A friend recently woke up with a sore throat. He took a covid test at home, which came back negative, then went to work and a happy hour with two colleagues. Several days later he learned that they developed symptoms and tested positive for the virus. He had a false negative and infected them?
It seems likely, but it is impossible to know. Many Americans have probably had similar experiences. Home covid tests were supposed to help people with cold or flu symptoms determine whether they should isolate or seek treatment, but their usefulness has been greatly diminished by frequent false negatives.
One study published last spring in the Journal of the American Medical Association found that home testing detects just 64% of cases as same-day PCR tests, and just 10% of asymptomatic cases. They often give false negatives during the first few days after infection, when people are most contagious.
Much of the blame lies with the Food and Drug Administration, which has prioritized minimizing false positives over false negatives. There is usually a trade-off between the two outcomes in medical diagnostic test development: adjusting the formula in rapid tests to minimize one will usually result in more of the other.
The FDA specifies that home tests must demonstrate only 80% sensitivity in clinical trials, meaning less than 20% of people who are truly positive test negative, though they generally do worse in real-world studies. . At the same time, the agency says that tests must be at least 98% specific, meaning that less than 2% of people who are truly negative test positive.
As an example, the FDA has refused to authorize a test from start-up Global Diagnostic Systems, which returned no false negatives in clinical trials, but had a 5.2% false positive rate. The test uses a highly absorbent swab and highly sensitive antibodies to detect the Covid antigen. Their process also does not dilute the sample and produces an easy-to-read test line for positive results.
By contrast, other tests require users to place their swab in a tampon and squeeze the sample out of the swab or add drops to it. These approaches dilute the sample and make it difficult to detect the presence of Covid. Users may also miss very faint lines on test strips.
Abbott’s BinaxNOW test instructions, for example, place a magnifying line next to its test strip and advise users to “look very closely” because “the bottom line can be very faint.” Abbott says that eight of the 30 users in his “human factors” study had trouble seeing some results: “As the intensity of the line improved, the ability to read the result correctly ranged from 83% to 60%. , with a general rate of 70%. .”
Thanks to its relative ease of use, the Global Diagnostics test should also be less likely to give false negatives in the real world. GDS shared with the FDA a modeling study by Yale researchers that showed that the test would result in far fewer infections than other tests under various epidemiological and behavioral assumptions.
Why, then, doesn’t the FDA relax the specifications and give the test a green light? An FDA spokesperson says the agency “cannot comment on any filings that a company has or has not made.” In general, however, the agency is concerned about false positives that delay proper diagnosis and treatment for the actual cause of the disease. Someone who gets such a result could also self-quarantine with other infected people.
The latter scenario is likely to be rare and would put only the uninfected at risk. On the other hand, false negatives could also delay treatment for people who are at risk of getting seriously ill from covid. Treatments like the antiviral Paxlovid are most effective when taken soon after symptoms start.
False negatives further contribute to the spread of the virus and put vulnerable people at risk. The FDA has tacitly acknowledged that the high false negative rate is a problem. In November, the agency instructed people who test negative to take multiple tests over three to five days to increase the chances of detecting the virus. But people can hardly be expected to isolate themselves and test themselves repeatedly for several days each time they develop cold symptoms.
This strategy is also expensive. After the public health emergency ends in May, Americans will have to start paying out-of-pocket for home tests, which cost about $10 each. Manufacturers make more money by producing less sensitive tests that must be taken repeatedly. But many people would prefer tests that give fewer false negatives, even if they have a slightly higher risk of giving a false positive. Why not give them the option?
The FDA has relaxed its standards for authorizing children’s Covid vaccines and bivalent boosters despite potential risks and scanned evidence of benefits, while limiting access to home tests that could reduce the spread of the virus and help protect Americans at risk. Once again, public health officials seem to ignore their contradictions.
Magazine editorial report: Paul Gigot interviews Dr. Marty Macary. Images: AFP/Getty Images Composition: Mark Kelly
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