Take Results of Those Antigen Tests You're Going to Get with a Big Grain of Salt

 

I already mentioned this issue when I blogged about testing a few weeks ago. But it bears repeating and emphasizing now the the Biden administration is distributing free antigen test kits to everybody.

Starting yesterday, a day ahead of schedule, the Post Office began taking orders for the test kits. You can get four per address, which to be honest doesn't sound like enough for bigger households.

Something to know about these tests is that although they are fast, they are not very accurate compared to PCR tests, which take days to process. So says a review of studies (sometimes called a meta-analysis) published not quite a year ago.

They compiled the results of different studies done on different brands of rapid tests, which compared antigen test results to PCR test results on samples taken from the same people at the same time. Here is part of their "plain language" summary:

In people with confirmed COVID‐19, antigen tests correctly identified COVID‐19 infection in an average of 72% of people with symptoms, compared to 58% of people without symptoms. Tests were most accurate when used in the first week after symptoms first developed (an average of 78% of confirmed cases had positive antigen tests). This is likely to be because people have the most virus in their system in the first days after they are infected.

In other words, if a test subject does not have symptoms, there is almost a one-in-two chance the test will say they don't have COVID when they do—that's essentially no better than flipping a coin. The tests are a more accurate for people who have symptoms, but there is still a one-in-four chance of a false negative. Worse, it may be that existing antigen tests are less sensitive to Omicron.

The danger is that if people don't know this they will assume the negative result of their antigen test is accurate and go about their business, potentially spreading the disease. That's what happened to this college kid who took six rapid tests, got six negatives, went home to his family, then tested positive on a PCR test after exposing everybody.

I won't advise you what to do, but for myself, I will not use these tests unless I have symptoms. Even if I have symptoms I will retest if I get a negative result, and then get a PCR test anyway.



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