i have seen different reference ranges at different transplant centers.
I have seen the bottom number to be either 2 or 5 mcg/L.
The upper limit is 20 mcg/L.
So I have seen reference ranges of 2 - 20 and 5 - 20.
Normally, the lab is done a good 11-12 hours after taking prograf so it shows the trough level at it's lowest.
I agree with Mike as to reference ranges.
Around 5~8 is what was normal trough level for me when I used to take it.
What was normal for you is not really illustrative of what the reference range is.
My normal was 4.5 - 5.5 but that is not the reference range on any labs I've seen. My lab sheet showed 5 - 20.