I understand insurance may be a cause that a test cannot be administered to someone that doesn’t fit their criteria. What I want to know is if there is a scientific reason a test will work on someone who is 45 but not on someone who is 40. Or someone who is 50 but not someone who is 42.
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Any given test will have a false positive rate – meaning that it will tell you that you have cancer, when you really don’t. Say that’s 1 in 10,000 – that if you get 10,000 tests, 1 of them will show cancer when you don’t actually have it.
Similarly, rates of most cancers go up with age. When you’re 40, the rate of a given cancer may be 1 in 100,000.
So if the test is *perfect* at detecting actual cancer, and you tested 100,000 40 year olds, you’d expect to find 1 cancer case. But you’d also get 10 false positives.
Given the cost of testing, and the psychological and actual cost of false positive diagnosis, it’s not worth testing 40-year-olds.
But, when the rate of that cancer is 5 in 10,000 when you’re 60, that test makes sense now.
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