Can you trust the results of a rapid home COVID-19 test? What the experts say

By Julie Appleby and Phil Galewitz, KHN

Julie Ann Justo, an infectious disease clinical pharmacist for a South Carolina hospital system, hoped that Christmas week would finally be the time when her family could safely come together for a reunion.

Prior to the celebration, eligible family members were vaccinated and strengthened. They quarantined and used masks in the days leading up to the event. And many took comfort in the negative results of speed COVID-19[femininetests effectués quelques jours avant le rassemblement en salle de 35 personnes dans le sud de la Floride pour s’assurer que personne n’était contagieux.

Mais en une semaine, Justo et au moins 13 membres de sa famille élargie ont été testés positifs pour COVID-19, beaucoup ressentant les symptômes typiques d’un virus des voies respiratoires supérieures, comme un mal de gorge et un nez qui coule.

Comme beaucoup d’autres Américains, la famille de Justo a appris à ses dépens qu’un seul résultat négatif d’un test rapide à domicile, qui prend environ 15 minutes, ne garantit pas qu’une personne n’est pas malade ou porteuse du virus.

Il y a tellement de variables. Les tests peuvent survenir soit trop tôt, avant que suffisamment de virus ne soient présents pour être détectés, soit trop tard, après qu’une personne a déjà transmis le virus à d’autres.

Et la plupart des tests rapides, même selon leurs instructions, sont destinés à être utilisés par paires – généralement à un jour ou deux d’intervalle – pour une précision accrue. Malgré cela, quelques marques sont vendues à l’unité et, avec des tests parfois chers et rares, les familles comptent souvent sur un seul dépistage.

Alors que les tests d’antigène à domicile restent un outil utile – et sous-utilisé – pour freiner la pandémie, disent les experts, il est souvent mal utilisé et peut donner une fausse confiance.

Certaines personnes considèrent à tort les tests à domicile “comme une carte de sortie de prison gratuite”, a déclaré Dr William Schaffner, spécialiste des maladies infectieuses à la Vanderbilt University School of Medicine à Nashville, Tennessee. “‘Je suis négatif, donc je n’ai plus à m’inquiéter.'”

C’est encore plus vrai maintenant que la nouvelle variante plus transmissible domine le pays.

“Omicron est si transmissible qu’il est difficile d’utiliser n’importe quel type de stratégie de test en termes de rencontres et de réussir”, a déclaré Dr Patrick Mathias, vice-président des opérations cliniques du département de médecine de laboratoire et de pathologie de la faculté de médecine de l’Université de Washington.

Les tests rapides sont assez bons pour détecter correctement l’infection chez les personnes présentant des symptômes, a déclaré Mathias, avec une plage de précision de 70% à près de 90% estimée dans plusieurs études. D’autres études, dont certaines sont antérieures aux variantes actuelles ou ont été réalisées dans des contextes plus contrôlés, ont montré des taux plus élevés, mais, même dans ce cas, les tests peuvent encore manquer certaines personnes infectées. Cela augmente le risque de propagation, la chance augmentant considérablement à mesure que le nombre de personnes assistant à un événement augmente.

Les résultats des tests antigéniques sont moins précis pour les personnes sans symptômes.

Pour les asymptomatiques, les tests rapides, « en moyenne, [correctly] detect infection about 50% of the time,” said Shama Cash-Goldwasser, adviser for Prevent epidemics to solve to save lives, a nonprofit group led by Dr. Tom Frieden, former director of the Centers for Disease Control and Prevention.

Looking back, Justo said her family took precautions, but she recognizes the missteps that put them at increased risk: Not all family members were tested before getting together due to a shortage of test kits. Some of his family members who could only find rapid tests tested once due to the need to ration tests. And there were several children under the age of 5 who are not yet eligible for a COVID-19 vaccine. They were then among the first to show symptoms.

“We were probably relying too much on negative rapid tests to congregate indoors with others without other layers of protection,” she said.

Even if everyone tested themselves properly before the party, health experts said, that would not mean all attendees are “immune” from contracting COVID-19. The tests simply reduce the risk of exposure; it does not eliminate it.

Other risk assessment factors at a gathering: Is everyone vaccinated and boosted, which can help reduce the risk of infection? Did the participants correctly follow all the steps outlined in the test kit instructions, which may differ by brand? Did someone test too soon after exposure or, conversely, not close enough to the event?

A critical detail “is the timing of the test,” Schaffner said at Vanderbilt. Another, he said, is the tests’ ability to detect true positives and true negatives.

Test too soon, such as a day or two after exposure, and the results will not be accurate. Likewise, testing several days before an event won’t tell you much about who might be infectious on the day of the gathering.

Schaffner and others recommend that self-testing begin three days after known exposure or, if feeling ill, a few days after symptoms appear. Since the time to detection of an infection is uncertain, it is always a good idea to use both tests in the kit as directed – the second 24-36 hours after the first. For an event, make sure one of the tests is done on the day of the gathering.

Antigen tests work by looking for proteins on the surface of the virus, which must be present in adequate amounts for a test to be detected. (Lab-based PCR tests, or polymerase chain reaction tests, are more accurate because they can detect smaller amounts of virus, but they take longer to get results, maybe even days, according to backlog of labs.)

COVID-19 markers can persist as remnants long after the live virus is gone, so some scientists question the use of tests – whether antigen or PCR – as a measure of timing where patients can end their isolation, particularly if they seek to shorten the recommended period. The CDC recommends five days of isolation, which can end if their symptoms resolve or resolve, with no fever.

Some patients will test positive 10 or more days after their first symptoms, although they are unlikely to remain infectious by then.

Yet that means many people are using rapid tests inappropriately — not only relying too much on them as protection against COVID-19, but also as an indicator of when an infection is over.

Rapid home tests should be used over several days to increase the chance of getting an accurate result.

“Each individual test doesn’t have much value as a serial test,” said Dr. Zishan Siddiqui, chief medical officer of the Baltimore Convention Center Field Hospital and assistant professor of medicine at Johns Hopkins University. And, because testing is less reliable in people without symptoms, he said, asymptomatic people shouldn’t rely on a single rapid test to reunite with friends or family without taking further action. attenuation.

Worse still, a recent study looking at the omicron variant found that rapid tests could not detect the virus within the first two days of infection, even though laboratory PCR tests found evidence of COVID-19.

The study looked at 30 adults vaccinated in December 2021. “Most cases of omicron were infectious for several days before being detectable by rapid antigen tests,” according to the study, which was not evaluated by peers.

False negatives are also more likely when the extent of disease in a certain area, called community spread, is endemic, which is true for most of the United States today.

“If there’s a lot of community spread, it increases the likelihood that you have covid” at a gathering, Cash-Goldwasser explained, because one or more attendees who tested negative may have received a false result. . Positivity rates are now over 25% in some US cities, indicating that a lot of virus is circulating.

So right now, “if you get a negative result, it’s important to be more suspicious,” she said.

Vaccinations, boosters, masking, physical distancing, ventilation, and separate testing are all imperfect strategies to prevent infection. But layered together, they can provide a more effective barrier, Schaffner said.

“The rapid test is useful” – his own family used them before gathering for Thanksgiving and Christmas – “but it is a barrier with holes in it”, he added.

The virus moved through these gaps to crush the party and infect the Justo family. While most of the participants had largely mild symptoms, Justo said she was short of breath, tired and suffered from headaches, muscle aches and nausea. It took about 10 days before she felt better.

“I definitely spent a lot of time looking back at what we could have done differently,” Justo said. “Fortunately no one needed to go to hospital, and I put that down to vaccinations – and I’m grateful for that.”

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