Thread by @DearPandemic

1/ Are there really false-positives from COVID tests? If so, how is it possible for a test to pick up something that is not there?

A: Yep, they exist.

2/ Two types of tests are commonly used to identify an active #COVID19 infection, & both of them can occasionally lead to a “false-positive” result, or a positive test result when there is actually no infection present.

3/ The two types of tests are:
1. PCR tests 🧬, which detect itty bitty pieces of the virus’ genetic material, &
2. Antigen tests 🦠, which detect signature proteins on the outside of the virus. (Note: Don’t confuse antigen with antibody tests 🩸 ! That's different.)

4/ 🧬🧬A PCR test works using a chemical reaction that matches genetic material collected in the sample to two fragments of genetic code that are totally unique to SARS-CoV-2.

5/ If there’s no match, the test is negative. If there’s a match on one fragment but not the other, the test is inconclusive & will usually be repeated, & if there is a match on both fragments, the test is positive.

6/ Scientists think that false-positives in PCR tests happen most often because the bits of genetic code we are matching TO are very small fragments. Small remnants of the virus in the sample might match, even though they are not whole viruses & couldn’t infect anyone.

7/ An infected person is likely no longer able to get others sick about 10-14 days after their first symptoms appear, but they may still have little bits of inactive viral material in their body for quite some time.

8/ We've heard reports of people with positive PCR tests *months* after their symptoms have abated (shout out to Lu in Italy! Hang in there). This is likely due to inactive viral material still hanging around in the body after infection.

9/ 🦠🦠Antigen tests give a positive result when they detect a particular protein on the surface of the virus, like a fingerprint. Antigen tests have some major advantages, including being fast & cheap. If you’ve ever had a rapid strep test, you’ve had an antigen test.

10/ Antigen tests can return false-positives if they find a similar protein, if the sample is handled incorrectly, or if something goes wrong with the test chemicals.