Friday, May. 21, 2021
Does the standard coronavirus test use a single globally-agreed measure of viral presence for determining positivity?
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
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Fact briefs are bite-sized, well-sourced explanations that offer clear "yes" or "no" answers to questions, confusions, and unsupported claims circulating online. They rely on publicly available data and documents, often from the original source. Fact briefs are written and published by Gigafact contributor publications.
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