![]() In RT-PCR tests, we first convert the virus’s RNA to DNA, and then amplify it to make millions of copies. Every virus particle contains this RNA, which is a sequence of about 30,000 “letters” or nucleotides. The RT-PCR test detects the RNA that is the genetic code of the SARS-CoV-2 virus. For more details, a good description can be found here. The main drawback of these tests is that they sometimes fail to detect the virus. The antibodies in the test kit are designed to bind to other molecules that create a small band of color on a test strip, so you can see the results as a colored band in just 15 minutes. The rapid antigen test contains molecules called antibodies that bind to a specific molecule, the nucleoprotein, which is present on the surface of the SARS-CoV-2 virus. If possible, you should immediately get a followup test using RT-PCR, which is more accurate.Īddendum: for those who want to understand the difference between the rapid antigen test and RT-PCR, here’s a bit more on those. If you do get a positive test, you probably have Covid-19, but there’s still a roughly 30% chance that you don’t.If you get a negative test, you can relax: over 99.5% of negative results are correct, meaning you truly don’t have Covid-19.The chance that you’ll have an incorrect reading, either positive or negative, is very small, less than 1%. The rapid antigen test is generally very accurate, and certainly worth taking if you have any reason to think you might have Covid-19.What’s the take-home message from these studies? Well, I’d summarize it in three points: That’s better than the 42% found in the Toronto study, but it still means that many positive results from the rapid tests turn out to be incorrect. ![]() ![]() What about those false positives? Here the news was better than the Toronto study, but still far from perfect: about 28% of the positive results from the antigen test were false. So how good was the rapid antigen test in this study? First, its sensitivity was 92.7%, meaning that it correctly identified 92.7% of people who had Covid-19, whether or not they had symptoms. In this study, released in late January as a preprint on medRxiv by a group of my Hopkins colleagues led by Zishan Siddiqui, the investigators looked at 1054 participants, and tested all of them with both a rapid antigen test and RT-PCR.Įven though this study looked at far fewer subjects (1000 versus 900,000), they checked everyone with PCR, which allowed them to measure both sensitivity and specificity i.e., they could count how many infections the antigen test missed. They only used PCR to double-check the positive tests, so that study doesn’t answer the question of how many infections might have been missed.įortunately, the second study answers that question. One thing lacking in the Toronto study is that the investigators didn’t test everyone with PCR.
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