Covid research hits a wall: Why antibody tests won’t help us much

Many tests are inaccurate, some look for the wrong antibodies and even the right antibodies fade away, said experts at the Infectious Diseases Society of America, which issued the new guidelines. Because current tests cannot determine if someone is immune, the society said, they “cannot inform decisions to discontinue physical distancing or lessen the use of personal protective equipment.”

Antibody testing generally should be used only for population surveys, not for diagnosing illness in individuals, the panel said. But its guidelines described two situations in which antibody testing could be used when the normal diagnostic tests for the virus — called PCR tests — failed or were likely to fail. People who felt sick weeks or months ago and now wonder if they are immune to COVID-19 “probably shouldn’t bother” getting an antibody test, said Dr Angela M. Caliendo, a testing expert at Brown University’s Alpert Medical School and a member of the society’s expert panel.

Many New Yorkers fell ill in the spring, could not get tested then and now wonder whether they had the disease and are immune; the panel concluded that current antibody tests cannot answer that question. “We don’t really know if a positive test means you’re protected,” Dr Caliendo said. “If you were sick back in March, you probably don’t have antibodies any more; and if you were only mildly sick, you may not have gotten antibodies.” Moreover, she said, “if you live in a low-prevalence area, you have a much higher likelihood of getting a false-positive test, meaning you may think you are protected but you aren’t.”

Despite the flaws of antibody tests, recent studies of patients who definitely were infected suggest that they have long-lasting immunity and that it is very unlikely they will get reinfected. That may be because white blood cells known as B and T cells, which are “primed” to recognise and attack the coronavirus, remain in circulation long after antibodies have faded away. But B and T cells are not analysed by common antibody tests. Other experts echoed the society’s recommendation against individual antibody tests. Michael T. Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy, called diagnostic clinics that charge individuals for antibody tests “snake oil salesmen.”

Antibody tests — which analyse blood for proteins produced by the immune system that bind to the outside of the virus and neutralize it — are useful mostly for surveys of large populations to see roughly how many people have had the disease, the society said. Even for that purpose, only tests that are correctly positive more than 96 per cent of time and correctly negative at least 99.5 per cent of the time should be used, according to the guidelines.

Very few of the dozens of tests made in this country, Europe and Asia that the panel looked at met those standards, Dr Caliendo said. None can be done at home or immediately in doctors’ offices, and the best are assays known as Elisa or CIA (chemiluminescence immunoassay). Tests using paper strips were less consistently accurate, the panel found. With two exceptions, antibody tests should not be used to diagnose individual infections, the society said. When a patient has all of the symptoms of COVID-19, including X-ray evidence of pneumonia, but still comes up negative on repeated diagnostic PCR tests for the virus, an antibody test may be useful.

Ideally, it should be given not immediately after symptoms appear, but three to four weeks later — “the sweet spot,” Dr Caliendo said. (PCR nasal swab tests may come up negative after the virus migrates from the nose and throat to the lungs, Dr Caliendo explained; but the body may not produce enough antibodies to be detectable in the blood until more than two weeks have passed.) The tests can also be used for diagnosis when a doctor suspects a child has multisystem inflammatory syndrome, a rare but serious complication of COVID-19 in children. Because it is not known how long after the initial infection this inflammation begins, doctors should do both a PCR test and an antibody test, the guidelines said.