Fasting Before a Blood Test: Everything You Need to Know

Some blood tests will require a patient to fast beforehand. In these circumstances, your doctor will tell you not to eat or drink anything other than water in the hours before the test.

The prospect of not eating or drinking for even a short period of time may seem overwhelming, but knowing when and how long to fast before a blood test might help alleviate any anxiety.

In this blog, we will explain everything you need to know about fasting before a blood test. 

Why Is Fasting Required in Blood Tests?

Fasting before some blood tests can help ensure that all your tests come back as accurately as possible. All food and drink besides water have vitamins, minerals, fats, carbs, and proteins, which might affect blood-level readings and affect the results of your test. 

How Long to Fast Before a Blood Test?

The length of time you must fast will vary based on the test. Most tests will instruct you to have just water for the eight hours preceding the test. However, some tests may also require a 12-hour fast.

What Types of Blood Tests Require Fasting?

Not all blood tests will need you to fast beforehand. Fasting is likely required for the following blood tests:

• Blood glucose test: This test measures blood sugar levels and can help confirm or rule out diabetes.
• Gamma-glutamyltransferase test: A gamma-glutamyl transferase (GGT) test aids in diagnosing liver illness. GGT is an enzyme found in the liver that aids in its function.
• Cholesterol test: Blood cholesterol tests, also known as lipid profiles, are used to determine the amount of fat in the blood. If a person has recently eaten, the level of these fats will increase. As a result, patients are advised not to eat for 9–12 hours before the test. Different fats tested for include: 
  • high-density lipoprotein cholesterol, also known as “good” cholesterol
  • low-density lipoprotein cholesterol, also known as “bad” cholesterol
  • triglycerides
• Iron blood test: It aids in the diagnosis of iron deficiency-related health problems. This test determines the amount of iron in your blood. To achieve reliable findings, patients should avoid eating on the morning of the test.

If your doctor has suggested a new blood test for you and does not specify whether or not you should fast or for how long, ask them if fasting is necessary. Some tests, such as a fecal occult blood test, may not need fasting but do restrict specific meals. When preparing for a test, always follow your doctor's instructions.

What Happens If You Don’t Fast Before a Blood Test?

If you do not fast before a required test, the findings may be inaccurate. This can lead to wrong diagnosis and health concerns. 

What If You Accidentally Eat or Drink During a Fast for a Blood Test?

If you forget and accidentally eat or drink anything, contact your doctor or lab to see if the test may still be performed. They can then notify you if your test has to be rescheduled.

Things to Avoid During Fasting Before Blood Test

When fasting for a blood test, you should avoid the following things:

• Alcohol: Alcohol can influence blood sugar and fat levels, causing blood tests that need fasting to produce erroneous findings.

• Smoking: Smoking can also alter blood test results, so avoid smoking.
• Chewing gum: Even if it does not include sugar, chewing gum should be avoided since it can speed up digestion, which can alter results.
• Exercise: Most exercises speed up digestion and have an impact on the outcomes of your blood tests. Therefore, you should avoid exercising during the required fasting period.

Final Takeaway

Before a blood test, speak with your doctor to see if fasting is necessary. If the test requires fasting, consult your doctor about how long you should fast or what specific foods you should avoid. Follow all the instructions from your doctor.

If fasting becomes too difficult and you break the fast due to acute hunger or thirst, you should contact your doctor to reschedule the blood test appointment.

If blood test findings are faulty, you may obtain an incorrect diagnosis. This is why it is critical to follow best practices about fasting before blood testing.

If you need a blood test, you can walk into our urgent care center in Oceanside, CA, and we will be happy to help you. With our on-site on-site lab service, we can test and diagnose most health conditions in minimal time. 8-2-8 Urgent Care doctors will also let you know everything you need to know about fasting before a blood test.