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A ‘99% Accurate’ Antibody Test
What to know about the Abbott antibody test being rolled out in the EU and some parts of the United States
This week, the European Union approved an antibody test for Covid-19 that its manufacturer says is “99%” accurate. Produced by the global diagnostics company Abbott, the test received a “CE mark” signifying that it meets the EU’s safety standards. It’s also already being rolled out in some U.S. cities, after receiving Emergency Use Authorization from the U.S. Food and Drug Administration.
If you’re going to go out and get tested, it’ll help to know what it really means for an antibody test to be 99% accurate, for you and for the general population.
Antibody tests are used to identify people who were previously infected with Covid-19 by detecting antibodies — molecules that the immune system produces when it encounters a specific pathogen, in this case the SARS-CoV-2 virus, which causes Covid-19. On April 27, Abbott published a statement about its new antibody test, known as the “SARS-CoV-2 IgG assay.”
In a news article published in the British Medical Journal on April 29, an Abbott spokesperson explained how the company supports its 99% accuracy claim. In a study of 73 samples from Covid-positive patients, the spokesperson said Abbott’s test had “100% sensitivity,” and in a study of 1,070 samples from Covid-negative patients, the test had greater than “99.6% specificity.”
You’ll probably notice that the spokesperson didn’t use the term “accuracy” but rather “sensitivity” and “specificity” to talk about the study results. Umair Irfan, who’s done tremendous work covering the nitty-gritty of diagnostics at Vox, does a great job explaining what these words mean:
Tests are evaluated on two key metrics: specificity and sensitivity. Sensitivity is the likelihood that a test will detect your target. A highly sensitive test will have a low false negative rate. It’s especially good at turning up a positive result when your target is actually there.
Specificity is the likelihood that the test won’t be fooled by something other than the intended target. A test with high specificity will have a low false positive rate. It excels at producing a negative result when your target isn’t there.
Crucially, he points out that it’s really hard to make a test both specific and sensitive. “Advancing on both fronts is possible in some cases, but it’s difficult, and it’s almost impossible to make a test that’s perfectly sensitive and specific.” Abbott, however, claims to have specificity and sensitivity greater than 99%.
So what does this mean for you if you get tested? Unfortunately, there’s no simple answer, since even a test with 99% accuracy must be interpreted together with other data.
The probability that an antibody test can correctly tell you whether you ever had Covid-19 isn’t just dependent on specificity or sensitivity, but also the prevalence of the disease — the proportion of people who are actually sick — in the population being tested. Both Irfan and this Scientific American explainer have really clear and solid breakdowns of the math behind this idea, but this quote from the latter story sums up how the Abbott test would fare in a population where the prevalence was 1% (and remember, prevalence can vary greatly by region):
What if there were a nearly perfect test with 99% sensitivity and specificity? Seems awesome, doesn’t it? If that test were used in a country with 1% prevalence, then the predictive value of a positive test would be only 50%— a coin toss!
Despite these odds, Abbott’s test is becoming available in some places, like at CityMD Urgent Care Clinics in New York City. But by now you may be wondering what the point of getting antibody testing is, since even the most accurate test on the market can’t tell you with complete certainty whether you had Covid-19 or not, and having antibodies doesn’t necessarily mean you are immune to the virus.
The New York Times explains in a useful Q&A that testing positive for antibodies shouldn’t change your behavior anyway — you should still practice social distancing and good hygiene, since we don’t know what antibodies say about immunity — but it could make you eligible to donate your blood plasma to help treat other people who are sick.
More importantly, antibody testing can be especially useful on the population level, helping determine the actual spread of disease within a population. The more scientists discover about immunity to Covid-19 and how long it lasts, antibody tests could help guide whether and how we reopen society. For now, however, what they actually tell us is very limited.
