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Why pcr takes time - why pcr takes time. Why does it still take so long to get a COVID-19 PCR test result? 

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It's the test some employers want employees to take before returning to work after having Covid, and the one some people scramble to schedule to know whether they can travel or stop isolating. But experts say the PCR test isn't the best one for every situation.

When you should take a PCR vs. More Videos How to get your at-home Covid test for free. Reality Check: The real cost of Covid relief fraud. Pfizer CEO pledges vaccines to low-income countries at zero profit. NYC mayor rejects mask mandate amid high Covid alert. North Korea declares national emergency over reported Covid cases. Parents, children, superstars, and politicians killed by Covid This is why you should still have confidence in the FDA for vaccines.

What we can learn from the WHO Covid mortality numbers. Shanghai police barge down door in resident's apartment. Ashish Jha predicts what's next with Covid See Dr. Gupta's reaction to judge ending mask mandate. Mount Sinai launches post-Covid care center for long-haulers. PCR -- or reverse transcription-polymerase chain reaction -- tests can detect small amounts of the coronavirus' genetic material in a specimen collected from a human.

The test then works by amplifying, or making copies of, that genetic material if any is present in a person's sample, according to the US Centers for Disease Control and Prevention.

The test's amplifying property enables it to detect very small amounts of coronavirus in a specimen, "making these tests highly sensitive for diagnosing COVID," according to the CDC. While this sensitivity can be advantageous for detecting coronavirus after a recent exposure, it also means PCR results can be positive even after you're no longer contagious.

A worker at a drive-up coronavirus testing clinic administers PCR coronavirus tests in Puyallup, Washington state, on January 4. A PCR test might say you're positive for coronavirus for three or four weeks after you've recovered because it's still "picking up past infection and the small fragments of the virus are still being amplified," said CNN Medical Analyst Dr. After seven to 10 days, "that PCR test is not an appropriate test.

Regardless of your situation, here's what you need to know about the differences between PCR and antigen tests called rapid lateral flow tests in the UK and when you should use them. Read More. When a PCR test is key. The prime time to take a PCR test is when you have had a known or suspected exposure to someone with Covid or are experiencing symptoms, and you want to find out if you have a coronavirus infection, said Dr. Knowing your Covid status as early in an infection as possible can help you figure out whether you're infectious to other people, what to tell recent close contacts for their own safety, and what to share with your doctor so they can talk with you about your symptoms and prescribe any medical care if needed, said Emily Somers, an epidemiologist who holds professorships in internal medicine, environmental health sciences, and obstetrics and gynecology at the University of Michigan's schools of medicine and public health.

How to get free at-home Covid tests from the government. That's important because "up to two days before symptoms, or the one day or two days after symptoms begin people can have the highest risk of infecting others," Ko said. When people are first exposed, it takes time for the virus to replicate enough for a test to pick up on it, Ko explained.

PCR tests detect it earlier in the course of infection after exposure than the less sensitive rapid antigen test, which works by detecting a specific protein on one of the coronavirus' spikes. The US Food and Drug Administration has authorized use of both PCR tests and rapid antigen tests in laboratory-based or point-of-care settings such as a physician's office, urgent care facility, pharmacy, school health clinic and temporary locations like drive-through testing sites.

Antigen tests are also commonly available as self-tests. When rapid antigen tests are crucial. Contrary to what some still think is best, a rapid antigen test or rapid lateral flow test is what you should be taking after you have had Covid for several days and want to confirm you're probably no longer infectious to other people.

It follows a curve after someone has been exposed. Gupta take an at-home Covid test. Watch Dr. Gupta take an at-home Covid test The CDC's current isolation guidance is that if people have access to a coronavirus test and want to take it, the best approach is using a rapid test toward the end of their five-day isolation period.

Those who continue to test positive at that point should continue isolating until they reach 10 days after their symptoms began. People who test negative can stop isolating but should wear a mask around others until day Aaron Salvador swabs his nose with a coronavirus rapid antigen test kit in Washington, DC, on December 29, But evolving evidence has led public health experts to think that rapid tests do better than PCR tests at capturing when people are exiting the period of maximum infectiousness, since PCR tests are too sensitive, he said.

This doesn't mean that being contagious at this point is totally impossible -- it just means that once you have had Covid for several days and then test negative with a rapid antigen test, the viral RNA is likely at levels low enough to not be infectious even if you still test positive on a PCR test. Going somewhere? Here's how to test yourself for Covid before you do.

There have been many online reports of employers requiring people to show proof of a negative PCR test before they return to work, which is "probably not coherent thinking" and has people not returning to work until weeks after their initial rapid antigen or PCR test, Ko said. In addition to being more convenient, the most logical return-to-work testing strategy is using rapid antigen tests, Ko said.

However, "if somebody has had multiple antigen tests that are negative, but they have symptoms of Covid, having a PCR test can help to confirm if they have it or not. Get CNN Health's weekly newsletter. Some places, such as Antarctica and Kosovo , require travelers to show proof of a negative PCR test before entering. However, it's going to potentially prolong the amount of time between when somebody can enter the country compared to when they were infectious.

     

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  PCR is the most reliable and accurate test for detecting active infection. PCR tests typically take hours to perform, but some are faster. Traditional PCR has advanced from detection at the end-point of the reaction to detection while the reaction is occurring. Figure 1: Real-Time PCR Evolution. The longest analysis is for the PCR test, which varies from lab to lab but often requires about to 2 hours, he says.    

 

How Long Does It Take to Get COVID Results by Test Type?.

    PCR is the most reliable and accurate test for detecting active infection. PCR tests typically take hours to perform, but some are faster. Traditional PCR has advanced from detection at the end-point of the reaction to detection while the reaction is occurring. Figure 1: Real-Time PCR Evolution. The longest analysis is for the PCR test, which varies from lab to lab but often requires about to 2 hours, he says.