Coronavirus Rapid Testing Falters in People Without Symptoms : Study

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  • #Coronavirus Rapid Testing Falters in People Without Symptoms : Study - The New York Times
    https://www.nytimes.com/2020/11/02/health/coronavirus-testing-quidel-sofia.html

    Un #test_antigénique réalisé chez des sujets asymptomatiques a donné 68 % de faux négatifs, mais cela ne concerne quasiment que des cas avec des charges virales basses (ct > ou = 30), donc ayant a priori peu de chances d’être contagieux (mais, problème non signalé par l’article, pas de possibilité de traçage prospectif ni rétrospectif).

    Par ailleurs le nombre de faux positifs a été relativement important (plus grand que le nombre de vrais positifs).

    In a head-to-head comparison, researchers at the University of Arizona found that, in symptomatic people, a rapid test made by Quidel could detect more than 80 percent of coronavirus infections found by a slower, lab-based P.C.R. test. But when the rapid test was used instead to randomly screen students and staff members who did not feel sick, it detected only 32 percent of the positive cases identified by the P.C.R. test.

    [...]

    When asked about the Sofia’s poorer performance in asymptomatic people, Dr. Harris said, “I think everybody expected that.”

    But he and his colleagues argued in their manuscript that some of the asymptomatic people who tested positive with P.C.R., but negative with the rapid test, might have been missed for good reason: They were carrying too little of the coronavirus to spread it to others.

    Some P.C.R.-based tests can be configured to produce a number called the cycle threshold, or C.T., which increases as the amount of virus in the body decreases.

    Of the 13 asymptomatic people who were not identified by the Sofia test, 12 had C.T. values in the 30s.