If the test is cheap, you could run it 2 or 3 times. Then the false positive rate would be pretty low and you could proceed with more intense treatments/diagnostics.
It might still be useful to know you have weird protease activity that isn’t cancer derived, but the more of these tests we do, the more likely it is that for every person, there’ll be at least one non-cancer oddity that looks like cancer signal for at least some test.