Why isn’t everybody screened for cancer of all types more urgently?

Question

With the odds of about 1 in 2 people getting cancer at some point in their lives, why isn’t it protocol for everybody to get screened for cancer of all types more often? Like maybe every few years starting at the age of twenty? It seems most times it get caught is when somebody is complaining of a symptom, often times too late.

In: Biology

Answer 1

– Because not every kind of cancer has a good screening test. Read about the statistical concepts of sensitivity and specificity.

– Because not every person is equally likely to get any (or all) cancers. Read about pre-test probability.

– Because not every screening test is equally valid for every person. Read about positive and negative predictive value.

In order for a screening test to be useful, it has to have a reasonable chance of catching real cases (minimizing false negatives) while ignoring healthy people (minimizing false positives). The statistical concepts I mentioned above all build on each other to determine how useful a test will be for any specific group of people. All cancers are different, so the inputs required to make a screening test valid for any particular cancer are also different. Applying all screening tests to all people (even those very unlikely to have a particular type of cancer) would increase the number of false positives and false negatives found. How would we figure out which people that tested positive were actually positive? And how would we sort through the negatives to determine which people had falsely tested negative and actually needed treatment?

The costs of a false positive or false negative screening test can be vast and not limited to financial costs. What if a screening test tells you that you have a type of cancer, but it’s a false positive? What if you undergo chemo, or radiation, or surgery because of a positive test, and permanently alter your future health trajectory unnecessarily? What if your mental health or relationships are affected? What if you found out after everything that your test was incorrect, and you never had cancer to begin with?