is there any way to determine or calculate one's exact metabolic rate and daily (or weekly) caloric expenditure?

I know about the general methods of finding your BMR by calculating you weight by 11-14, and if you are active then multiply by 15-18. But these methods give quite big ranges, sometimes with 400-500 calries difference.

So what I am sking is... what is the best way to figure out my metabolic rate within a 50 calorie difference? Do I do that with a blood test?

thanks