The Hokey Pokey

Let us suppose it takes 1 billion UV-tagged DNA chains to trigger a positive RT-PCR test result and the polymerase chain reaction process follows a perfect doubling. If we start with a swab containing 1 viral RNA molecule we'll hit that detection threshold in 30 test cycles. If we increase the initial sample to 10 viral RNA molecules we'll hit detection at 27 test cycles. With 100 viral RNA molecules on our swab the Ct drops to 24 cycles. With 1,000 viral RNA molecules on our swab the Ct drops to 20 cycles, and with 10,000 viral RNA molecules the Ct drops to 17 cycles (slide #1).

Somebody happy to give their nose a jolly good poke and fetch out 10,000 viral fragments will yield a positive result at a Ct of 17, whereas somebody who gingerly dabbed about retrieving just 1 viral fragment will generate a positive result at a Ct of 30. These differences may arise because of viral load and they may arise simply through the random variable that is the nose poke. Because we cannot count the RN…