FDA warns of COVID-19 antigen test false positives as report refers to Quidel on accuracy.

FDA has issued an alert to clinical labs and healthcare providers about the potential for COVID-19 antigen tests to deliver false positives after receiving reports from nursing homes and other settings.The agency's notice reminds users to follow the instructions, noting that improper storage and reading the results at the wrong time can lead to false results. At the same time, the agency said false positives could be an inevitable consequence of using tests with specificities below 100%. If a test with 98% specificity is used to screen a population in which 10% of people are infected, two out of every 10 positive results will be false. One strategy has involved the widespread use of rapid tests, which forgo sophisticated equipment and can return results in minutes. Purchased in bulk by the federal government and shipped nationwide, millions of these products have already found their way into clinics, nursing homes, schools, athletic teams’ facilities and more, buoying hopes that the tests might hasten a return to normalcy. A new study casts doubt on whether rapid tests perform as promised under real-world conditions, especially when used in people without symptoms. In a head-to-head comparison,(unpublished to date) researchers at the University of Arizona found that, in symptomatic people, a rapid test made by Quidel could detect more than 80 percent of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. 32 percent is a very low sensitivity. Much of coronavirus testing has so far relied heavily on a laboratory technique called polymerase chain reaction, or P.C.R., which can amplify very small amounts of genetic material from the virus, and which is sensitive enough to detect the pathogen even when it is scarce. But these tests are slow and expensive, sometimes taking days to return results. At the University of Arizona, which hosts nearly 50,000 students and staff in a typical year, there are enough laboratory resources to run only about 1,500 P.C.R. tests each week, said David Harris, a stem cell researcher and an author on the study.. Quidel’s test, called the Sofia, looks for bits of coronavirus proteins, or antigens, instead of genetic material. It is less accurate than P.C.R.-based tests. But it takes only 15 to 30 minutes to run, using a toaster-size instrument, and costs about $23, compared with $50 or more for a typical P.C.R. test> Quidel’s test, called the Sofia, looks for bits of coronavirus proteins, or antigens, instead of genetic material. It is less accurate than P.C.R.-based tests. But it takes only 15 to 30 minutes to run, using a toaster-size instrument, and costs about $23, compared with $50 or more for a typical P.C.R. test.