Coronavirus Disease 2019 (COVID-19) – Serology

As of August 6, 2020, COVID-19 serology should not be used as a diagnostic test, except in very rare circumstances, due to high rates of false negative and false positive results. Serology also cannot be used to assess whether a person is immune to COVID-19.

This test provides qualitative detection of serum IgG antibodies to SARS-CoV-2, the causative agent for novel coronavirus disease (COVID-19). IgM serology is currently not being offered at PHO Laboratory, as the current assays available do not provide additional sensitivity to serology testing and have reduced specificity compared to IgG tests.

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 COVID-19 IgG antibodies correlates with immunity. It can take at least 7-14 days from symptom onset to develop a measurable COVID-19 IgG response with some individuals never seroconverting. It must be noted that COVID-19 IgG antibodies do not correlate with recovery or infectivity. 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 has not yet been well characterized, with studies showing a reduction in IgG levels and seronegativity after 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

At this time, with the low prevalence of disease in Ontario, even with high test specificity, there is a risk that a positive COVID-19 serology result is a false positive. Conversely, there is a high risk of a false negative result if sera is taken too early (ie. <3 weeks after symptom onset) or too late, from a patient with 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 the diagnosis of 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:

  • Pediatric patients suspected to have multisystem inflammatory syndrome in children (MIS-C) 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 heath action, will be considered following consultation and approval by a PHO Microbiologist prior to specimen collection. Specimens submitted for testing for indications other than MIS-C or without prior approval will be rejected.

Serology should NOT be used for:

  • The diagnosis of acute infection or determining infectivity of the patient
  • Determining immune status of the patient (i.e. protection against future infection)

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

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.

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 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.

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. For pediatric patients suspected to have MIS-C, 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), physician name and address, specimen type and date of collection, reason for test, patient setting, clinical symptoms and medical history, COVID-19 PCR result(s) and date(s) of testing, travel, and other exposure history.

OHIP and other non-PHO requisitions will not be accepted for specimen collection or submission. The dedicated PHO Laboratory COVID-19 Serology Test Requisition must be used and submitted.
 

Requisitions and Kit Ordering

Test Frequency and Turnaround Time (TAT)

COVID-19 IgG serology is performed daily Monday to Friday. 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

Although the true prevalence of COVID-19 in Ontario is not yet well defined, it is estimated to be low. The positive predictive value (PPV) of serology testing is low when the prevalence is low and may yield false positive results. In the interim, PHO Laboratory uses orthogonal testing to reduce the risk of false positive results. This is a 2-stage process consisting of an initial screening test with the Abbott Architect SARS-CoV-2 IgG assay (chemiluminescent microparticle immunoassay (CMIA)), followed by a supplemental test with the Ortho-Diagnostics VITROS anti-SARS-CoV-2 IgG assay (chemiluminescent immunoassay (CLIA)). Both assays are intended for qualitative detection of IgG antibodies to SARS-CoV-2 in human serum.

Algorithm

Specimens are initially tested using the Architect SARS-CoV-2 IgG assay. Specimens that are reactive are then tested using the VITROS anti-SARS-CoV-2 IgG assay. A final interpretation is based on the results of both the screening and supplemental tests.

Architect IgG Result

VITROS IgG Result

Interpretation

Non-reactive

Not Tested

Non-reactive/Negative

Reactive

Reactive

Reactive/Positive

Reactive

Non-reactive

Inconclusive

Interpretation

COVID-19 IgG testing should NOT be used to determine a patient’s immune status or infectivity. Results should be interpreted in the context of clinical and exposure history.

A negative COVID-19 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 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 COVID-19
  • False-positive 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)

An inconclusive COVID-19 IgG test result:

  • Occurs when an initial reactive IgG result cannot be confirmed
  • This can occur if the level of IgG is too low or from non-specific binding of antibody to the antigen used in the assay
  • If there is high clinical suspicion of a previous COVID-19 infection, consider retesting in 2-3 weeks
  • Specimens with insufficient volume to be tested by the supplemental test will also be reported as “inconclusive”
Updated 5 Aug 2020