Medical tests do not always provide certain results. Quartz illustrated this with the accuracy of a simulated antibody test that identifies 90% of those infected and 95% of those not infected:
That means that if you took the test and got a positive result, there’s a 45.1% chance it’s correct. If you got a negative result, there’s a 99.6% chance your result is accurate.
Of course, this doesn’t mean don’t take the test. Detecting 90% of infections with false positives is a good thing. However, it does help to understand what the numbers mean before you do anything with them.