With kids going to back to school and many Americans heading back to the office, every cough or sneeze can cause panic during the ongoing COVID-19 crisis. Even vaccinated individuals need to be vigilant with the increasing number of breakthrough COVID-19 cases being reported.

According to the Centers for Disease Control and Prevention, fully vaccinated people who have come into close contact with someone with suspected or confirmed COVID-19 should be tested three to five days after exposure, and they need  to wear a mask in public indoor settings for 14 days or until they receive a negative test result.

But what test should you take? There’s been a lot of progress in COVID-19 testing since the pandemic began when people were lined up in droves to have a cotton swab inserted into their nostrils. Today, there are hundreds of tests that have been offered by the Food and Drug Administration through their emergency use authorization. According to Yale Medicine, the tests have not been given full FDA approval so there aren’t any gold standards as there are with other diagnostic tests.

There are two types of diagnostic COVID-19 tests: molecular and antigen. Molecular tests are considered to be more accurate than the rapid, antigen tests. Antigen tests can be done in 15 minutes at doctors' offices, pharmacies, and even at home. They are appropriate when fast results are needed for screening purposes, but most healthcare providers rely on the molecular tests when more information is needed or because the patient has COVID-19 symptoms.

The most widely used molecular test is called the polymerase chain reaction or PCR.

“PCR and similar tests look for the COVID virus’ RNA,” explains Dr. Sheldon Campbell, associate director of Yale Medicine’s Clinical Microbiology Lab. “They tend to be quite sensitive, but even among these, they are on a continuum of sensitivity and vary a whole lot.”

A test that is deemed highly sensitive will be able to catch more people who have a disease and generate fewer false-negative results, says Yale Medicine. Samples are obtained by using varying techniques with nasal swabs. The deeper you go up into the nose for a specimen, the greater the sensitivity. Some centers allow healthcare providers to take samples from the back of the throat or use saliva for the test. The samples are then sent to laboratories for test results which are usually available within one to seven days.

Antigen tests are much simpler, say Yale Medicine experts, and resemble pregnancy tests. They work by detecting pieces of protein from the SARS-CoV-2 virus, the virus that causes COVID-19. After a sample specimen is obtained from the nose, or sometimes the throat, it is placed on a treated test strip or cartridge. A colored line will indicate the results. The results are typically generated within 15 minutes. However, because this test needs a higher level of virus than PCR tests, it generates more false negatives. 

Antigen tests are available in pharmacies and doctors’ offices but can also be purchased for home use, says Yale Medicine. PCR tests are also available at pharmacies, physicians’ offices, and designated testing locations.

A third category of COVID-19 testing is the ID Now technology that is available in some testing locations. ID Now is a rapid molecular test that is more accurate than the rapid antigen test and results are available in about 15 minutes.

The Yale Medicine experts say that while home testing is convenient, you are better off leaving this important procedure to experts.

“I would think it’s better to have the test done by someone whose job it is to do it, especially if there are free testing locations available,” says Campbell.

What test you should take depends on what information you require. Campbell says that for traveling, it’s best to take PCR test because it is more sensitive, and you want to detect the virus early. For routine testing in schools and workplaces, the antigen test is sufficient and more accessible.

For vaccinated people who recently attended a crowded event and are worried that they may have come into contact with someone who has COVID-19, the expert suggests waiting a few days before getting either the PCR or the antigen test.

“We think that you have to have a fair amount of virus present to be infectious to others, and we know that in the course of infection, the viral load goes up and down,” Campbell told Yale Medicine.