If a Coronavirus antibody test is positive, what are the odds you actually had Coronavirus?
% of the population has been infected at some point
The test has % specificity (or in other words, it has a false positive rate of %)
The test has % sensitivity (or the false negative rate is %)
Prefill specificity and sensitivity with values for the: / / / test
If you take the test and it is positive, there is a 83.47% chance that you have had Coronavirus
If you take the test and it is negative, there is a 100% chance that you have not had Coronavirus
Even though a test itself might be 99% accurate, so few people have had Coronavirus (we think) that as many people will get a positive test due to that 1% inaccuracy as will get one for actually having the disease. This is called the "base rate fallacy" and you can read more about it on Wikipedia.
I built this calculator so you can plainly see what the specificity/sensitivity values for tests actually mean when applied to the current population, and see how test accuracy changes not just with specificity and sensitivity, but also with the infection rate.