Blood draws for several kinds of diagnostic tests require you to fast for several hours. Eating or drinking anything besides water can skew the results, leading to you needing to be retested.

You may be asked to fast ahead of several types of blood tests, such as a lipid panel and a blood glucose test. When you fast, you don’t eat food or drink any liquids other than water for an extended time.

Consuming food and beverages causes your body to absorb sugar, fat, protein, and other nutrients. These substances affect the levels of various substances in your blood, which can make the test results inaccurate.

Fasting before a blood test typically lasts for 8 to 12 hours, and you can eat again immediately after your blood is drawn.

Read on to learn more about the most common blood tests that require testing and how to prepare for the best experience.

A handful of common diagnostic tests require you to avoid all food and beverages besides water.

In addition to fasting for 8 to 12 hours, your doctor may ask you to avoid alcohol, smoking, chewing gum, and strenuous exercise for 24 hours before your blood test. These can also influence certain results, like blood sugar levels.

Let’s go over the most common blood tests that require fasting.

Fasting blood glucose (sugar) test

The fasting blood glucose test is used to help diagnose diabetes and prediabetes and see whether treatments are effective. Eating and drinking can cause a temporary spike in your blood sugar levels, leading to inaccurate readings. Another version of a blood glucose test involves drinking a sugary liquid after a blood draw and then having a second blood draw 2 hours later.

Lipid panel

A lipid panel checks for your level of cholesterol and triglycerides. High levels put you at risk of developing heart disease or a stroke. This test doesn’t always require fasting, but when it does, eating and drinking can raise lipid levels for several hours, which skews results.

Metabolic panel

Frequently part of a physical exam, a metabolic panel checks your blood sugar, electrolytes, fluid balance, and sometimes your kidney and liver function. If it does, it’s called a comprehensive metabolic panel. Depending on what your doctor wants to measure, fasting may not be necessary for this test.

Liver function tests

There are several kinds of liver function tests, which measure the enzymes alanine aminotransferase (ALT) and aspartate aminotransferase (AST), the protein albumin, and the pigment bilirubin. Fasting ensures accurate enzyme and protein measurements, though it’s not always required.

Vitamin B12 test

A vitamin B12 test is used to help doctors diagnose anemia and other conditions. It also measures the amount of vitamin B12 in the blood.

Iron tests

There are several types, including ferritin and serum iron, and they’re used to see if iron levels in your system are too low or too high. Eating and drinking can also alter iron levels.

Gamma-glutamyl transferase (GGT)

A GGT test shows the level of GGT enzyme in your system. A high amount may indicate liver disease, bile duct problems, or alcohol overuse.

Renal function panel

A renal function panel tests the health of your kidneys. It’s sometimes ordered as part of a metabolic panel and doesn’t always require fasting, though other tests may still require it.

The most important step when you’re getting ready for a blood test is to speak with your doctor in advance to confirm the specific preparations for your test. Here’s a handy checklist to help you prepare.

Preparing for your blood test

  • Speak with your doctor to confirm if you need to fast, for how long, and what kinds of liquids you can drink during this time.
  • Tell your doctor which medications you are taking and whether you can keep taking them during the fast. In most cases, you can keep taking your medications.
  • Try to schedule your blood test for the morning. This means much of your fasting period will occur at night, when you are sleeping.
  • Work out the latest time you can have a meal before going to sleep to minimize your time spent thinking about food.
  • Bring a snack to your medical appointment so that you can eat as soon as your blood draw is complete.

You can only drink water when fasting for a blood test. The nutrients in these other beverages can get into your bloodstream and skew results. You also need to avoid flavored still or carbonated water, because they can also affect the test.

Drinking plain water will not affect the results and will also make your veins fuller, making it easier for the medical professional performing your test to get a sample of your blood.

It’s generally safe for you to fast for 8 to 12 hours before a blood test while pregnant. Your doctor can provide you with the best guidance for your situation.

If you forgot to fast before your fasting blood test, it’s important to let your doctor know right away. They may ask you to postpone taking the test to another day, or to let the medical professional performing your blood draw know, so that your results can be interpreted correctly.

Eating and drinking affect the results of just a few blood tests. Fasting is often necessary before tests for glucose (blood sugar), triglycerides (part of a lipid panel), and several other tests to provide the most accurate results. Not following your doctor’s suggestions may mean you need to get retested at a later date.

After your blood is drawn, it will be sent to a lab to be evaluated. In many cases, you will hear back from your doctor about the results on the same day.

If you’re concerned about fasting before a blood test, let your doctor know. They may be able to suggest another test that could serve a similar purpose.