Diabetes is asymptomatic for years and is prevalent in every demographic. Leaving it unattended it can cause damage to blood vessels, nerves, kidneys, and eyes through chronic high blood sugar, by which time complications may be advanced; detecting it on time can prevent or delay this with treatment and lifestyle changes.
Hyperlipidemia is asymptomatic for years and is prevalent in every demographic. Leaving it unattended it can cause artery blockage through cholesterol buildup, by which time heart attack or stroke may occur; detecting it on time can prevent this with diet and medication.
Kidney disease is asymptomatic for years and is prevalent in every demographic. Leaving it unattended it can cause kidney failure through gradual loss of function, by which time dialysis may be needed; detecting it on time can slow progression.
Glaucoma is asymptomatic for years and is prevalent in every demographic. Leaving it unattended it can cause irreversible vision loss through optic nerve damage, by which time blindness may be permanent; detecting it on time can preserve vision.
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I'm SO glad you're not my family doctor!
On the other end of the spectrum, what doesn’t make sense is testing a random person off the street for Ebola. The prevalence approaches zero and symptoms are fairly noticeable, so any positive test is definitely wrong.
Most diseases are in between and have to be evaluated case by case, not buckshot.
You may be particularly interested to hear that there’s little evidence to support regular checkups in most adults beyond blood pressure testing and cervical cytology.
> Given the lack of favorable evidence and the potential adverse effect, primary care providers should consider the fact that general health checks, beyond the screening interventions shown to have benefit, likely have little or no effect on important health outcomes. Some of the interventions with demonstrated benefit have sufficiently large effects that a uniform application is warranted (blood pressure measurement and cervical cytology screening). In others, the trade‑off between benefits and harms is so close that patients should be involved in fully shared decision making regarding their participation (breast and colon cancer screening).
https://pubmed.ncbi.nlm.nih.gov/31642821/
I suspect your doctor would agree with me. See if they’ll test you for Ebola, for instance. Not because you have symptoms but just cuz.
In your list, 1-4 are common enough, the tests are accurate enough, the costs of intervention are low enough and the benefits of early intervention are high enough to justify screening, which is why they do generally screen for them at least in hgiher risk groups. The other two are more mixed, which is why mass screening is less common.
All the evidence for full body scans is that they are not justified for asymptomatic people. The false positives are high, the costs of these false positives are high, and the imporved outcomes are too low to justify them. If you want one, go ahead, but realise that almost anything it finds is likely to be false either positive or not likely to ever cause a problem, and you'd have to deal with the worry and invasive tests and even surgery in aid of something that may never cause any trouble.