Some blood tests require fasting to give accurate results, especially those measuring sugar and certain fats. Typically, this means no food or calorie-containing drinks for 8–12 hours before the test.
In most cases, you are allowed to drink small amounts of plain water. Water keeps veins easier to find and prevents dehydration. But juices, tea or coffee with sugar or milk, soft drinks and even chewing gum with calories can affect results and are best avoided.
If you take regular medicines, ask your doctor whether to take them as usual or adjust for the test day. Some can be taken with a small sip of water; others may need special instructions.
Time your fast to be as comfortable as possible. For example, eating an early dinner and scheduling a morning test lets you sleep through most of the fasting period.
If you accidentally break the fast, tell the lab or doctor honestly. It’s better to repeat the test correctly than to make decisions based on distorted numbers.