Florida Department of Health Quietly Requires Labs to Report Cycle Threshold of Troubled PCR Test https://t.co/LQ4oNLbn2s
— LastAmericanVagabond (@TLAVagabond) December 7, 2020
The Florida Department of Health has issued new guidance requiring all laboratories conducting Covid19 tests to report the “Cycle Threshold” (CT) level for tests.
“Cycle threshold (CT) values and their reference ranges, as applicable, must be reported by laboratories to FDOH via electronic laboratory reporting or by fax immediately,” a memo issued by the Florida Department of Health reads.
“If your laboratory is not currently reporting CT values and their reference ranges, the lab should begin reporting this information to FDOH within seven days of the date of this memorandum.”
Reverse transcriptase quantitative polymerase chain reaction (PCR) tests use nasal swabs to collect RNA from a person’s nasal cavity. The RNA is reverse transcribed into DNA, and then amplified through 40 or more cycles. The result is then reported as a simple “yes” or “no” to the question of whether a person is infected with Covid19.
But tests with cycle thresholds above 35 are too sensitive, health officials say. “I’m shocked that people would think that 40 [cycles] could represent a positive,” says Juliet Morrison, PhD, a virologist at the University of California, Riverside. A more reasonable cutoff would be 30-35, she says.
Officials at the Wadsworth Center, New York’s state lab, have access to CT values from tests they have processed. Earlier this year, at The New York Times’ request, they re-analyzed their numbers. In July, the lab identified 872 positive tests, based on a threshold of 40 cycles. With a cutoff of 35 cycles, about 43% of those tests would no longer be considered positive. 63% percent would no longer be judged positive if the CT threshold was limited to 30.
In Massachusetts, from 85 to 90 percent of people who tested positive in July with a cycle threshold of 40 would have been deemed negative if the threshold were 30 cycles.