Yes, you can drink water before the test. It will not affect the results. However, do not drink any other beverages, do not take any vitamins (particulary Vitamin D) and certain medications like MAO inhibitors, steroids or certain antiobiotics. Are you on any oral contraception?!
They affect it too.
Basically, fast for 12 hours, drink water if you want and ingest nothing else, and you'll have an accurate reading.
Good luck.