Before a fasting blood test, a patient may only consume water and must abstain from any other foods or liquids, according to Quest Diagnostics. Fasting is necessary before certain blood tests in order to obtain accurate results.
According to Quest Diagnostics, water is okay to drink before a fasting blood test and is actually recommended. Patients who must fast overnight or for a few hours should drink a lot of water to stay hydrated. The extra water can plump up their veins and make it easier to for a lab technician to draw their blood. Generally, the longest period a patient may need to fast is eight hours.