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.