The Case for the Rapid (but Imperfect) Test

Dana G Smith
Published in
2 min readAug 6, 2020

Image: Yuri Samoilov via flickr/CC BY 2.0

Imagine if every morning before you showered you spit into a test tube, and by the time you got out, you knew if you had Covid-19. That result would dictate whether you went to work or saw friends or if you stayed home and isolated.

This technology exists, and some experts say it could be a pandemic game-changer, but critics of the idea, including the FDA, say the tests aren’t reliable enough.

The tests detect antigens, proteins on the surface of the virus that trigger an immune response. The antigen tests are faster to use and cheaper to produce than PCR tests, which look for the virus’s RNA. They could even function like at-home pregnancy tests — antigens in the saliva react with molecules in a strip of paper, changing its color to indicate a positive test. However, the FDA requires tests to detect at least 80% of positive cases, and these rapid tests are closer to 50%.

That sounds problematic. But Michael Mina, MD, PhD, a Harvard epidemiologist and immunologist, says the difference in test accuracy is at either the very beginning or end of the disease when only a small amount of the virus is present and people aren’t really infectious. The tests are much more precise when people have millions of viral copies in their nose and throat, which is also when they’re most infectious, even without symptoms.

Mina says a less precise test that immediately detects the most infectious cases is an improvement. Currently, about nine in 10 people with the virus go undiagnosed because of testing shortages. “High frequency testing using a less sensitive test goes much further than low frequency testing using the best test in the world,” said Mina in an interview with This Week in Virology.

Dana G Smith

Health and science writer • PhD in 🧠 • Words in Scientific American, STAT, The Atlantic, The Guardian • Award-winning Covid-19 coverage for Elemental