About those Covid-19 tests

 I haven’t had one, not that I haven’t tried. Here’s the deal with them: There are three kinds of tests—

PCR virus test. PCR stands for polymerase chain reaction, a system that analyzes DNA—the DNA of the virus in this case. It can detect an active infection from a sample you get by swabbing your nose or back of your throat. Because the samples go to a lab for analysis, it takes several days to get the results back. With PCR tests, false positive rates are extremely low and are usually caused by mishandling the specimens in the lab. False negative results can occur during the first week of infection, usually because the virus isn’t yet present in detectable quantities. According to a study in the Annals of Internal Medicine, the probability of a false negative PCR result is 100 percent on the first day of infection and decreases to 20 percent by the eighth day. This test is considered the most reliable.

Antigen test. An antigen is a substance that causes your immune system to produce antibodies against it. The antigen test looks for certain molecules on the surface of the virus. This test also requires nasal swabbing. Positive results from antigen tests are highly accurate, but the test has about a 50% rate of false negatives. In other words, negative results don’t rule out infection. Though less accurate than a PCR test, you get results in minutes, sort of like a pregnancy test. A number of companies are producing these tests. 

Molecule test. It's a do-it-yourself test that checks for virus RNA molecules. You swab your nose, stick the swab in a container containing a chemical that changes color depending on the results, which take 30 minutes. The FDA just approved one of these tests: the Lucira COVID-19 All-In-One Test Kit. A positive result probably means you have the virus; a negative result does not necessarily mean you don't.

As to my experience: We were all set to have tests prior to a trip but cancelled the trip as well as our tests. At the same time, in the interests of science, I’d signed up to take the test for the Stanford CATCH study, which is tracking the spread of Covid-19 in the San Francisco Bay Area. But the couriers who deliver the test kit and pick up the sample couldn’t find my house.

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