A fasting blood test requires you to avoid eating or drinking anything for some time before your test. The amount of time to fast varies for different tests. It’s generally in the range of 8 to 12 hours.

Fasting allows a doctor to get accurate readings for measurements that are influenced by the food you eat, such as your blood sugar levels or cholesterol levels.

If you accidentally ate before a fasting blood test, you should let your healthcare provider know right away so that you don’t receive incorrectly interpreted results.

In some cases, you may still be able to receive your test as scheduled, but in other cases, you may need to move your test to a different day.

Let’s look at why fasting is necessary for some tests, what type of tests require fasting, and what you can do to make the fasting process easier.

Some blood tests require you to fast because the results can be easily influenced by the foods that you eat. The micronutrient, carbohydrate, protein, and fat levels in your food can all skew the results of some tests.

An example of a test that requires fasting is a blood glucose test, which measures your blood sugar levels. Eating foods high in carbohydrates can significantly raise your blood sugar levels within 15 minutes.

The amount of time you need to fast depends on the type of test you’re receiving. A doctor will tell you how long you should abstain from eating.

  • Blood glucose test. A fasting blood glucose test usually requires an overnight fast that lasts for about 8 to 10 hours.
  • Blood cholesterol test. Some types of cholesterol tests don’t require fasting. Some like a direct LDL cholesterol test may require up to a 14-hour fast.
  • Triglyceride level test. You likely won’t need to fast for a triglyceride test, but a 12-hour fast may be required in some situations.
  • Serum iron test. You may be requested to fast for 12 hours and to avoid taking iron supplements for 24 hours before this test.
  • Vitamin B12 tests. You’re often not required to fast before a vitamin B12 test. In some situations, a doctor may recommend a fast that’s about 6 to 8 hours.
  • Vitamin B complex test. A blood test looking at all your B vitamins is typically performed the morning after an overnight fast.
  • Renal function panel. You may be told to fast for 8 to 12 hours before a renal function test.
  • Gamma-glutamyl transferase test. A doctor may instruct you to fast overnight and avoid alcohol for 24 hours.

If you break your fast, the results of your test may be inaccurate. You can call your doctor to see if the test can still be done.

Some tests may be still be analyzed knowing that you’re not in a fasted state. It’s important to be honest with the test administrator so they can interpret your results properly. Some types of tests may need to be rescheduled.

There are many conditions a doctor might want to test for using a fasting blood test. Just three of many examples include diabetes, high cholesterol, or low iron.

The following early symptoms may indicate you have one of these conditions:

Many types of blood tests don’t require you to fast. A healthcare provider will let you know if, and for how long, you need to avoid eating.

For all types of blood tests, including fasting tests, you can still drink plain water. You should avoid other drinks like coffee, tea, juices, and alcoholic beverages.

The following tips may help make fasting for your blood test easier:

  • Stay hydrated. Drinking plenty of water before your test makes your blood vessels easier to find.
  • Schedule a morning test. If your blood test is in the morning, you’ll likely only need to skip one meal.
  • Eat before fasting. Eating directly before your fasting window reduces the amount of time you need to avoid food. For example, if your test is at 9 a.m. and you need to fast for 12 hours, you may want to eat your last meal around 8:30 p.m. the night before.
  • Avoid exercising during your fast. Exercise speeds up digestion and causes you to burn extra calories.
  • Keep yourself distracted. Keeping yourself busy may help take your mind off your hunger.

You can eat and drink immediately following your blood test. You’ll likely be hungry after fasting, so you may want to pack a snack with you to eat directly after your test.

Some blood tests require you to fast in order for your healthcare provider to get an accurate reading.

If you eat during your fast, you should let your doctor know so they can decide if you should reschedule. In some cases, you may still be able to have your test at the scheduled time.