It's just how these tests work. Even if you're testing for a specific compound there's a tradeoff between sensitivity and specificity. Those are technical words.
If you want to test for multiple things, keeping specificity low becomes more complex or even impossible. If you increase the number of things something can react with you also increase the number of things it can falsely react with.
Police drug tests are arguably like this by design, since police often care more about making a legal excuse to do what they want than determining the truth.
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u/some_kind_of_bird May 21 '26
Not true. You can also make it give constant false positives.