Most of the coronavirus tests the U.S. does are worthless. But there's a solution that could actually work — and stop the spread

These are not the same thing as the antibody tests you might have heard of, which detect virus-fighting substances in the blood of people who were infected previously (and may therefore have immunity). Rapid antigen tests are meant to detect ongoing, active infections.

So why isn’t the U.S. government mass-producing antigen tests and distributing them freely to everyone? The major hurdle, so far, has been what news stories tend to describe as “accuracy.” But “sensitivity” is a better way to think about it.

PCR tests make thousands of copies of the virus’s RNA and thus can detect it at very low levels. Antigen tests rely on a molecule that binds to the coronavirus’s protein spikes; at very low levels of infection, there may not be enough viral particles in a sputum sample to trigger a positive result.

The fear is that if we rely on antigen tests, we’ll miss many cases. But there are two reasons this fear may be unfounded

These are not the same thing as the antibody tests you might have heard of, which detect virus-fighting substances in the blood of people who were infected previously (and may therefore have immunity). Rapid antigen tests are meant to detect ongoing, active infections.

So why isn’t the U.S. government mass-producing antigen tests and distributing them freely to everyone? The major hurdle, so far, has been what news stories tend to describe as “accuracy.” But “sensitivity” is a better way to think about it.

PCR tests make thousands of copies of the virus’s RNA and thus can detect it at very low levels. Antigen tests rely on a molecule that binds to the coronavirus’s protein spikes; at very low levels of infection, there may not be enough viral particles in a sputum sample to trigger a positive result.

The fear is that if we rely on antigen tests, we’ll miss many cases. But there are two reasons this fear may be unfounded

Respiratory therapist Diana Vega of the University Medical Center of Southern Nevada collects coronavirus tests during a preview of a new testing site on Monday. (Ethan Miller/Getty Images)

The first is that we’re already missing tons of cases. According to CDC antibody data, our current PCR system is testing only enough people to detect about 10 percent of the total number of infections. “If everyone took an antigen test today — even identifying only 50 percent of the positives — we would still identify 50 percent of all current infections in the country,” Jha has explained. That’s “five times more than the 10 percent of cases we are likely currently identifying because we are testing so few people.”

The second is that the amount of coronavirus in the body increases exponentially in the early days of infection. At first, during the incubation period, no test would be sensitive enough to detect it. About three to five days later, a PCR test would pick it up. After another eight to 24 hours, according to Mina, a rapid antigen test would show a positive result.

So the question isn’t whether an antigen test would detect an infection. It’s when.

This is a crucial distinction. Say the PCR test delivers a positive on day four, and the antigen test delivers a positive on day five. As best as experts can tell, both day four and day five are before or near the beginning of the window when people can transmit the virus to others. But if the person who takes the PCR test doesn’t find out whether he or she is positive until day eight, or even day 14, while the person who takes the antigen test finds out on day five, the antigen test is far more useful for stopping the spread of the virus, even though it’s less sensitive.

What if your viral load was too low on day five for the antigen test to detect it? Your infection probably wouldn’t be very transmissible yet. Then the next test you take, on day seven, would pick it up. And the person who took the PCR test still wouldn’t have the results.

“The vast majority of PCR positive tests we currently collect in this country are actually finding people long after they have ceased to be infectious,” Mina recently explained. “All we’re doing with all of this testing is clogging up the testing infrastructure, and essentially finding people for whom we can’t even act because they are done transmitting.”

Current FDA guidelines stipulate that any new coronavirus test vying for emergency clearance must perform nearly as well as a PCR test. But as fall approaches — and with it colder weather, increased indoor activity, a return to school and the potential for even larger waves of infection — the time may have come to rethink those regulations.

Rapid antigen tests have their challenges. Equitable distribution on a mass scale would probably require billions of dollars in government investment; cheap and easy at-home tests would work only if everyone takes them conscientiously.

But experts say they’re starting to make more sense than the status quo.

“If you had asked me this a couple months ago, I would have said we just need to be doing the PCR tests,” Susan Butler-Wu, a clinical microbiologist at the University of Southern California, told the New York Times Wednesday. “But we are so far gone in this country. It is a catastrophe. It’s kitchen sink time.