Antibody study suggests coronavirus is far more widespread than previously thought


Blood tests to detect past exposure to the virus are starting to hit the market but as NPR's Richard Harris tells Steve Inskeep of morning edition even test that claim to be more than ninety percent accurate will often miss the mark one of the tests supposed to do well the test cannot be used to diagnose the disease instead they identify antibodies that appear in your blood about a week after you've been infected he said about is a part of your immune system's reaction to the virus so I just do not know the weather people with antibodies are definitively protected from the disease and if so for how long but that hope that prospect is really driving a lot of this excitement so for example I talked to Deborah Vander gassed and tipped in Iowa she runs a daycare center for children with developmental and behavioral disabilities they're a lot like little kids everywhere we laugh about you know the the sanitizing everything because you know the three impacted justice two seconds later center gassed is eagerly awaiting the rollout of the blood test in her county she thinks about her staff who are being hyper vigilant not to spread the disease if some of the people I have already been established to have antibodies they wouldn't have to go home and I sleep for two weeks they can continue working she says the test isn't available in her area but it is starting to take off nationally Dr Jeremy Galbraith runs a mobile medical service in Austin Texas he says he got a supply of antibody tests made by a major Chinese manufacturer he's already run a few hundred tests in the last few days we you know also the test for people who may have suspected that they had corona virus back in February or March when testing with a nasal swab PCR was very limited Gabbar says he only test people when he has other evidence that they might have been exposed if they had an illness that sounds like it could have been coronavirus and they have a positive antibody test then it's very likely that this is a what we call a true positive that they indeed had come in nineteen the testes using boasts a specificity of ninety nine percent which means it only falsely says a blood sample has antibodies when it doesn't just one percent of the time but despite that impressive statistic a test like this is not ninety nine percent correct and in fact in some circumstances could be much much worse that's because of this counter intuitive fact the validity of a test depends not only on the test itself but oddly on how common the diseases in the population you're sampling it is kind of a strange thing Dr Gilbert Welch is a scientist at Brigham and women's hospital in Boston hi antibody test is much more likely to be wrong in in the population with very little code the Greeks Boettcher Richard I think we need to slow down here why with the accuracy of a test depend on how common the disease is in a population yeah that it's surprising but here's a simple way to look at it say you are running a test it gives five falsely positive results in a hundred people sounds like pretty good odds right but yeah but consider this Steve if five percent of our population is infected then you run the test on a hundred people you should get five true positives but you also have those filed false positives well says there's no way to know which is which the test will be wrong half the time half the people will be falsely reassured so it's basically a coin flip and it gets worse the food and drug administration does not regulate these tests but the White House coronavirus task force set in informal standard they're supposed to have no more than ten false positives per hundred if you were to use a test that meets that standard in a population where only one percent of the population had been infected with rotavirus a positive result would be wrong a shocking amount more than nine times out of ten and you can see that one way to limit this problem is to focus on populations with the disease is more common Dr Jordan laser a pathologist at Northwell health on Long Island New York says it would make sense to start with health care workers should be wonderful for health care workers to know their immune status and give them just a peace of mind even so laser says it would still be a mistake to rely on these results definitely don't use these tests to change your practices in terms of personal protective equipment definitely do not become more comfortable in doing your job and taking care of complications it really would be more of a psychological benefit but you know these tests can still be incredibly useful as long as individual false positive results don't matter and one situation with that is the case is serving a broad populations and in fact these tests will be used to figure out just where

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