Why False Positives Matter, Too
In the high-stakes world of coronavirus testing, one mistake has taken center stage: the dreaded false negative, wherein a test mistakenly deems an infected person to be virus-free.
These troublesome results, experts have said, can deprive a person of treatment and embolden them to mingle with others, hastening the spread of disease.
But false negatives are not the only errors bedeviling coronavirus diagnostics. 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.
Although false positives may seem relatively harmless in comparison with their false-negative cousins, “people can absolutely get hurt,” said Dr. Benjamin Mazer, a pathologist and diagnostics expert at Johns Hopkins University.
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.
Some rapid tests, which forgo sophisticated laboratory equipment and can deliver results in under an hour, have been criticized for returning high numbers of false positives, especially when used to screen people without symptoms. Even laboratory tests that rely on a very reliable technique called polymerase chain reaction, or P.C.R., have been known to return the occasional false positive.
In places where the virus is relatively scarce, false positives may even outnumber actual positives — eroding trust in tests and, under some circumstances, prompting outbreaks of their own.
Unnecessary isolation
A positive result on a coronavirus test sets off a cascade of consequences. According to guidelines published by the Centers for Disease Control and Prevention, people who test positive should immediately isolate for at least 10 days after their symptoms start (if they experience symptoms at all).
That is 10 days spent away from friends and family, and 10 days of potential productivity in a school or workplace lost.
The ever-increasing availability of virtual platforms might make that transition easy for some, Dr. Mazer said. But for people whose work can only be done in person, or who lack a financial buffer, “this could be a huge deal,” he said. “They could lose their paycheck. They could lose their job.”
Caregivers, too, may find themselves put in a precarious position by false positives, which could force them to separate from children, older family members or other vulnerable contacts. Should another person need to come in and take their place, the risks of exposure to the virus could increase for all involved.
And for facilities with limited space, placing a person in unnecessary isolation could shift equipment, medical care and even follow-up tests away from someone who might need those resources more.
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.
In a worst-case scenario, “if someone isn’t actually positive, we could be sending them to their death,” said Dr. Valerie Fitzhugh, a pathologist at Rutgers University.
In early October, officials in Nevada ordered the state’s nursing homes to halt the use of two rapid coronavirus tests until further notice, citing concerns about accuracy. (Nearly 40 percent of the country’s known Covid-19 deaths have come from nursing homes, according to an analysis by The New York Times.) A rash of false positives had been uncovered, according to the directive, which had risked placing uninfected residents into units with infected people, giving the coronavirus more opportunity to spread. Such a scenario “could result in causing harm to a population that we have collectively worked so hard to protect,” officials noted in a follow-up directive. Under pressure from the federal government, Nevada officials revoked the rapid testing prohibition on Oct. 9.
Missed or inappropriate treatment
False positives can also be disastrous from a treatment standpoint, said Linoj Samuel, a clinical microbiologist at Henry Ford Health System in Detroit. People with the flu or Covid-19, for example, often show similar symptoms, but may only be tested for 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.
“That’s definitely a price you pay,” Dr. Samuel said.
A false sense of security
Mounting evidence suggests that most people who have fought off the coronavirus once retain a degree of immunity that can thwart the virus a second time. Scientists do not know how long this immunity lasts, or how it varies from person to person, but the C.D.C. has issued guidelines that 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.
That could be helpful advice for people who actually contracted the coronavirus. But people who unknowingly receive a false positive on a test might be misled into thinking that they are invincible, said Saskia Popescu, an infection prevention expert at George Mason University.
However, if these people never actually encountered the virus, they would be just as vulnerable as before — if not more, should they choose to forgo masks or physical distancing after assuming their bodies are already bolstered against the virus.
The Coronavirus Outbreak ›
Words to Know About Testing
Confused by the terms about coronavirus testing? Let us help:
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- Antibody: A protein produced by the immune system that can recognize and attach precisely to specific kinds of viruses, bacteria, or other invaders.
- Antibody test/serology test: A test that detects antibodies specific to the coronavirus. Antibodies begin to appear in the blood about a week after the coronavirus has infected the body. Because antibodies take so long to develop, an antibody test can’t reliably diagnose an ongoing infection. But it can identify people who have been exposed to the coronavirus in the past.
- Antigen test: This test detects bits of coronavirus proteins called antigens. Antigen tests are fast, taking as little as five minutes, but are less accurate than tests that detect genetic material from the virus.
- Coronavirus: Any virus that belongs to the Orthocoronavirinae family of viruses. The coronavirus that causes Covid-19 is known as SARS-CoV-2.
- Covid-19: The disease caused by the new coronavirus. The name is short for coronavirus disease 2019.
- Isolation and quarantine: Isolation is the separation of people who know they are sick with a contagious disease from those who are not sick. Quarantine refers to restricting the movement of people who have been exposed to a virus.
- Nasopharyngeal swab: A long, flexible stick, tipped with a soft swab, that is inserted deep into the nose to get samples from the space where the nasal cavity meets the throat. Samples for coronavirus tests can also be collected with swabs that do not go as deep into the nose — sometimes called nasal swabs — or oral or throat swabs.
- Polymerase Chain Reaction (PCR): Scientists use PCR to make millions of copies of genetic material in a sample. Tests that use PCR enable researchers to detect the coronavirus even when it is scarce.
- Viral load: The amount of virus in a person’s body. In people infected by the coronavirus, the viral load may peak before they start to show symptoms, if symptoms appear at all.
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