system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how does the "control" line on a diagnostic test work (like a pregnancy test)? What makes it react if the sample itself is negative and how does that prove the test is working?
gpt
The control line is basically testing for the test ingredient. Say the test is looking for some Hormone A. It has a supply of Substrate B, which is known/designed to react with Hormone A to form Product C. Now if you test only for Product C and find nothing, do you know if there was no Hormone A? Maybe the Substrate B degraded and is no longer sensitive to Hormone A, so no Product C was formed.

So if you have two tests in one, one to test for Product C and one to test for Substrate B, then you can be sure that the lack of Product C wasn’t because Substrate B was missing, but because Hormone A was missing, since the Substrate B test came back positive. This means you’re not producing Hormone A (ie, you’re not pregnant).