What a Negative COVID-19 Test Really Means


It’s nonetheless unclear how good COVID-19 exams are at discovering these presymptomatic circumstances, however the timing of the take a look at issues. As quickly because the coronavirus finds its manner into a new host, it hijacks cells to repeat itself. The quantity of virus builds over this time, peaking at or proper earlier than symptom onset, which might take two to 14 days however often takes a mean of 5 or 6. Accordingly, public-health authorities have suggested getting examined about 4 days after publicity.

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This is a affordable advice, given the knowns, however surprisingly little knowledge exists on how early COVID-19 exams can detect an infection earlier than symptom onset. One mannequin utilizing COVID-19 circumstances from seven beforehand revealed research means that the false-negative charge is 100 p.c on day considered one of publicity, which falls to 38 p.c on day 5 (when signs on common seem) after which a minimal of 20 p.c on day eight. But in combing the literature, the researchers behind this mannequin discovered just one case the place a affected person was examined earlier than feeling sick—as a part of a cluster at a chalet within the French Alps. Moreover, the mannequin doesn’t deal with a entire different set of people that by no means develop signs in any respect. “It gave no information about people who are permanently asymptomatic,” Justin Lessler, an epidemiologist at Johns Hopkins University who co-authored that research, informed me.

More knowledge are more likely to come quickly. On Tuesday, the Food and Drug Administration launched suggestions for labs and producers that wish to validate COVID-19 exams in individuals who don’t have signs. (Currently, no exams are FDA-authorized for screening asymptomatic individuals.) Testing massive numbers of asymptomatic individuals—reminiscent of by pooling samples—to determine potential silent carriers will even be an important a part of the reopening technique for colleges and companies.

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For people, nonetheless, the FDA cautions that destructive outcomes don’t rule out an infection. It asks that asymptomatic exams embrace this assertion: “Negative results must be considered in the context of an individual’s recent exposures, history, presence of clinical signs and symptoms consistent with COVID-19.” And this, within the face of imperfect COVID-19 exams, is vital to deciphering a destructive consequence. It will depend on your chance of getting COVID-19 within the first place.

Consider once more the choice to go to aged kinfolk after a destructive take a look at. “If you have symptoms or you work in a place where you’re at high risk for exposure, then even with a negative test, you might want to think really hard about it,” Steven Woloshin, a co-director of the Center for Medicine and Media on the Dartmouth Institute, defined to me. “If you’re at low risk because you live in some remote area, you’re practicing social distancing, you always wear a mask, and you feel fine,” a destructive take a look at might be a true destructive. So even with widespread testing, social distancing and masks will proceed to be essential for controlling the unfold of COVID-19.