The excellent and informative articles "Want a Test? Depends Where You Live" (The Coronavirus Pandemic, March 12) and "Don't Jump to Conclusions" (Heard on the Street, March 11) about testing for the new coronavirus didn't cover some important nuances.
The test kits in use in the U.S. described in the articles detect viral genetic material—RNA, in the case of coronaviruses—which can be infectious material or noninfectious fragments. Once the patient has recovered and the RNA has been cleared, the tests will be negative. If we're trying to ascertain what proportion of the population has been infected and experienced asymptomatic, mild or more serious infections, such post-infection testing yields "false negatives."
Additional, essential information will need to come from "serological tests" that measure antibodies in blood, which will tell us whether a person has been recently infected with SARS-CoV-2 and recovered. (Note that antibodies take about 10-14 days from exposure to the virus to appear.)
Understanding the full scope of the outbreak will provide critical information about how the virus spreads and about the case fatality rate, the fraction of infections that result in death. Serological tests are currently being used in Singapore and China, and two are being developed at the U.S. Centers for Disease Control and Prevention.
Henry I. Miller, M.D.
Redwood City, Calif.