Dear Doctor: I had a coronavirus test, and it was negative. Then I had the antibody test, and I was surprised when it came back positive because I hadn’t been sick. How do these coronavirus tests work? Are they accurate?
Dear Reader: There are two testing goals related to SARS-CoV-2, the coronavirus that causes COVID-19. One is to learn whether or not you are currently infected. The other is to see whether or not you have been infected in the past.
By far, the most common test being conducted at this time is for the coronavirus itself. This involves the use of a nasal swab, which is a long, thin stick with soft brushlike material on the tip. The swab is carefully inserted deep into the nose, to the passageway that connects to the back of the throat. (It’s uncomfortable, but not painful.) The goal is to gather a sample that includes cells and secretions from the back of the throat and from all along the length of the nose. Although there’s a less-invasive COVID-19 test that relies on just a throat swab, similar to the test used for strep throat, it’s not considered to be as accurate as the nasal swab. A newly developed saliva test, which requires just a sample of spit and returns a result within 30 minutes, has been submitted for regulatory review. Each of these tests look for genetic markers specific to SARS-CoV-2. Meanwhile, on July 3, the Centers for Disease Control and Prevention announced the development of a single test able to diagnose influenzas A and B as well as the novel coronavirus.
When you become sick with a virus, the immune system sends specialized proteins known as antibodies to mount an attack. Since antibodies are specific to the invader they are fighting, it’s often possible to identify when someone’s immune system has fought off a particular pathogen. It usually takes at least one week, and can take up to three weeks or longer, for the body to develop antibodies. Although it’s rare, despite being infected by a pathogen, some people don’t develop antibodies at all.
An antibody test is performed by screening a blood sample for specific markers of the antibody in question. Unfortunately, in the case of the novel coronavirus, the accuracy of these tests is in question. The CDC has found that up to half of antibody test results may be inaccurate, giving either a false positive or a false negative. A recent study examined 40 different types of antibody tests and found a wide disparity in their accuracy. The researchers suggested that until these tests are improved, people should not rely on the results.
As for having antibodies to SARS-CoV-2 despite not having been physically ill, that’s entirely possible. A hallmark of this virus is that some people, despite being infected, don’t develop symptoms. However, they can still pass along the virus, which is known as asymptomatic transmission. That’s why, in order to protect the people around us, it’s so important for each of us to wear a face covering in public, social distance and wash our hands.
Eve Glazier, M.D., MBA, is an internist and associate professor of medicine at UCLA Health. Elizabeth Ko, M.D., is an internist and assistant professor of medicine at UCLA Health.
