If a person buys into the idea that insulin resistance is the root problem rather than glucose levels, then 15 is higher than a person might like. My understanding is that would not be an excessive number but would not be a healthy level.

Keep in mind when a blood test comes back the reference numbers they call within a "normal" range are just numbers derived from a population averages. The assumption is that the average person is healthy. But if the population is unhealthy in some aspect, that portion of the "normal" range could be far from optimal. There is no scientific reason for many of those ranges other than statistics gained from the population that took the blood test. It is a statistical number.

My blood test from 2019 shows a reference range of 2.6-24.9 That is a pretty wide range. The doctors that I follow suggest optimal is single digits. Anything under 10 being optimal. My test then was 7.6 and that was after being on a low carb diet for around a year. 7.2 in 2020.

I have a fairly low insulin level because of the way I eat to control my blood glucose levels. I simply do not need much insulin to process what limited carbohydrates I consume. Even at that I am still insulin resistant. If I were completely healthy, eating at the very low level of carbohydrates that I do, my insulin level would probably be more down around 2 or 3. But since my cells are still resistant to insulin it takes more of it to do the job of keeping my blood glucose levels in normal range.  I never had an insulin test before so don't know what it would have been prior to changing my diet, but I would guess something over 30. Really the insulin test would have been useless at that time because I was taking insulin shots so the insulin levels could have been extremely high.

So it kind of depends on who you want to believe. If you believe mainstream doctors have a good handle on diabetes and blood glucose management then being "normal" would be between the 2.6-24.9. If a person believes some of the alternative thoughts of the doctors I follow (one of them being an insulin research person that specializes in insulin, Dr Benjamin Bikman), then the upper end of that range is not at all optimal. From what I have learned from them is that the level you are at is not excessive but also probably not optimal for optimal health.

So it kind of boils down to which doctors and which line of thinking you think is correct.

Here is one line of thinking.  https://www.youtube.com/results?search_query=dr+ken+berry+blood+tests

About minute 20 on this video.   https://www.youtube.com/watch?v=n9GzYO0cOBU&t=1285s