In American English, they usually just ask the question you got asked. If you were supposed to get blood drawn when you hadn't eaten or drunk anything, they'd tell you to fast, "no fluids except water, and no food." If you're getting surgery, however, there's usually a period of anywhere from eight to twelve hours prior that you can have "nothing by mouth," which means you can't drink water, either. I see mumblemumble pulled that up for you, as well as "nil per os," which I'd never looked up. I know NPO, because I've had a few surgeries and that's what appears on the paperwork, but it's explained as "nothing by mouth." Most US paperwork assumes people are barely literate, and it's unfortunately right in that assumption.