The U.S. Food and Drug Administration has authorized a test that determines whether someone has antibodies for the coronavirus responsible for COVID-19. Results are available within 15 minutes. The tests, which require drawing blood, must be done in a certified lab, not a doctor’s office or a drive-through testing center.
The body produces antibodies specific to a virus when it attacks. Presence of those antibodies in their blood indicates that a person has been infected by the virus in the past or perhaps is still fighting it, although the test is not particularly good at the latter because antibodies don’t always appear early in the course of infection. Nonetheless, the FDA says in its eight-page "emergency use authorization" letter to the maker, North Carolina-based Cellex Biosciences Inc., that “based on the totality of scientific evidence available to FDA, it is reasonable to believe that your product may be effective in diagnosing COVID-19.”
Testing for antibodies matters immensely because people can gain immunity as a result of their body’s successful fight against disease. Whether that’s the case with the coronavirus, however, or for how long such immunity might last, is not yet certain. Some people have strong antibody responses, and others do not. Antibody tests in the aggregate should also provide a better idea of just how much of the population has been afflicted with COVID-19. That matters when calculating the death rate from infections.
Other nations, including China and Singapore, have been running their own antibody tests for some time. Other companies have also been allowed by the FDA to make and distribute antibody tests in the United States, but none have been allowed to claim their tests are useful for diagnoses. In the case of Telluride, a tiny elite ski resort in southwest Colorado, all 8,000 residents are being given an antibody test from United Biomedical for free.
Yasemin Saplakoglu at Live Science reports on the Cellex test:
The test looks for two types of antibodies: immunoglobulin M and immunoglobulin G. Immunoglobulin M is the first antibody that the body makes in response to a foreign substance and can appear a couple of days after infection.
In contrast, the body produces large quantities of immunoglobulin G later on in the infection process. Immunoglobulin G is specific to the novel coronavirus. Positive results could either mean either a person is currently infected or was recently infected by the coronavirus, according to the letter.