What Is Considered Fasting for Blood Work?
Fasting 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.
How Long to Fast Before Blood Work
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 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.
Do You Always Have to Fast Before Blood Work?
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.
Fasting Tests for Diabetes
If you have diabetes or your doctor thinks you might, certain blood tests can tell a lot about how your body processes blood sugar.
Fasting blood glucose test
Doctors use this test to diagnose diabetes. Your doctor will ask you to fast for eight hours beforehand. The test is usually done early in the morning, so you don't have to go too long without eating. A blood glucose level of 126 milligrams per deciliter or higher is a sign of diabetes. A healthy score is below 100 milligrams per deciliter.
Oral glucose tolerance test
This is another test used for diagnosis. You'll also need to fast for eight hours for this one before it's done. A nurse will start the test by taking a sample of your blood. Then, you'll drink a sugary liquid and stay in the doctor's office. Two hours later, a nurse will take another blood sample for testing. If that shows a blood glucose level of 200 milligrams per deciliter or higher, you'll be diagnosed with diabetes. A level of 140-199 milligrams per deciliter is a sign of prediabetes. That's when your blood sugar is higher than normal, but you might be able to keep diabetes at bay with some lifestyle changes.
Non-fasting blood tests for diabetes
It's not necessary to fast for all diabetes blood tests. These tests include:
Random blood glucose test. Doctors use this test to diagnose people who have serious diabetes symptoms. It can be given at any time on a moment's notice.
Oral glucose challenge test. Most pregnant women have this test to check for gestational diabetes between 24 and 28 weeks. There's no need to fast, and you can have this test done any time of the day.
A1c test. This test measures your average blood glucose level over the past three months. That timeframe means it doesn't matter if you eat before the test.
Other Fasting Blood Tests
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:
A lipid profile checks the level of cholesterol and other blood fats, such as triglycerides. High levels put you at a risk of heart disease or having a stroke. Not all situations require fasting. You may not need to fast if you're younger than 25 years or 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 panelis often part of a routine physical. The tests check your blood sugar, electrolytes, 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.
Why Is Fasting for Blood Work Necessary?
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.
Can you drink water when fasting for blood work?
Yes, drinking water (but no other liquids) is okay while fasting for blood work.
Forgetting to Fast Before Blood Work
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.
Eating After Blood Work
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.
Risks of Fasting for Blood Work
While fasting for blood work is usually safe, it can sometimes cause side effects. For people with diabetes, fasting can trigger low blood sugar (hypoglycemia) and cause dizziness, trouble concentrating, fatigue, and, in severe cases, fainting and seizures. If you have diabetes and are fasting for a blood test, be sure to have juice, hard candy, or glucose tablets with you to take right after the test.
Should you take diabetes medication before a fasting blood test?
Ask your doctor if you should take your diabetes medicine.
Fasting for Blood Work During Pregnancy
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.
Can You Do Fasting Blood Tests at Home?
Some fasting blood tests are available outside a doctor's office for you to do at home. These include:
Blood glucose meter
If you have diabetes, you'll use this test to check your blood glucose levels at home. Your doctor will tell you how often you'll need to test and if you should do it at specific times. For example, you may need to test your levels before snacks or meals.
Cholesterol test
You can purchase a test kit online, collect a small blood sample, and return the sample to a lab to be analyzed. You'll then get your test results in a few days.
Home blood tests are convenient, but incorrectly using one can affect your results. You should still consult your doctor for guidance.
Takeaways
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.
Fasting for Blood Work FAQs
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 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 8-12 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 8-12 hours. You may need to avoid alcohol longer than that.
Do you have to fast for thyroid blood work?
Most doctors don't suggest fasting before a thyroid blood test.
