COVID Questions: Different Tests

Tom Friedrich, professor in the UW School of Veterinary Medicine, explains a consent form and the process of volunteers spitting in a small vial as part of a trial of a new COVID-19 saliva test
Tom Friedrich, professor in the UW School of Veterinary Medicine, explains a consent form and the process of volunteers spitting in a small vial as part of a trial of a new COVID-19 saliva test. (Jeff Miller / UW-Madison)

Note: UW-Madison will be publishing answers to questions about COVID-19 and the pandemic each week in a COVID questions column. If you have a question, please email it to covid19update@uc.wisc.edu. This post was originally published here.

Q: What is the difference between a PCR test and an antigen test? 

A: Tests for SARS-CoV-2, the virus that causes COVID-19, have two main parts. First there is the kind of sample taken to look for the virus. This could be a nasal swab, the infamous nasopharyngeal swab (the deep nasal swab sometimes jokingly called a brain swab), or a sample of saliva. Any of these sample types could be used with any of the following test types, though in practice each test type has one or more preferred sample types.

PCR tests look for the virus’s genetic material. PCR is an extremely sensitive technique — it can detect very small amounts of genetic material in a swab or saliva sample. In most people, it seems that contagiousness begins a day or two before the onset of symptoms and ends after symptoms resolve. But in most infected people there seems to be a period of time after symptoms end in which there is a small amount of virus remaining. This small amount can be detected by very sensitive PCR tests, but it is not enough to transmit to another person. That is, after symptoms resolve, you might be positive by PCR, but not contagious. (For this reason, it would be helpful if PCR test results reported the amount of virus detected, and not just “positive” or “negative.” Then doctors could have a better sense of whether someone might be contagious to others.)

This is where antigen tests come in. They can use the same sorts of samples that PCR tests do, but instead of looking for genetic material, they look for pieces of the virus itself, called antigens. Antigen tests are rapid — they can give results in just minutes. Right now, it seems like antigen tests are almost as good as PCR at detecting SARS-CoV-2 — IF the person being tested has symptoms and has had them for a week or less. The data on this is not as solid, but antigen tests seem to be less reliable in people who do not have symptoms. We need more studies to know whether antigen tests will be a good tool for screening people without symptoms.

In any question about testing, I feel it’s important to add that we all need to keep up our other prevention strategies, like distancing and masking, whether we get tested or not. A negative test today does not mean that you will be negative tomorrow. It’s particularly important to remember this as we want to gather with friends and family for the holidays. We should not view a negative test on, say, Friday as a license to gather without precautions on Saturday or Sunday.

– Thomas Friedrich, Professor, Pathobiological Sciences, UW School of Veterinary Medicine; Virology Services unit head, Wisconsin National Primate Research Center

See more answers to COVID questions at https://news.wisc.edu/tag/covid-questions/. Also, visit the COVID-19 impact site.


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