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Note: Information in this article was accurate at the time of original publication. As the highly contagious Omicron variant continues to sweep across the country, people—even those who are fully vaccinated—are wondering if the onset of cold and flu symptoms is really a sign of COVID infection.
As Omicron surged, appointments at sites where results are processed by a laboratory became difficult to find in a timely manner. Often called rapid tests, such kits are sold in drugstores and online, allowing people to test themselves—and get results—in their own home in a matter of minutes.
Unfortunately, at-home tests have also become difficult—if not impossible—to find. The federal government hopes to alleviate the bottleneck by requiring private insurance companies to start reimbursing customers for at-home tests and by distributing 1 billion free rapid tests to Americans. If you can get your hands on a test, you might find the different options confusing. In which situations is a laboratory-based often called PCR test best? Are they all equally accurate? Do some produce results faster than others?
And how far does that Q-tip go up your nose with each kind? Some of these questions are easy to answer, while others are more difficult—particularly when it comes to accuracy. Therefore, they have not been as rigorously tested or vetted as other medical tests with full FDA approval.
And since the virus is new, all the tests are also new, meaning we have neither a long track record of comparing results, nor a true gold-standard test yet. Furthermore, with each new variant , new questions arise.
Recently, there has been discussion about whether throat swabs or saliva samples are better at detecting Omicron compared to the more mainstream method of nasal swabs—or if rapid tests are less effective at detecting Omicron. Sheldon Campbell, MD, PhD , a Yale Medicine pathologist and microbiologist, cautions against getting caught up in what he calls mostly anecdotal data. Campbell, referring to a swabyourthroat hashtag that is trending. Below, Dr. They are broken into two categories: molecular and antigen more below.
You can get antigen test results in about 15 minutes, but they tend to be less accurate. Health care providers typically rely on molecular tests, particularly when people have COVID symptoms, whereas antigen testing is often used when quick results are needed or for general screening and surveillance.
Campbell says. A test that’s highly sensitive will catch almost anyone who has the disease and not generate a lot of false-negative results. How does the test work? A molecular test looks for genetic material from the virus. The test uses sophisticated chemicals and equipment to reproduce millions to billions of copies of viral-related DNA from even the smallest sample. Because of that, the test is considered highly sensitive, leading to very few false negatives.
How is a sample obtained? Usually with a swab inserted into your nose. There are three different methods for nasal collection:. In general, the deeper you go for a specimen, the greater the sensitivity, says Richard Martinello, MD , a Yale Medicine infectious disease expert. Where can you get one? Molecular tests are offered at pharmacies, doctors’ offices, and designated testing locations, such as health clinics, as well as locations set up by private or state and local public health systems.
How quickly can you get results? Because the tests are sent to a lab, it depends on lab capacity. Results are typically available within a range of one to seven days, depending on your location. How accurate are they? Martinello says. But limiting false negatives might be extremely important, especially with the rise of more transmissible variants. Whereas molecular tests require specialized equipment for processing samples, an antigen test is simpler, because it requires smaller devices that are easy to transport.
Their design is similar to, say, a pregnancy test. The sample you provide is treated with a reagent and analyzed on the spot by a health care professional. Unlike molecular tests, these require a higher level of virus in the test sample before the test will turn positive.
This means that an antigen test may sometimes lead to a false negative. As with molecular tests, a sterile swab is inserted into your nose or throat to obtain a specimen see details above —although throat swabs may be less common these days. How is the test processed? The sample is self-applied to a test strip or cartridge. Similar to a home pregnancy test, results show a colored line to signify positive or negative.
Antigen tests are offered in pharmacies, doctors’ offices, and can be purchased to use at home. Results are typically available in 10 to 15 minutes. Complicating things a bit is the availability of ID NOW, a rapid molecular test used by some testing locations, such as pharmacies, that can read results on-site—in about 15 minutes. The home tests that give instant results are all antigen.
However, there are home kits that require mailing a sample to a lab that use molecular technology. Home tests are useful if you need an immediate answer, Dr. And since they are less sensitive than a molecular test, the FDA recommends doing serial testing—or taking multiple tests—over several days to improve the chance of catching asymptomatic infections.
Rather, test again when your symptoms have gone away and you are ready to go back to work. The overarching principle of testing in the medical world, Dr.
Campbell explains, is that you should only test when a result will change what you do. Given the high case rate levels right now, if you are exposed to COVID and then develop symptoms, chances are you have the virus, he says. If your symptoms can be managed safely at home and you can isolate, then that is what you should do.
Another thing to remember, Dr. Campbell says, is that if an antigen test comes back positive right now, it is most likely accurate. People who are antigen negative at the very worst are less contagious.
But at this moment, when incidence is so high, false positives are extremely unlikely. If you are exhibiting a fever, cough, trouble breathing, or other COVID symptoms, you should get tested, regardless of vaccination status, health experts advise. If testing is available, consider testing three to five days after a high-risk exposure, adds Dr. Still, for the diagnosis of people seriously ill with a presumed case of COVID , doctors will generally use a PCR test, because false-negative tests might result in inadequate treatment.
If you are traveling, you might be required to get tested, too. The location you are visiting might require a certain type of test and approved testing locations as well. If you can get PCR, great. But for surveillance, such as at schools or workplaces, antigen tests work well, Dr. Would PCR be better in that setting? Sometimes, speed is the most important thing. Say you are vaccinated but plan to attend a crowded event and then visit an immunocompromised or elderly relative.
If you have a limited number of tests, you should use them right before you visit vulnerable friends or relatives—or right before you go to an event with lots of people, adds Dr.
Detecting variants requires genetic testing done in a lab. Across the country, a selection of positive COVID samples are sent to specialized labs, where they are anonymously sequenced to identify variants so that public health officials can monitor COVID trends. And not all positive specimens are tested; only sample amounts are taken.
Such tests are offered at doctors’ offices and clinics and need to be sent to a lab, and should come back in a day or less. These tests will become available closer to flu season, in October. The federal government launched a website for people to order free COVID test kits four per household to be shipped to their homes.
Visit the site by clicking here. Note: Information provided in Yale Medicine articles is for general informational purposes only.
No content in the articles should ever be used as a substitute for medical advice from your doctor or other qualified clinician. Always seek the individual advice of your health care provider with any questions you have regarding a medical condition.
Skip to Main Content. Yale Medicine experts explain the ins and outs of lab- and home-based tests. Updated: Jan. Below, we take a closer look at the two categories. There are three different methods for nasal collection: Nasopharyngeal: A health care professional inserts a long swab deep into your nostril to collect fluid from the back of your nose.
Mid-turbinate: This method, which someone can be coached to do themselves or is done by a professional, involves placing a soft swab straight back into the nostril less than one inch to collect a sample. Anterior nasal swab: This test, which can either be self-administered and possibly supervised by a trained health care provider, or done by a health care professional, involves putting a swab three-quarters of an inch into the nostril and twirling it around at least four times to get a sample.
Saliva: You drool into a sterile, leak-proof cap container. For now, this type of testing is only offered at select locations. Most specimens are sent to laboratories. But where does it fit in the molecular vs antigen test result accuracy spectrum? But given the current shortage of tests, Dr. Campbell advises using them sparingly.
How accurate is pcr testing – none: –
Some of these folks even had exposure history and the signature hallmarks unique to this virus, like multiple peripheral opacities on CT scan and loss of sense of taste and smell. Contrary to common belief, tests of all kinds have some degree of false positives and false negatives. With influenza almost nonexistent in the U. Most folks believe that a positive test means they are sick and negative means that they got a green signal to be free, but the truth is far from it, and the answer is somewhat complicated.
It was only recently that I uncovered the extent of this issue after some research. False negatives can be up to percent on day one of exposure asymptomatic and down to 20 percent on day 8 of exposure day 3 of symptoms and then starts going up again.
Statistics tell us that the false-negative rate goes up even higher as the prevalence of disease goes up, as is the case with COVID right now. A meta-analysis from Europe actually found an average false negative of 54 percent. These numbers are actually from monitored and regulated studies where things are done more meticulously than real-world scenarios.
Performing many of these tests quickly in the clinical setting because of soaring demand, with each test resulting in sampling error, pressure on labs to provide quick turnaround, and rationing of scarce reagents in labs increase this percentage even higher. Some mutations could be potentially affecting the accuracy as well.
With false-negative tests , people get a false sense of security, bring their guard down, and stop isolating. The resulting higher prevalence in the community leads to an even higher false-negative rate. Unprecedented pressures on hospitals, physicians, nurses, case managers, and hospital administrators contribute to this. Medicare rules are black and white, and it does not consider a patient having COVID even with typical symptoms if the test is negative. This may seem very complicated at first, but if we can understand the basic variables that may affect a test result, we can get a rough estimate of the post-test probability of a positive or negative test without any complicated calculations.
Postmarketing surveillance and research show that most RT-PCR tests have a high specificity of 90 to 95 percent, but their sensitivity is low at 60 to 70 percent. False-positives are not that much an issue unlike antigen testing and mostly because of lab contamination, but false negatives are. The best time to do the test is between 1 to 7 days from the start of symptoms, with day 3 being the best. On the other hand, a case can be made that all patients with almost typical symptoms be assumed to be positive and should isolate without any testing.
Testing can be limited to patients with atypical symptoms or high-risk patients who could benefit from monoclonal antibodies if diagnosed earlier within 10 days. The population can also be educated about the limitation of this testing, especially false-negatives, and also focus on the red flag symptoms of confusion, shortness of breath, persistent chest pain, cyanosis, etc. This will also relieve a tremendous burden on testing sites and the need by family members to drive these patients around town and, in the process, exposing themselves and other health care workers.
This will also free up some critical health care workers and valuable resources, including testing kits and reagents for higher risk and hospitalized patients. I wish the FDA, CDC, epidemiologists, experts, and media put more time and effort into public education on this serious pitfall of testing.
Balram Khehra is an internal medicine physician. Image credit: Shutterstock. Learn more. Comments are moderated before they are published.
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