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  5. Why Specificity Matters in COVID-19 antibody testing

Why Specificity Matters in COVID-19 antibody testingTest quality and accuracy are paramount to minimize risks of inaccurate results.

A good antibody test is one that has a high degree of sensitivity and specificity.

Sensitivity vs Specificity

  • Test sensitivity indicates the ability of the test to correctly identify patients that have the disease. A test’s sensitivity is also known as the true positive rate. If a diagnostic test correctly identified 100% of all positive results, it would be as sensitive as possible.
  • Test specificity indicates the ability of the test to correctly identify patients that do not have the disease. If a test correctly identifies all people without the disease as negative, it would be as specific as possible.

Specificity in COVID-19 testing

For SARS-CoV-2 antibody testing, the CDC suggests use of tests with a specificity ≥99.5% to minimize the potential for false-positive results.1

There are numerous tests that claim to detect antibodies to the SARS-CoV-2 virus; only a few are highly accurate:

Bubble chart showing sensitivity and specificity of more than 200 SARS-CoV-2 antibody tests in the market.

How do highly specific antibody tests support good performance, even with low disease prevalence?

Let’s look at what happens in two cities, one with a 5% disease prevalence, another with 10% prevalence.
Infographic part 1: City A has a disease prevalence of 5% and City B has a 10% prevalence.

What happens when the population is tested with an assay that has higher specificity of 99.8%?

A highly specific test minimizes inaccurate results. With a higher disease prevalence, fewer people will experience incorrect results.
Infographic part 2: Antibody tests with high specificity minimize inaccurate results. This model assumes assay sensitivity of 100%.

What happens when the specificity is reduced to 96%?

A 96% specificity may seem high, but a difference of as small as 3 to 4 percent can create dramatic changes in test results.
Infographic part 3: When reduced by just a few points, lower specificity leads to more incorrect results. . This model assumes assay sensitivity of 100%.

The model above assumes assay sensitivity of 100%

References:
1
Accessed July 8, 2020: https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antibody-tests-guidelines.html
2
Accessed July 8, 2020: https://www.fda.gov/medical-devices/emergency-situations-medical-devices/eua-authorized-serology-test-performance

* This test has not been FDA cleared or approved. This test has been authorized by FDA under an EUA for use by authorized laboratories. This test has been authorized only for detecting he presence of antibodies against SARS-CoV-2, not for any other viruses or pathogens. This test is only authorized for the duration of the declaration that circumstances exist justifying the authorization of emergency use of in vitro diagnostics for detection and/or diagnosis of COVID-19 under Section 564(b)(1) of the Act, 21 U.S.C. § 360bbb-3(b)(1), unless the authorization is terminated or revoked sooner. Product availability may vary by country and is subject to varying regulatory requirements.