Having a few minutes to spare, I thought I would look this up for you, but the answer wan't as easily available as I thought. In the USA they use a different unit and the only UK references I could find were in articles about hip replacement. Perhaps this is why you want to know. Apparently there is some contamination in the blood from the materials used in these replacements. However, I did come across the phrase, "Since the upper limit for normal patients without implants is 5.0 nmol/L …..", perhaps we have our answer. So, it seems that you are probably at the upper limit for normal!
As for what "nmol/L" means, it means "nano moles per litre", or in plain English, think of a "nmol" as a "speck" or maybe a "needle in a haystack" and you'll get the general idea.
Sally