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…