Blood tests are a single evidence point in a chain of thinking. You do a blood test to confirm your initial theory, not as an exploratory exercise. You need it

Blood tests are a single evidence point in a chain of thinking. You do a blood test to confirm your initial theory, not as an exploratory exercise. You need it to be accurate, because otherwise you'll have to do it multiple times. (which unless the number of tests required is cheaper than the original, its pointless)