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Has your doctor ordered fasting blood work? If so, you might wonder how long to fast before a blood test and whether there are other rules to follow. Keep reading to learn why some blood tests require you to fast — and how to fast to ensure accurate results.

Why Do You Fast for Some Blood Tests?

Blood tests (also called blood work) help your doctor monitor your health and diagnose health problems. They measure various cells or substances in your blood and compare them to normal ranges.

Not all blood tests require you to fast. The levels of some blood components, like cells or platelets, don’t change if you eat or drink. But tests that measure nutrients in your blood like triglycerides (fat) or glucose (sugar) might appear higher than normal after eating.

Fasting before bloodwork gives your doctor an accurate picture of your blood. These are some common tests that require fasting:

• Fasting blood glucose test. This measures sugar in your blood. Note that you don’t have to fast for an A1C test, which measures your three-month blood sugar average.
• Lipid profile. This measures fats in your blood, like triglycerides, HDL (good cholesterol), LDL (bad cholesterol), and total cholesterol levels.
• Liver function test. This checks your liver health by measuring various proteins and enzymes in your blood.
• Basic metabolic panel. This combination of tests measures your electrolyte minerals, kidney function, and blood sugar.
• Vitamin B12 and iron tests. These are nutrients in your blood. Their levels can change if you’ve recently eaten certain foods.

If your doctor has ordered a blood test but didn’t mention fasting, double-check and ask whether you should fast or not.

What Are the Rules for Blood Work Fasting?

Fasting before blood work means you shouldn’t eat or drink anything except water for at least eight hours before a blood test. For some tests, you may need to fast for 12 hours. Ask your health care provider how long to fast before your blood test if you’re unsure.

Besides food, you should also avoid chewing gum and mints. These might have sugar or other ingredients that can get into your bloodstream and cause inaccurate test results.

You should also avoid all beverages, including regular and decaf coffee and all types of tea. Compounds in coffee and tea might affect your test results. And any caffeine in coffee or tea (or even sugar-free soft drinks) acts as a diuretic, reducing the amount of fluid in your body and blood.

It’s OK to drink plain, uncarbonated water, and, in fact, it’s recommended to hydrate with water before a blood test. It puts more fluid in your veins, making it easier and faster to draw your blood. Just make sure you skip the lemon or other flavoring you might usually add.

You should also avoid alcohol for at least 24 hours before a fasting blood test. You should know that smoking and exercise may also affect test results. Ask your doctor if you should avoid these before your test and if so, for how long.

You can stop fasting as soon as your blood test is over. If you’re hungry or craving that cup of coffee, you can stop to eat and drink on the way home. Or bring a snack and eat it once you leave the lab.

What if you accidentally eat or drink?

Sometimes your schedule is on autopilot, and you eat and drink out of habit. If you slip up, let the lab or your doctor know if you eat or drink — even a few bites or sips. Depending on the test and what you ate or drank, you may have to reschedule your test.

Although it might seem like a tiny bite of food or sip of juice might not hurt, these can affect your test results.

Taking Medications When Fasting

Ask your health care provider if you can continue to take most prescription medication while fasting.

If you have to take your medicine with food, you might want to wait until after your test to take it. Taking it on an empty stomach might reduce its effectiveness or give you an upset stomach.

And if you take blood sugar-lowering medications, ask your doctor if you should skip a dose or take it after you eat. Many are OK to take while fasting. Others, like insulin, might lower your blood sugar too much on an empty stomach.

While fasting, you should check with your provider if it is OK to take most over-the-counter medicines, like pain relievers or reflux medications. But ask your doctor about any supplements. Some, like gummy vitamins, contain sugar, while others have certain vitamins or minerals which may affect your test results.

Tips to Make Fasting Before Blood Work Easier

It’s hard to go up to 12 hours without eating or drinking anything except water. To make things easier, try these tips:

• Try to schedule your blood test early in the morning. That way, you’ll fast while you sleep.
• If your test is at 8:00 am, eat dinner later, around 7:00 pm. Just make sure you’re finished eating by 8:00 pm so you can fast for 12 hours.
• If you’re in the habit of eating breakfast or drinking coffee first thing in the morning, leave yourself a reminder. Set an alert on your phone or put a note in the kitchen, so you don’t automatically eat or drink.
• Drink a glass or two of water before your test to ensure you’re well-hydrated. Water might also help fill your stomach if you’re hungry.
• Pack a snack, like some fruit and nuts, so that you can eat as soon as possible after your blood test.

Having blood tests is an essential part of staying healthy. Following these guidelines will help ensure the most accurate test results if your doctor orders fasting blood work. And if you have any questions or concerns about your test, don’t hesitate to contact your health care team.