Please ask your doctor about this. Also ask if you can drink fluids/water before any blood test. Most blood tests require fasting, and have nothing to do with fluid intake.
If you're allowed to drink water, drink a lot of it (at least 2 glasses) about one hour before your blood draw! You'd be amazed at how many tests still allow you to drink water. It's so much easier to perform blood tests on well hydrated patients!!!