The list of close contacts of President Donald Trump who have tested negative for Covid-19 continues to grow, but experts say it doesn’t mean they are off the hook: A single negative test doesn't mean a person doesn’t have the virus.
“A negative is not a Get Out of Jail Free card,” Dr. Alan Wells, medical director of clinical laboratories at the University of Pittsburgh Medical Center.
Covid-19 diagnostic tests look for bits of the coronavirus in a person’s body at that exact moment. But “it takes a number of days for the virus to replicate enough to be detected,” Wells told NBC News.
That's why the Centers for Disease Control and Prevention advises anyone who may have been exposed to quarantine for 14 days, which is believed to be the extent of the incubation period.
Vice President Mike Pence and his wife, Karen, announced Friday morning that they had both tested negative for Covid-19. A memo from White House physician Dr. Jesse T. Schonau said the vice president is not considered a close contact of anyone who has tested positive, including Trump, and does not need to quarantine.
A White House spokesperson tweeted Friday that Ivanka Trump and Jared Kushner, who both attended Tuesday's debate with Democratic nominee Joe Biden, had tested negative.
Generally, the best time to test is about five to seven days after an exposure, said Susan Butler-Wu, a clinical microbiologist at the University of Southern California.
“If you’ve just been exposed, it’s very unlikely that even if you’re incubating that you’re going to test positive,” Butler-Wu said. “There’s just not enough time.”
Dr. Joseph Petrosino, director of molecular virology and microbiology at the Baylor College of Medicine in Houston, said it also depends on the type of test being given. For example, molecular or PCR tests that are sent to a lab for analysis are the most reliable because they are more sensitive, picking up positive cases earlier, when there is less virus in the body. But PCR tests must be done in a lab and take several hours to run. Adding in transportation time and backlogs, results may take days in some cases.
Download the NBC News app for full coverage of the coronavirus outbreak
Trump and the vice president — and close contacts in the White House — reportedly have been tested with a type of rapid test, called the Abbott ID NOW, which analyzes samples on the spot, so it doesn’t need to be sent out to a lab. Results are given in about 15 minutes.
Wells says the drawback with rapid tests in general is that they are less sensitive. Studies have shown that Abbott’s test in particular can give too many false negatives.
“The ID NOW is about 70 to 75 percent as sensitive as the lab PCR test,” Wells said, referring to a study done by his colleagues at UPMC. “We’re looking at missing 1 in 4 that are picked up.”
In May, the FDA put out an alert warning doctors that people testing negative with the Abbott ID NOW test may need a second lab-based test. It's unclear whether Trump or his close contacts have received a second test.
In a statement Friday, Abbott said that “more than 11 million Americans have taken the ID NOW test, helping to stop the spread of the virus,” and cited their own study, finding a 95 percent sensitivity rate when the test is given within seven days of symptom onset.
Petrosino said that ideally, people who have possibly been exposed should be retested every 24 to 48 hours over the next two weeks.
“Everyone known to be in close contact with positive individuals should self-isolate until this period is over,” he said.