As South Carolina and the rest of the nation experience yet another surge in positive coronavirus cases (due in no small part to the expanded availability of testing), questions are emerging as to the reliability of the Palmetto State’s testing numbers.
And the efficacy of test results …
Specifically, should South Carolina be following the lead of Florida in trying to determine the extent to which its “positive” tests are really positive?
Wait … what is a cycle threshold value?
Basically, it is a measurement of the extent to which a person is infected with Covid-19 – and more importantly, an indicator of the extent to which their infection is contagious.
Most Covid-19 tests rely on what is known as the polymerase chain reaction (PCR) method – in which specially calibrated machines identify, isolate and amplify the virus’ ribonucleic acid (RNA) in order to yield detectable amounts of viral material.
In other words, they scan a sample multiple times until they are able to detect traces of Covid-19.
This “magnification” process typically runs between 37 to 40 cycles. If there is no viral material detected by the end of the fortieth cycle, your Covid-19 test comes back negative.
For positive tests, though, “cycle threshold” refers to the first cycle in which viral material was detected.
Does this number matter? Yes. The earlier in the cycle a test comes back positive, the more viral material is present – i.e. the higher your “viral load” is. And the more contagious you are.
By a significant factor, too …
“A test that registers a positive result after 12 rounds, for a CT (cycle threshold) 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,” Robert F. Service noted in a recent article for Science.
This inverse measure is not precise – and has been known to yield inconsistent results – but research has consistently demonstrated that lower CT numbers generally correlate to more serious (and more contagious) cases of Covid-19.
Which makes sense …
The more viral material people are exposed to, the worse their experiences with the virus are generally going to be … and the easier it will likely be for them to spread that material.
Generally speaking, if your cycle threshold number is 25 or lower you have what is considered to be a high viral load – meaning you are more susceptible to potentially serious Covid-19 complications and more likely to be contagious.
If your cycle threshold number is above 35, however, you have a low viral load – meaning it is far less likely that you could transmit the virus. Also, individuals with CT scores above 35 are far less likely to experience the onset of severe symptoms – especially if they are younger and in good physical shape.
“We have to stop thinking of people as positive or negative, and ask how positive?” Harvard University epidemiologist Michael Mina told Service.
That is exactly what Florida is attempting to do by requiring labs to report “cycle threshold (CT) values and their reference ranges” in reporting positive cases.
Some have speculated this is “a first step toward changing the definition of a COVID-19 case in Florida,” but we see it as less about changing definitions and more about providing better, more actionable data.
After all, ascertaining which positive cases are more dangerous than others can help health officials better target their resources. And more importantly, they can help individuals who test positive for Covid-19 better assess their risk – to themselves and those around them.
“There is a growing body of research from respected institutions … that clearly demonstrates that many people who test positive are not actually infectious,” Doherty said. “Further, they’ve shown how it’s possible to identify which positive results can be safely ignored without causing an unnecessary disruption to people’s lives. If governor Murphy truly believes in the science and wants to do what’s right for New Jersey, he should do what Florida just did and update how positive COVID-19 results are reported.”
Accordingly, this news outlet would call upon the S.C. Department of Health and Environmental Control (SCDHEC) to follow Florida’s lead and begin requiring laboratories to submit this cycle threshold data – and to begin providing this information to the public as part of its daily case total tabulations.
We would also call upon the agency to release any cycle threshold data it has previously collected in the hopes of providing some sense as to the severity of the positive cases it has been reporting since March.
Clearly this is a metric worth following as we continue to monitor rising Covid-19 cases …
Again, our objective is not to try and reclassify what constitutes a positive case – or in any way, shape or form to downplay the severity of the virus – it is simply to provide context for risk, specifically the risks associated with additional societal shutdowns related to the virus.
After all, if we are able to rule out transmission potential for many (if not a majority of positive Covid-19 cases), isn’t that worth knowing?
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