting, clinical symptoms (including onset date) and medical history (including COVID-19 vaccination status), COVID-19 PCR result(s) and date(s) of testing, travel, and other exposure history.
 

Requisitions and Kit Ordering

Test Frequency and Turnaround Time (TAT)

SARS-CoV-2 IgG serology is performed two days a week. Turnaround time is up to 5 days from receipt by PHO Laboratory.

Reporting

Results are reported to the ordering physician or health care provider as indicated on the requisition.

As a disease of public health significance, all reactive/positive results will be reported to the local public health unit.

Test Methods

To maximize test sensitivity, PHO Laboratory uses two SARS-CoV-2 IgG assays: the Abbott Alinity SARS-CoV-2 IgG assay (chemiluminescent microparticle immunoassay (CMIA)) that detects anti-nucleocapsid (N) IgG, and the Ortho-Diagnostics VITROS anti-SARS-CoV-2 IgG assay (chemiluminescent immunoassay (CLIA)) that detects anti-spike (S) IgG. Both assays are intended for the qualitative detection of IgG antibodies to SARS-CoV-2 in human serum.

Algorithm

Specimens are tested using both assays. A reactive/positive result on either assay is considered a reactive/positive result.

Interpretation

SARS-CoV-2 IgG testing should NOT be used to determine a patient's immune status, vaccination status, or infectivity. Results should be interpreted in the context of clinical and exposure history.

A negative SARS-CoV-2 IgG test result:

  • Does not rule out current or previous SARS-CoV-2 infection
  • If clinical suspicion is high, consider retesting in 2-3 weeks
  • Negative results may occur if the specimen is collected too soon or too late following infection, if the patient is immunocompromised, or if the patient is too young to produce an effective adaptive immune response (eg. neonate)

A positive COVID-19 IgG test result:

  • Indicates recent or prior infection with SARS-CoV-2 virus
  • An individual with evidence of seroconversion over a 4-week interval, regardless of nucleic acid amplification testing (e.g. real-time PCR), is considered a "confirmed" case if the individual has not received a COVID-19 vaccination1
  • An individual with antibody detected in a single serum specimen is considered a "probable" case if they have not yet received a COVID-19 vaccination AND had symptoms of COVID-19 AND had a high-risk exposure or epidemiological link AND antibody was detected within four weeks of symptom onset1
  • False-positive results may occur from cross-reaction due to prior infection with other human coronaviruses, including SARS-CoV-1 and certain seasonal coronaviruses (e.g. human coronavirus OC43)

If the patient is symptomatic, a respiratory specimen should be collected and tested for SARS-CoV-2 using a molecular assay. Additional information can be found here: COVID-19 PCR Testing.

There is currently no data available to determine if these commercial assays can or cannot detect IgG antibodies produced in response to infection by SARS-CoV-2 variants of concern (VOCs).

1Refer to the Ministry of Health COVID-19 Case Definition: https://www.health.gov.on.ca/en/pro/programs/publichealth/coronavirus/docs/2019_case_definition.pdf

Updated 11 May 2021