system
Given the following post, respond as if you're commenting on Reddit:
human
Why do antigen rapid tests not work after 15 minutes?

I've used two different types of antigen rapid tests. Both say that the results aren't valid if more than fifteen minutes have past since testing (dropping the solution onto the test kit.). Why is this so? Do the coloring/colored molecules that do the binding no longer work, or weaken, after 15 minutes? Or does a positive turn into a negative?
gpt
These tests work by detecting the virus with manufactured antibodies that recognize some specific feature of the virus, like the infamous spike protein. There are two antibodies that together form a sandwich around the antigen (the feature the antibodies are looking for). The sandwich is formed when all the parts come together like a lock and key, or ball and cup. It’s very similar to an enzymatic reaction in that everything matches both physically (like the grooves in a key and the pins of the lock), but also electromagnetically - sometimes there are concentrations of positive charges that expect to meet negative charges (and vice versa) in these components that fit together. So the better the fit, the more obvious and fast the reaction. Where the fit is good, the test can be considered positive because all those antigen-antibody sandwiches are present in a high concentration (and therefore visible as a line on the test). 

However, over time, these sandwiches will still continue to form even in the absence of one of the components (even in the absence of the antigen), because positive and negative charges are so drawn to each other. They make a bad, but detectable, reaction. Because this isn’t an ideal sandwich, it happens slowly, very slowly, compared to a true positive. That’s why they give a cut-off time to the test - eventually it’s going to turn positive, but the speed at which it does is the telling part.