Medically Reviewed by Jabeen Begum, MD on July 29, 2024
Written by Regina Boyle Wheeler , Jody BravermanFasting for blood work means you don't eat or drink anything but water for 8-12 hours before you have a blood test. You also need to avoid chewing gum, smoking, exercising, and taking certain medications and supplements. Your doctor will ask you to do this for certain blood tests because foods and drinks can affect your results.
You will normally need to fast for 8-12 hours before a blood test. But your doctor will tell you exactly how long, as fasting periods are different for different tests. Your doctor may also tell you not to drink alcohol for a longer amount of time before your test.
If possible, schedule your test in the morning so that most of your fasting time takes place when you're asleep.
Not all blood tests require fasting. You only have to fast before a blood test if your doctor tells you to. If you are unsure, you can always ask your doctor the day before your test to check.
Blood tests help doctors check for certain health problems and find out how well your body is working. Doctors also use them to figure out how well treatments are working.
These tests typically require fasting:
Fasting blood glucose measures the amount of glucose (sugar) in your blood to test for diabetes or prediabetes.
A lipid profile checks the level of cholesterol and other blood fats, such as triglycerides. High levels put you at risk of developing heart disease or having a stroke. Not all situations require fasting. You may not need to fast if you’re younger than 25, if you require only a partial lipid panel, or if your doctor is looking for a "non-fasting" result. Ask your doctor if you need to fast for your test.
A basic or comprehensive metabolic panel is often part of a routine physical. The tests check your blood sugar, electrolytes, and fluid balance, and kidney function. The comprehensive test checks your liver function, too.
The vitamin B12 test measures how much of the vitamin is in your blood. It can help diagnose a specific type of anemia and other problems.
Iron tests are used to see if iron levels in your system are too low or too high.
Gamma-glutamyl transferase (GGT) shows the level of the GGT enzyme in your system. A high reading may indicate liver disease, bile duct problems, or alcohol abuse.
Renal function panels assess the health of your kidneys. Your doctor might order them as part of a general health screening or if there is a problem with your kidneys.
Nutrients in food and drinks go into your bloodstream and can change things measured by the tests, skewing your results.
For instance, if you eat or drink before a fasting blood glucose test, your blood sugar probably will be higher than if you hadn't had anything. When you're fasting, doctors get a baseline result so tests can be compared to give a true picture of your sugar levels over time.
If you mistakenly eat or drink anything besides water before the tests, tell the person taking your blood. Your doctor needs to know this so they can interpret your tests correctly. For the best results, they may ask you to reschedule.
Once your blood is taken, your fast is over. You might want to bring a snack and a drink with you so you can eat as soon as possible after the test.
Some blood tests during pregnancy require fasting. It's normally safe for you to fast while you're pregnant, but discuss it with your doctor if you're concerned.
Food and drinks can affect blood work results, which is why your doctor may tell you to fast before some blood tests. It's important to carefully follow your doctor's instructions to get accurate results. If you are unsure of what to avoid or how long to avoid it, ask your doctor for more information.
Can you drink water when fasting for a blood test?
Yes, you can drink water. In fact, it's important to drink plenty of water to stay hydrated. This makes your veins plumper and makes it easier for the person taking your blood to find them.
Does black coffee affect a fasting blood test?
You should not drink coffee or any other beverage besides water. Coffee contains caffeine, which can affect the results of some tests.
What can you have when fasting for blood work?
You can only have plain water.
What are the rules for fasting for a blood test?
Avoid any food or drinks, alcohol, smoking, chewing gum, exercising, and some supplements and medications for 8 to 12 hours before your test. Your doctor will give you specific instructions based on the test you're having.
How many hours do you need to fast for a blood test?
Normally, fasting is required for 8 to 12 hours. You may need to avoid alcohol longer than that.
