False Positives

COVID-19 Questions & Myths

17. What is the real false positive rate of COVID-19 PCR tests? (I have heard it is 50-90%!), and what is a “cycle threshold” in PCR testing?

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.

18. Are false positives and over-testing driving up the case numbers? If testing stops will the problem go away?

As noted above, the false positive rate of COVID-19 PCR testing is extremely low. Many think that Alberta performs more tests than other provinces. This was true early in the pandemic when Alberta’s provincial laboratory was able to do more tests than other provinces. Then Alberta trailed behind Ontario and PEI by approximately 20,000 tests until mid-January. As of January 18, 2021, on the CBC News Coronavirus Tracker, Alberta (67,518 tests/100,000 people) performed the highest number of tests with Ontario  (58,685 tests/ 100,000 people) and PEI (54,515 tests/ 100,000 people) next. If increased testing of well people was causing the problem, we would not see the increases in hospitalizations and deaths after 1-3 weeks of newly detected cases..

Reference:
i. Tracking the Coronavirus, CBC News