The Two Coronavirus Tests You Need to Know About

Understanding the key differences between diagnostic and antibody tests

In the early stages of the Covid-19 pandemic, much of the conversation focused on diagnostic testing, a crucial tool for identifying people who could transmit the disease. Diagnostic testing is still an enormous priority now, but more recently, a second type of testing has emerged on the scene: antibody testing, a very different tool that provides different information. It can be confusing to keep track of the two, so here’s a brief guide to what makes each important.

Diagnostic tests are used to diagnose people who currently have Covid-19 — and they’re especially important because many people who have the disease show no symptoms. The diagnostic test that the United States currently relies on involves taking a nasal swab from a person and then searching for traces of the virus in that sample using a laboratory technique called polymerase chain reaction, or PCR. This is the gold standard for diagnostic testing, but it takes a long time to complete — about six to eight hours. As a result, there’s a growing backlog of people waiting to get their results.

Diagnostic testing across the United States continues to fall short of what’s needed to effectively track the spread of Covid-19. According to the Covid Tracking Project, roughly 150,000 people have been tested each day over the past week. Recent research from Harvard University suggests that 500,000 to 700,000 tests must occur each day in order to safely reopen the United States by mid-May. Scientists are racing to develop faster and cheaper tests, but the vast majority of them have not yet been approved by the Food and Drug Administration.

Antibody tests search a sample of blood for antibodies, which are molecules produced by the immune system in response to a Covid-19 infection. In theory, antibody tests can identify whether a person has been infected and has started to build immunity against the disease (even if they weren’t aware they had the illness). This type of testing is considered crucial to reopening the country, since the presence of antibodies could mean that a person is immune to Covid-19, at least temporarily.

However, the science behind antibody tests for Covid-19 is not yet considered sound. On April 8, the World Health Organization warned that there is no evidence that antibody tests can show immunity to Covid-19 and said…