How do blood labs determine what is “normal”? Do they use average Americans who are obese and have diabetes and high blood pressure etc, or do they use “healthy” people?

Question

For example: Normal isn’t the same as optimal. Smoking used to be normal.

So if it says normal Vitamin D levels are 60 does that mean it’s the optimal level or just what level is common?

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Answer 1

Normal in this context is just meaning “within healthy ranges”

So values falling outside of healthy ranges would suggest the presence of disease, injury or other irregularity

Sometimes, having a higher than normal or lower than normal result may actually be expected too and would be noted.
For example a lot of people will have a decreased vitamin d level in areas where the sun is only up for a few hours during winter, this would be “normal” for the population but it would not be seen as “normal” in regards to a blood test because it is outside healthy ranges. Luckily it’s easily fixed by supplanting and/or using lamps