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This fact brief was originally published as an experiment to test the concepts behind fact briefs.
Readers should be aware that while there is still a lot of useful information in fact briefs like this one, not all of them reflect Gigafact's current methods and standards for fact briefs. If you come across any that you feel are out of date, don't hesitate to contact us at support@gigafact.org.
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FACT BRIEF

Does the standard coronavirus test use a single globally-agreed measure of viral presence for determining positivity?

By Jacob Alabab-Moser
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NO

The standard tests for confirming infection with the coronavirus, known as polymerase chain reaction or PCR tests, yield a specific measurement of the viral "load" in any given sample. Labs calculate what's called a "Ct" value for each sample. A low value indicates a high risk that someone is infectious to others. A high value indicates little presence and little risk.

But there is no single agreed number to apply universally, as varying methods affect values among tests and laboratories. Most public health authorities recommend against reporting out specific Ct values without careful interpretation. Some experts say this has resulted in overly restrictive quarantine policies, which could be more flexible with greater testing frequency and rigor.

As of March 2021, Taiwan authorities allow positive travelers to leave isolation if their test result has a Ct value of 34 or higher.

This fact brief is responsive to conversations such as this one.
Sources
Association of Public Health Laboratories Ct values: What they are and how they can be used
Clinical Infectious Diseases To interpret the SARS-CoV-2 test, consider the cycle threshold value
Taipei Times Rules to release positive cases to be relaxed today
Centers for Disease Control and Prevention Frequently asked questions about coronavirus (COVID-19) for laboratories: Interpreting results of diagnostic tests
New York Magazine What really happened with that weird Yankees COVID-19 outbreak
ABOUT THE CONTRIBUTOR
Gigafact
Between 2020 and 2022, under close editorial supervision, Gigafact contracted a group of freelance writers and editors to test the concepts for fact briefs and provide inputs to our software development process. We call this effort Gigafact Foundry. Over the course of these two years, Gigafact Foundry writers published over 1500 fact briefs in response to claims they found online. Their important work forms the basis of Gigafact formats and editorial guidelines, and is available to the public on Gigafact.org. Readers should be aware that while there is still a lot of relevant information to be found, not all fact briefs produced by Gigafact Foundry reflect Gigafact's current methods and standards for fact briefs. If you come across any that you feel are out of date and need to be looked at with fresh eyes, don't hesitate to contact us at support@gigafact.org.
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About fact briefs published on Gigafact
Fact briefs are short, credibly sourced summaries that offer “yes/no” answers in response to claims found online. They rely on publicly available, often primary source data and documents. Fact briefs are created by contributors to Gigafact —a nonprofit project looking to expand participation in fact-checking and protect the democratic process. See all fact briefs here.