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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Below a certain age, the negative consequences of false positives (totaled across the population) are worse than the total number of false negatives – missing those few very rare cases.
Below a certain age, your risk of cancer is so low that any “positives” are very likely to be false positives, so you are exposed to extra testing, biopsies, etc (and their risks) unnecessarily.
However, if you have a family history of cancer, your doctor will likely recommend screening earlier and/or more often than they would recommend to their “average” patient.
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.
There are multiple factors involved in making something suitable for screening.
It includes that others have said.
Another factor not mentioned yet is cost-effectiveness. For the number of test you do, how many of the target disease will be picked up, and then how many years of life saved/disability avoided. So if you are thinking about looking for colon cancer you should target the age range that is affected most (and probably a bit beyond that age range at either end of it). There’s no point in doing the screening test in a group that isn’t affected; in the colon cancer example you wouldn’t do it on children as they don’t get colon cancer (for clarity there are exceptionally rare cases, most of which wouldn’t be picked up by the stool test used in my country to indicate who needs a colonoscopy)
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