Screening tests such as "blood tests and imaging" have two costs and one benefit:
- the cost (dollars and possible health problems) of the actual screening, which can in some cases be free
- the cost (dollars and possible health problems) of the followups required when the screening is positive
- the benefit of less people dying or their treatment being less expensive when things are found before there are symptoms.
Obviously, deciding whether or not to screen will be different for every disease because each of these costs and benefits will be different for every disease. But it's really important to realize that screening brings false positives. These cause distress and anguish to people who believe they are dying and they cost a lot of money because the investigations required are expensive and intrusive.
Recently Ontario has stopped recommending women do monthly breast self exams. These are free. However they do not save any lives. Women who actually have lumps find them without dutifully checking every month or week for them. But with the regular screening, many find things that are not cancers. They need to take time off work, get biopsies, which can be inconclusive, get scans, even have surgery to remove lumps so they can be sent to pathology, and so on. This costs a lot of money directly to the hospital and indirectly in people being off work. All this expense is pointless: there is no difference in survival rates between groups who do BSE and those who do not. They didn't even take into account the misery of thinking you have cancer when you don't.
(A news article about the changes, which also includes not having doctors do breast exams routinely if there are no symptoms, and not having mammograms before age 50. The Task Force Guidelines referred to in that article.)
For screenings that are not just self-examination, consider also the radiation from scans, the chances of an allergic reaction to contrast dye, the chances of infection from a blood test, of catching an antibiotic-resistant infection while in the hospital recovering from having something removed, and so on. Both the screening and the procedures that happen when someone screens positive can hurt or kill people if you screen everyone, meaning millions of people.
Finally, while it may seem logical to you that screenings like this would save lives by catching things earlier, there is very little evidence to support that. Our bodies actually destroy small cancers all the time: just because you see a small cancer on a scan doesn't mean the huge treatment machine has to swing into action. That is only needed when a small cancer demonstrates it's becoming a big cancer. And in the majority of cases, there are symptoms. These things get found.