Things like which form of check it was, and the explanation for taking it, ought to issue into how a lot credence to offer a constructive or damaging outcome.

Katherine J. Wu


Swabbing for a coronavirus test in San Diego this week.
Credit…Ariana Drehsler for The New York Times

In mid-November, David Piegaro examined constructive for the coronavirus. His outcomes got here too late.

The evening earlier than, Mr. Piegaro, a member of the National Guard, drove to New Jersey to go to his household after receiving two damaging fast check outcomes, two days in a row. By the following morning, he had left. But the one in a single day keep was sufficient to unfold the virus Mr. Piegaro was unknowingly carrying to a number of members of his household, together with his grandfather, who ended up spending two weeks within the hospital.

“Those two negative results gave me more confidence than I should’ve had that I could see my grandfather,” Mr. Piegaro mentioned.

Since the beginning of the pandemic, the Food and Drug Administration has issued emergency inexperienced lights to greater than 200 varieties of coronavirus checks, every with its personal curiosities and quirks. Yet we have a tendency to speak about all of them in the identical binary method, with an identical phrases: constructive, damaging, true, false.

But relating to decoding outcomes, not all positives and negatives are equally dependable. Factors like whether or not you had signs, or the variety of folks in your neighborhood who’re contaminated, can affect how assured you ought to be in your check outcomes.

“It’s about context,” mentioned Andrea Prinzi, a medical microbiologist and diagnostics researcher on the University of Colorado Anschutz Graduate School. “Your test doesn’t end when you get your result.”

Experts say these phrases indicate a misleading permanence. “Negative” can mislead folks into pondering they’re safer from the virus than they really are.

Some folks might mistakenly imagine that testing damaging provides them a free cross to socialize — “I got a negative test and now I can go visit grandma,” mentioned Dr. Ashish Jha, the dean of the Brown University School of Public Health.

But sure varieties of checks, particularly the fast ones, aren’t dependable at choosing up on low ranges of the virus and may mislabel contaminated folks as “negative.” And no check can seize an individual’s standing sooner or later. People who check damaging one morning is perhaps constructive by the following, both just because the check missed the virus, or as a result of they had been newly contaminated.

A “negative” could also be higher described as a given check’s incapability to detect the virus at a single cut-off date: an empty freeze body.

Many coronavirus check outcomes already say “detected” or “not detected” as their default readout, a distinction that a number of specialists name extra helpful.

“‘Not detected’ really points to the moment,” mentioned Dr. Valerie Fitzhugh, a pathologist at Rutgers University. “It wasn’t detected today. That doesn’t mean I won’t have it tomorrow.”

Some checks, referred to as molecular checks, search for the coronavirus’s genetic materials, or RNA. (Whether carried out by a nostril swab or saliva, these checks are typically thought-about to be dependable, particularly if processed in a lab with a method referred to as polymerase chain response, or P.C.R.) Others, referred to as antigen checks, hunt as an alternative for items of coronavirus proteins, or antigens. Antigen checks are usually quicker, however are worse than molecular checks at figuring out coronavirus instances, particularly when the virus is current at comparatively low ranges, making it potential for somebody to be RNA “positive,” however antigen “negative.”

This might happen as a result of the virus is on its method out of the physique, or as a result of it has largely failed to realize a foothold. But it’s additionally potential that antigen checks won’t establish the virus on its upswing, within the early days of an infection.

Such discordant outcomes have compelled the phrase “negative” to play a large number of roles: “Negative” for antigen doesn’t all the time imply “negative” for the virus, complicated and irritating individuals who’d hoped to declare themselves infection-free.

Using phrases like “positive” and “negative,” which could evoke outdated stereotypes about sexually transmitted infections, may additionally dissuade folks from disclosing their standing to others, or following pointers round isolation, mentioned Hannah Getachew-Smith, a well being communication skilled at Northwestern University. A detection mind-set, nonetheless, may assist “detach testing from my life and my person.”

No check is ideal. But the chance of a false constructive or a false damaging can differ drastically relying on the circumstances underneath which a check was taken. The two most necessary of those are folks’s current well being historical past, and the way a lot the virus has unfold of their neighborhood. Scientists name the confluence of those elements pretest chance.

If an individual lately began feeling sick with Covid-like signs or interacted with somebody contaminated, there may be already good motive to suspect the virus is current.

People on this class who check constructive can normally be fairly assured in a constructive or “detected” outcome, mentioned Dr. Benjamin Mazer, a pathologist at Johns Hopkins University. They sought out the check with the concept that one thing was more likely to be amiss —  like dusting for fingerprints on the scene of against the law that’s already occurred.

In such a scenario, a “not detected” outcome might sound extra suspicious, and warrant a retest, particularly in an space the place the virus has been working rampant.

The reverse is true of a screening check — a routine test for the wholesome and symptomless. Pretty a lot anybody can go for a display, introducing extra noise into the seek for the virus. It’s extra like attempting to find a felon earlier than any wrongdoing has occurred. That means extra errors will inevitably crop up.

With screening, “you’ll need to be more cautious with your result,” mentioned Ms. Prinzi of the University of Colorado. Healthy folks with no recognized exposures who stay the place the virus is scarce are fairly unlikely to check constructive. If they do, F.D.A. pointers say that these folks may wish to verify the constructive outcome with one other check, particularly if the unique was an antigen check, which tends to provide extra false positives. (In the meantime, it’s nonetheless a good suggestion to self-isolate.)

In areas the place coronavirus instances are uncommon, false positives may outnumber true positives — a scenario that might trigger undue hurt and erode belief in testing. These errors may also go up when a much less correct check is used.

Mr. Piegaro, the National Guard member who examined constructive in November, mentioned that on reflection, he ought to have been extra skeptical of these first two damaging outcomes. Before taking his fast checks, he had made a number of journeys to his fitness center in Florida, some and not using a masks — an exercise he knew was excessive threat.

The fast check he had taken, the Abbott ID NOW, was additionally not supposed to be used in folks with out signs, and has a status for lacking the virus when it’s not current at excessive ranges within the nostril. (His constructive outcome was produced by a unique check, the Cepheid Xpert Xpress, which is about 55 occasions extra delicate. That pattern was additionally taken a day later.)

Now that totally at-home fast checks are trickling into the market, Americans might must confront these testing conundrums recurrently. Two of the three dwelling checks cleared by the F.D.A. thus far are antigen checks. One of these requires a prescription and is meant to be used solely in folks with signs; a 3rd is a prescription-only molecular check, made by Lucira Health.

Whether you obtain your outcome at a testing web site or in your front room, take into consideration the way you’ve been feeling and the place you’ve been. If there’s motive to assume your check ought to detect the virus, resembling signs, a current publicity or a present outbreak in your neighborhood, a constructive outcome might be appropriate. That chance will increase if the check you’re utilizing has a status of being very correct, resembling a laboratory check.

A shock constructive shouldn’t be dismissed, although, particularly because the variety of coronavirus instances continues to balloon — growing the pretest chance for tens of millions of individuals nationwide.

If you’ve actually been cloistered away, and also you’re not feeling sick, a damaging is extra probably a damaging. Still, no single check outcome ought to clear an individual’s path to journey, mingle unmasked or shirk different measures like bodily distancing, Dr. Jha mentioned.

Tests are there to identify the virus after it’s already taken maintain, and may’t by themselves thwart its skill to unfold, Ms. Getachew-Smith mentioned. While testing is highly effective, she mentioned, “it has to be coupled with other mitigation strategies.”