COVID-19: Roche Antibody Test – 14th May
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What do sensitivity and specificity tell us? Sensitivity tells us the proportion of people who test positive, out of the population who should have tested positive. In this case, a sensitivity of 100% tells us that everybody with a history of COVID-19 infection had a positive antibody test. (Tests with sensitivity or specificity that are truly 100% are very rare. I suspect that in this case, the sensitivity is very high, but maybe not actually 100%.) Specificity tells us the proportion of people who test negative, out of the population who should have tested negative. In this case, a specificity of 99.8% tells us that 2 in 1000 people (0.2%) tested positive with the antibody test, even though they should have tested negative. Surely this is all fine? 99.8% and 100% are both big numbers. But what does it mean for an individual who receives an antibody test result? Well, sensitivity and specificity can’t really help us. They tell us about what happens if we already know what the right answer should have been. But if you’re having a test, the whole point is that you don’t know the right answer. You get a test result, and you want to know how likely it is to be correct.