COVID-19 testing is complicated. Here are the answers to 6 big questions

A friend texted me recently because her daughter’s class had a coronavirus outbreak and my friend had questions about COVID-19 testing.

Testing has been on the minds of many other people and has been in the news lately as well. President Joe Biden recently announced that insurers must reimburse people for home coronavirus tests, and millions of tests will be available for people who do not have insurance. Some people make testing part of their routine before holiday gatherings.

Yet COVID-19 tests are not easy to find everywhere. Consumers complained that their pharmacies were sold out and that testing centers in some locations have long lines. And while you can get your hands on a test, a lot of people – like my friend – have questions. In particular, she wanted to know if her daughter’s recent vaccine injection would interfere with the results of her home test.

Here are some answers to this question and others.

What do COVID-19 tests measure?

There are two broad categories of diagnostic tests you might come across: antigen tests and nucleic acid amplification tests, better known as PCR tests. Both can determine if you have a current infection with SARS-CoV-2, the virus that causes COVID-19. You can find a list of tests available in stores or on test sites in the United States. here. (Antibody tests aren’t diagnostic tests – they can detect evidence of past infections, but not current cases.)

The PCR (polymerase chain reaction) test determines whether there is viral RNA in the swab or saliva sample being tested. These tests are the ones you see most often in the doctor’s office or other testing centers, and some home tests allow people to swab themselves and send the samples to a lab for PCR testing. These tests work by first converting the virus RNA in the sample into DNA. If viral genetic material is present, then multiple cycles of copying short pieces of certain viral genes should produce a signal. It would be a positive test result.

Most home kits are antigen tests, which probe for certain viral proteins. Most rapid antigenic tests look for the coronavirus nucleocapsid, or N protein. This protein helps condition viral RNA.

How accurate are the tests?

Both types of tests are precise but have strengths and weaknesses. Because of their taking turns copying, PCR tests are very sensitive and can collect small amounts of viral RNA, even before a person becomes contagious. The downside is that the test can still pick up RNA debris even after people stop making infectious viruses, which can make people appear to be contagious when they are not.

Home kits aren’t as sensitive as PCR tests, but “rapid antigen testing is really great, especially if you’re symptomatic, to determine if you have COVID at that time,” says Gigi Gronvall, immunologist and senior researcher at the Johns Hopkins Center for Health Security. “They compete with PCR for accuracy for this period” when people are symptomatic and most likely to be contagious to others.

But a negative test result doesn’t necessarily mean you aren’t infected. It just means that the test did not detect the virus at that time.

“All testing is a moment in time,” says Gronvall. “There will always be a time like watching a sunrise when the light has not yet come over the horizon. It’s not like the sun isn’t there. … You will not know the day or the time when the virus will start to reproduce, ”she said. For this reason, experts advise doing several rapid antigenic tests a few days apart.

It’s also something to keep in mind when deciding when to test while on vacation. Take a quick antigen test before you go to a party to make sure you are probably not contagious to others at the time. But if you’re worried you might have been exposed to the virus during the celebration, “the test as soon as you get home from the party won’t do much good,” says Gronvall. Health experts advise waiting five to seven days after exposure to be tested, but people often test positive within three to five days with the delta variant, she says. It is not known how long it may take to get a positive result if you have an infection with the omicron variant.

Do vaccines interfere with COVID-19 diagnostic tests?

At first glance, it seems logical that a vaccine could give wobbly test results. The mRNA vaccines from Pfizer and Moderna and the DNA vaccine from Johnson & Johnson all contain instructions for constructing the coronavirus spike protein. A PCR test could detect this signal, right?

It turns out that many PCR tests probe several viral genes, including those encoding the spike and N proteins, but also for an RNA polymerase that copies the genetic material of the virus and for the envelope of the virus, or protein E. A test is positive if it detects all of its target markers. But a signal from the spike protein in vaccines alone is not enough to send an alarm signal. In addition, the mRNA of vaccines does not stay in the body for long, from 24 to 72 hours. Thus, vaccines do not interfere with the precise results of PCR tests.

Most antigen tests detect the N protein, not the spike protein. So even if the vaccine works spectacularly and your body produces peak proteins at levels you would get with an active infection, the antigen test would ignore it, so it won’t interfere either. with the results of these tests.

Can the tests detect viral variants?

The delta variant cannot fool either of the two tests. Omicron, however, is missing nine building blocks of RNA called nucleotides in the N gene. It could lead to false negatives on a PCR test from Tide Laboratories used at more than 30 sites across the country, reports the United States Food and Drug Administration. This particular test only looks for the N gene, so missing pieces are a problem. But antigen tests can still detect the N protein despite the missing bit.

Omicron is also lacking some of the spike protein that causes what is called “S gene drop” in 26 PCR tests from various manufacturers, which means the spike gene is not even detected. if the person is infected. This is usually not a problem since these tests look for several viral genes. In fact, the missing peak signal may be a clue that a person is infected with omicron rather than delta and that their sample needs to be examined more closely. Alpha and a few other variants also caused the S gene to drop, so it’s not a slam dunk that the missing tip announces an omicron infection.

On the other hand, a second version of omicron which has been spotted in certain places does not cause the dropout of the S gene. Thus, a sample can test positive for the S gene and other markers and be mistaken for a delta variant infection, when it is actually omicron.

Should I use a rapid home test or take a PCR test?

“It’s really a question of when and why you need it,” says Gronvall.

When people have been exposed to someone with COVID-19, but do not have symptoms themselves, a PCR test could detect a low-level infection, perhaps before people become infectious to others. “It might save you some time” to isolate and prevent the virus from spreading to others, says Gronvall. But it may take a few days for some PCR test results to arrive, erasing this advantage.

Rapid tests could be useful for people who show symptoms of COVID-19 and need to know quickly if they are contagious to others. New antiviral drugs that may soon be on the market require people to take them within three to five days of onset of symptoms, so rapid testing can help people get treatment in that narrow window.

Logistics can also be a factor in the decision. Home tests are not officially recorded unless a person reports the results to a doctor, and even then the result may need to be confirmed with a second test. Insurance companies may require such confirmation to verify that people have had COVID-19 before paying for certain treatments or to monitor the long-term effects of the disease. International travelers will also likely need to take a PCR test to officially verify that they can travel safely, Gronvall said. “You might say you’re negative, but without proof they won’t take your word for it. “

How long are the tests good for?

If stored correctly, the tests should be fine for a while. For example, the Abbott BinaxNOW test I just purchased says to store it between 35.6 ° to 86 ° Fahrenheit (2 ° to 30 ° Celsius). It was made on November 2, and the little hourglass symbol on the box tells me it’s best to use it before June 19, 2022.

This long shelf life can be important as we learn to live with the coronavirus. Testing could become an integral part of our lives as people look for additional ways to socialize, work and travel safely. And my friend certainly won’t be the last parent to dab their little ones’ noses for answers, either.

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