Strategic COVID-19 Pandemic Committee

The false positive rate of COVID-19 PCR tests is extremely low. What is a “cycle threshold” in PCR testing?

COVID-19 Questions & Myths

A false positive is when someone who does not have COVID-19 tests positive for it. False positives could potentially make us think there are more cases of something than there really are. The false positive rate is the number of people who are uninfected but wrongly test positive as a proportion of all the people tested who do not have the virus.

The false positive rate of COVID-19 tests is low between 0.8 and 4% in most reports (published in a paper by Mayers & Baker). This is not enough to significantly affect COVID-19 case estimates.

The cycle threshold (CT) of PCR tests refers to the number of rounds of amplification needed to find a positive signal, so a higher number means there was a smaller amount of viral material in the sample. A test cannot create viral material.  Our test can detect small amounts of virus, which can indicate a poor sample, very early infection, or late possibly non-transmissible infection.

References:
i. Interpreting a COVID19 test result, Watson J, Whiting PF and Brush JE, British Medical Journal
ii. Impact of false-positives and false-negatives in the UK's COVID-19 RT-PCR testing programme, Mayers C, Baker K.