Because of sensitivity issues, some of these OTC tests are designed to be taken more than once, just in case your viral load is below detection the first time. The tests typically aren’t cheap, either, meaning that in some communities, the people at highest risk of infection won’t have easy access to them. In addition, outside the organized testing projects like the one Brigham was involved in, most of these OTC tests are not tied to doctors, public health departments, or any measure of accountability other than the entity requiring the test, such as an employer or a sporting venue.And while leaders at the companies who spoke with me about their OTC tests say they believe anybody who would take an at-home test would most likely sequester themselves if they tested positive, Shobita Parthasarathy, a science and public policy expert at the University of Michigan, says human behavior is hardly ever so cut and dried. OTC tests come in two flavors: antigen and molecular. Antigen tests detect small pieces of viral proteins that are either on the surface of infected cells or in bodily fluids. These small pieces are called antigens, and in the course of a disease like COVID-19, they are what the immune system recognizes as signs of infection. Molecular tests detect the virus’s genetic instructions, the nucleic acids that allow SARS-CoV-2 to copy itself once it gets inside a human cell. Antigen tests have a reputation for being unreliable, especially in people who are asymptomatic. They are considered less sensitive than molecular tests and can more often result in false negatives. That’s mostly because of detection thresholds: viral genes and viral antigens may both be needles in the haystack of our nasal swabs, but in a molecular test, that tiny amount of genetic material gets copied over and over, whereas antigen tests look for whatever bit of protein is physically in a sample. At the end of 2020, I had a significant exposure to someone with COVID-19 and experienced only mild symptoms. I tested negative with an antigen test but positive with a molecular one. Detection thresholds are most likely why.

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