Pathology News

COVID-19 IgG Serology Testing Now Available At Michigan Medicine

By Riccardo Valdez | 29 May

Serology testing for SARS-CoV-2 is now available for ordering at Michigan Medicine. A separate process for testing and screening of health care workers and Michigan Medicine affiliates will be communicated in the coming weeks.

The serology test detects specific antibodies generated as part of the immune response to SARS-COV-2 infection. Our current test methods detect IgG antibodies, which are usually seen within 2-3 weeks after symptoms begin.  Molecular PCR tests, in contrast, detect viral particles present during an active infection.

The serology testing methods being utilized at MM have been independently verified and separately validated by the Clinical Laboratory using over 300 samples.  These samples include many potential interferences (autoimmune conditions and other viral infections including seasonal coronaviruses) and samples from patients with PCR-confirmed SARS-CoV-2 with a wide range of time points after symptom onset.  The samples we have independently used reflect the types of samples that would be encountered in actual practice.  These assays have performed well and demonstrate similar performance to that claimed by manufacturers. In our experience, these tests have greater than 95% sensitivity at greater than two weeks after symptom onset and also demonstrate greater than 98% specificity.

The indications for SARS-CoV-2 serology testing include:

  • Evaluating patients for prior SARS-CoV-2 infection, at least fourteen days after the acute illness.
  • Evaluating patients with unexplained COVID-19-like symptoms to determine if those symptoms may have been caused by SARS-CoV-2.
  • Aiding the identification of potential SARS-CoV-2 convalescent plasma donors.

Serology testing is NOT INDICATED as a sole test for patients with acute symptoms, as antibodies are not produced in early stages of infection. Molecular PCR testing remains the appropriate test for diagnosis of acute illness. In addition, there is currently no conclusive evidence that the presence of antibodies confers protection from future or repeat COVID-19 infection.

A positive result indicates detection of IgG antibodies specific to SARS-CoV-2 in the blood, and likely indicates prior exposure and infection by SARS-CoV-2. However, the positive predictive value of a test is due to not only test performance, but also disease prevalence.  If the disease prevalence is only 1-2%, a positive screening test result with these assay characteristics would still more likely be a false positive than a true positive result.  As such, the meaningfulness of this test relies on testing individuals with a higher likelihood of positivity due to exposures or history of symptomatic illnesses.

A negative result indicates IgG antibodies to SARS-CoV-2 were not detected, which could be due to either absence of prior infection or testing too soon after an infection or in an immunocompromised individual.

Additional information can be found in the linked Fact Sheet.

Written by Dr. Ric Valdez, Dr. Don Giacherio, Dr. Carmen Gherasim, and Dr. David Manthei.

 
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