Ever since the coronavirus pandemic began, battles have raged over testing: Which tests should be given, to whom, and how often? Now, epidemiologists

One number could help reveal how infectious a COVID-19 patient is. Should test results include it?

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2021-07-25 21:30:04

Ever since the coronavirus pandemic began, battles have raged over testing: Which tests should be given, to whom, and how often? Now, epidemiologists and public health experts are opening a new debate. They say testing centers should report not just whether a person is positive, but also a number known as the cycle threshold (CT) value, which indicates how much virus an infected person harbors.

Advocates point to new research indicating that CT values could help doctors flag patients at high risk for serious disease. Recent findings also suggest the numbers could help officials determine who is infectious and should therefore be isolated and have their contacts tracked down. CT value is an imperfect measure, advocates concede. But whether to add it to test results “is one of the most pressing questions out there,” says Michael Mina, a physician and epidemiologist at Harvard University’s T.H. Chan School of Public Health

Standard tests identify SARS-CoV-2 infections by isolating and amplifying viral RNA using a procedure known as the polymerase chain reaction (PCR), which relies on multiple cycles of amplification to produce a detectable amount of RNA. The CT value is the number of cycles necessary to spot the virus; PCR machines stop running at that point. If a positive signal isn’t seen after 37 to 40 cycles, the test is negative. But samples that turn out positive can start out with vastly different amounts of virus, for which the CT value provides an inverse measure. A test that registers a positive result after 12 rounds, for a CT value of 12, starts out with more than 10 million times as much viral genetic material as a sample with a CT value of 35.