Virus World

In the high-stakes world of coronavirus testing, false positives are widely considered to be benign in comparison with false negatives, which can deprive infected people of treatment and embolden them to mingle with others, hastening the spread of disease. But false positives, which incorrectly identify a healthy person as infected by the virus, can have serious consequences as well, especially in places where the virus is scarce. False positives are generally very rare among tests that have been vetted by the Food and Drug Administration. But any test can be plagued by contamination, mishandling or technical glitches, leading a device to spot the coronavirus where it is not. In places where the virus is relatively scarce, false positives may even outnumber accurate positives — eroding trust in tests and, under some circumstances, prompting outbreaks of their own.

Adverse effects include:

• Unnecessary isolation: According to guidelines published by the Centers for Disease Control and Prevention, people who test positive should immediately isolate themselves for at least 10 days after their symptoms start (if they experience symptoms at all). That’s 10 days spent away from friends and family, and 10 days of potential productivity in a school or workplace lost.

• New outbreaks: Under certain circumstances, a false positive could seed new coronavirus cases. Crowded facilities, such as nursing homes, prisons or hospitals, might isolate coronavirus-positive people together. “If someone isn’t actually positive, we could be sending them to their death,” said Dr. Valerie Fitzhugh, a pathologist at Rutgers University in New Jersey.

• Missed or inappropriate treatment: People with the flu or Covid-19 often show similar symptoms, but may be tested for only one of them at a time. If a patient is given an incorrect diagnosis of Covid-19, that person could be deprived of treatment that could alleviate their illness, or be given a costly therapy that does little to speed their recovery.

• A false sense of security: Mounting evidence suggests that most people who have fought off the coronavirus retain a degree of immunity. Scientists don’t know how long this immunity lasts, but C.D.C. guidelines note that reinfection is unlikely within 90 days. People who don’t have symptoms within this time window do not need to be tested again, the agency has said. Should someone be dropped from routine testing, said Dr. Catherine O’Neal, an infectious disease specialist at Louisiana State University, it would imperil that person’s health and could put an entire group of close contacts at risk