Covid Testing: What You Have to Know

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Covid Testing: What You Need to Know

Virus tests are categorized based on what they look for: molecular tests, which look for the virus’s genetic material, and antigen tests that look for viral proteins. The various tests all use a sample collected from the nose, throat or mouth that may be sent away to a lab or processed within minutes. Testing should be free or paid for by your insurance, although some testing centers are adding extra charges. Here are the common tests and some of the pros and cons of each.

Laboratory molecular test: The most widely available test, and the one most people get, is the P.C.R., or polymerase chain reaction, test, a technique that looks for bits of the virus’s genetic material — similar to a detective looking for DNA at a crime scene.

Pros: This test is considered the gold standard of coronavirus testing because of its ability to detect even very small amounts of viral material. A positive result from a P.C.R. test almost certainly means you’re infected with the virus.

Cons: Because these tests have to go through a laboratory, the typical turnaround time is one to three days, though it can take 10 days or longer to get results, which can limit this test’s usefulness, since you may be spreading virus during the waiting period. Like all coronavirus tests, a P.C.R. test can return a false negative result during the first few days of infection because the virus hasn’t reached detectable levels. (One study showed that among people who underwent P.C.R. testing three days after symptoms began, 20 percent still showed a false negative.) Another frustration of P.C.R. testing is that it sometimes detects the virus’s leftover genetic material weeks after a person has recovered and is no longer contagious. The tests are also expensive, costing hospitals and insurers $50 to $150 per test.

Rapid antigen test: An antigen test hunts for pieces of coronavirus proteins. Some antigen tests work sort of like a pregnancy test — if virus antigens are detected in the sample, a line on a paper test strip turns dark.

Pros: Antigen tests are among the cheapest (as little as $5) and speediest tests out there, and can deliver results in about 15 to 30 minutes. Some college campuses and nursing homes are using rapid tests to check people almost daily, catching many infectious people before they spread the virus. Antigen tests work best when given a few times over a week rather than just once. “It tells you, am I a risk to my family right now? Am I spreading the virus right now?” said Dr. Michael Mina, an epidemiologist at Harvard University’s School of Public Health and a proponent of widespread rapid testing. Though, he cautioned, “if the test is negative, it doesn’t tell you if you’re infectious tomorrow or if you were infectious last week.”

Cons: An antigen test is less likely than P.C.R. to find the virus early in the course of the infection. One worry is that a negative rapid test result will be seen as a free pass for reckless behavior — like not wearing a mask or attending an indoor gathering. (The White House Rose Garden event is a good example of how rapid testing can create a false sense of security.) A negative antigen test won’t tell you for sure that you don’t have the coronavirus — it only tells you that no antigens were detected, so you’re probably not highly infectious today. (In one study, a rapid antigen test missed 20 percent of coronavirus infections found by a slower, lab-based P.C.R. test.) Antigen tests also have a higher rate of false positive results, so a positive rapid test should be confirmed.

Rapid molecular test. Some tests combine the reliability of molecular testing with the speedy results of a rapid test. Abbott’s ID Now and the Cepheid Xpert Xpress rely on a portable device that can process a molecular test right in front of you in a matter of minutes.

Pros: These tests are speedy and highly sensitive, and they can identify those exposed to coronavirus about a day sooner in the course of an infection than a rapid antigen test. A rapid molecular test isn’t quite as accurate as the laboratory version, but you’ll get the result much fast

Cons: Depending on where you live, rapid molecular tests might not be widely available. They are also less convenient and often slower than many antigen tests. And like all coronavirus tests, a negative result isn’t a guarantee you don’t have the virus, so you’ll still need to take precautions. Like its laboratory cousin, a rapid molecular test can detect leftover genetic material from the virus even after you’ve recovered.

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