The Science of Testing

Some science and explanation behind same-day COVID testing before indoor singing

Version: 2-Jan-2023

See also: COVID Testing Before Indoor Singing

What About “False Negatives”?

Symptomatic: It has become more common for people to notice symptoms a day or two before their rapid antigen test turns positive. This is likely due to immune activation, and may be even more pronounced when someone is vaccinated. This doesn’t mean that the antigen test doesn’t work. It just means that the amount of virus in their body was still too low for the test, but high enough to stimulate the immune system. But for our purposes, if a person has symptoms they should not attend a singing at all, regardless of their antigen test result.

Asymptomatic: It is possible to test people in a small window of time (for a few days after exposure) where they have already been infected, but they haven’t grown enough virus to turn the antigen test positive, and they don’t even have symptoms yet. But those people who don’t have enough virus to turn the antigen test positive are also not contagious to others. The negative antigen test in that case is not a failure (“inaccurate”), since no one gets infected at the singing.

For purposes of screening people for participation in singing, the relevant question is not whether they are PCR-positive, but whether they are contagious to others. Think of the rapid antigen test as a test for contagiousness.

It’s initially hard to wrap one’s head around the idea that we are not testing for: who will eventually get COVID; who will eventually have symptoms; who already has a positive PCR test; who will develop a positive rapid antigen test in the future.

We are only testing for who is contagious (infectious) during that brief window of time when we sing together.

Mina, 2020

Science!

  • For this to work, the prevalence of COVID in the community must be fairly low:
    • Less than 2% of the population is contagious, or below about 40 new diagnosed cases / 100k population / day, just as a rough guide. [1]
    • Requiring that singers be asymptomatic and have no recent direct exposure also helps to lower this pre-test probability.
    • Under these conditions, the negative predictive value of the test is >95%, even when the sensitivity of the test is no greater than 50-60%.
Woloshin et al., 2020
  • In fact, rapid antigen tests are >85% sensitive to detect COVID infection if the person’s viral load is high enough to be contagious to others (>106 copies/ml, or equivalently, a cycle-threshold Ct<25). (Gallichotte, 2020, Figure 2; Peeling, 2022; Pickering, 2021; Schuit, 2021; Wölfel, 2020) By comparison, PCR testing has a detection limit of about 103, and the time needed to transition between those two levels is on the order of one to two days.
  • Comparison of the antigen test to the presence of culturable virus is more important than comparison to PCR-positivity. PCR tests remain positive for weeks after the patient ceases to be contagious. Antigen test sensitivity is much higher (>90%) when compared to same-day culturable virus, the situation where you expect people to be infectious. (Chu, 2002; Korenkov, 2021; LaScola, 2020; Said, 4 Jan 2022; Cosimi, 2022; Van Kampen, 2021; Pekosz, 2021; Tariq, 2021)
Puhach, 2022: Fig. 1: Methods to measure infectious virus and RNA viral load.
Methods to measure infectious virus and RNA viral load. (Puhach, 2022: Fig. 1.)
Viral culture as contagiousness standard

Other questions about testing before indoor singing?

Contact David Brodeur

Sources

[1] My “back of the envelope” calculation is as follows:

  • 40 new reported cases per day per 100,000 population
  • each reported case represents 10 actual new cases (ascertainment bias)
  • each new case will be contagious to others for 5 days
  • 40/100,000 X 10 X 5 X 100% =2% prevalence of contagion

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