This week, the U.S. government approved the first blood test in the country that will look for the antibodies produced by the body to fight the novel coronavirus. Such a test can reveal whether someone is actively infected by the virus, as well as whether they were previously infected but recovered. While these tests are very much needed to better understand the scale of the outbreak, they have a few major limitations.
On Thursday, the Food and Drug Administration issued an Emergency Use Authorization to a covid-19 antibody test developed by the company Cellex. The test looks for two types of antibodies, IgM and IgG, created by the body’s immune system in response to the new coronavirus, known as SARS-CoV-2. The test, which requires blood drawn from a vein, can return results within 15 to 20 minutes, though the testing itself has to be done in a certified laboratory, not at a doctor’s office, unlike some other rapid covid-19 tests.
Serological or blood tests for covid-19 (the name of the disease caused by the virus) will be an essential part of tracking it, and many countries have already approved their own versions of these tests. Right now, most diagnostic tests look for the genetic footprints of the virus itself, usually obtained through a very uncomfortable nasal or throat swab. These tests can confirm an active infection by the virus. But they can’t tell us whether someone had previously been infected, an important distinction.
IgM antibodies are created fairly quickly into an infection. IgG antibodies, by contrast, take more time to form but remain in the body for longer. Right now, the evidence largely suggests that surviving an covid-19 infection will give many people immunity for months and possibly even years, thanks to these antibodies. Doctors are also starting to test whether antibodies from the blood plasma of covid-19 victims can help treat severe cases of the illness—a form of donated immunity.
Antibody tests could be used to identify people who are no longer at high risk of contracting covid-19, as well as to find potential plasma donors. Antibody testing could also confirm suspicions that many people infected with the coronavirus do not become sick at all and help clarify a person’s risk of a serious infection or death. Some experts also believe that you could implement wide-scale antibody screening of the population, allowing immune people back to the workplace sooner rather than later, in a sort of staggered lifting of the lockdowns countries have put into place to slow the spread of the virus.
But there are still many challenges ahead for antibody testing. One is that, compared to swab tests now, antibody tests aren’t necessarily as effective at confirming an active infection of the coronavirus, since even IgM antibodies can take several days into the infection to form (the FDA has said antibody tests should not be the only method of determining a covid-19 diagnosis). There’s also the matter of accuracy, a concern for any kind of test available right now.
In documents released by the FDA that were provided by Cellex, including a fact sheet meant for health care providers, the company notes that the absolute sensitivity (the odds that a test is correctly detecting a positive result for X disease) and specificity (the chance that a test correctly spits out a negative result) of their test are still unknown.
“Patients tested early after infection may not have detectable IgM antibody despite active infection; in addition, not all patients will develop a detectable IgM and/or IgG response to SARS-CoV-2 infection,” the fact sheet warns.
But even a highly accurate antibody test could lead to many false results if used widely on the wrong sort of population. For example, let’s say you test a town where the coronavirus has not been spreading—a 90 percent accurate test is going to deliver more false positives than true positives, leading some people to mistakenly think they are now immune and safe from infection. Conversely, the more true positives in a group, the less likely any one positive result will be false. This is a well-known problem for many screening tools, such as mammograms for breast cancer (it’s one reason why women are more likely to have a false-positive mammogram the younger they are).
There are ways around this limitation. For one, you could limit testing to smaller, high-risk groups, such as health care workers or in regions where large outbreaks have clearly occurred. But this drawback may hamper attempts to clear people as being safe from the virus. There’s also still the question about how much and for how long having these antibodies will protect any particular person against further infections. Studies using these antibody tests should help answer that question.
More practically, there’s still the same resource hurdle that other tests currently face. We wouldn’t have enough antibody tests to give to everyone even if that were the appropriate thing to do. Last week, the FDA gave permission to over two dozen labs and hospitals to begin developing their own antibody tests, though, unlike Cellex’s test, these tests will only be validated by the companies and labs themselves, without a formal approval by the FDA.
One of these makers, the Mayo Clinic, is set to start performing antibody tests next week. But for the time being, a representative for the Mayo Clinic told Gizmodo, testing will only be slowly rolled out within their facilities, in part due to the limited availability of supplies needed to create them. And they don’t expect to begin testing outside of their medical center anytime soon.
None of this is to say that antibody tests for covid-19 will be useless—only that they alone won’t turn the tide against the pandemic.