What Are Fasting Blood Tests? Preparation Tips

Published October 6, 2022
Doctor doing a blood test on a patient

Certain blood tests require you to fast beforehand. Simply put, this means don't eat or drink anything except water. Typically, you'll need to fast for 8-12 hours, but follow the instructions you are given on how long to fast before your test. If you forget and eat that day, reschedule it to ensure the best results.

Fasting isn't fun, but it's important to get accurate results for certain blood tests. What you eat and drink gets absorbed into your bloodstream and can skew blood test results. This means juice, coffee, soda and other drinks are off limits. Rest assured, you can make the whole experience more comfortable with just a little bit of planning.

What to Consume or Avoid Before Your Test

Your healthcare provider will provide guidance regarding what you can consume and what you should not consume before your test. In general, you can drink water and take medications, but avoid taking supplements.

Water

You can drink water before a fasting blood test. Drink 8-10 ounces of water beforehand since this will help you stay hydrated and can help keep fluid in your veins, making it easier to draw blood.

Medications

Typically you can take your medications before a fasting blood test, but ask your doctor to be sure. If your medications need to be taken with food, ask if you should wait to take them.

Supplements

If you take vitamins or supplements, wait until the test is over to take them. Certain supplements can interfere with fasting blood test results. Your doctor might ask you to stop taking your supplements for a few days in advance, so be sure to ask about this.

Tips to Prepare for a Fasting Blood Test

It's normal to feel anxiety about not eating and having your blood drawn, but there are ways to plan ahead and keep yourself comfortable that day.

  • Call your doctor's office if you have any questions about what you need to do to prepare.
  • Don't smoke, chew gum, or exercise before the test.
  • Give yourself enough time to get ready and drink water before leaving.
  • If you're wary of needles, bring headphones and your device to listen to music or watch a favorite show for distraction while you wait.
  • Schedule your appointment for the morning, this way most of your fasting time will be spent sleeping.
  • Wear comfortable, loose-fitting clothing so your arms are accessible.
  • You'll probably be hungry, so bring a snack and drink to have right after your test. If you need your morning coffee, have it afterwards.

Types of Fasting Blood Tests

Fasting for blood tests is required for a basic metabolic panel, blood glucose test, blood cholesterol test and others, and fasting can ensure the most accurate results. These blood tests are an important way for your doctor to assess your health or see if certain treatments are working.

Hand holding blood samples in laboratory

Basic Metabolic Panel

This test can be done as part of a regular checkup or it can be ordered for certain chronic conditions including kidney disease or high blood pressure. The basic metabolic panel typically measures certain substances in your blood that tell your doctor a lot about your health. The levels tested often include:

  • Blood glucose - this is the sugar found in your bloodstream from the foods you eat.
  • BUN (blood urea nitrogen) and creatinine - these are waste products that are removed from the bloodstream by the kidneys.
  • Calcium - this is a mineral that is important for proper functioning of your nerves, muscles and heart.
  • Sodium, potassium, carbon dioxide and chloride - these electrolytes help control your body's fluid levels and the balance of acids and bases, which control your body's pH level, important for every organ in your body.

Blood Glucose Tests

This can be part of the basic metabolic panel but it can also done separately. A blood glucose test is a way to see how your body handles blood sugar from what you eat and drink. It's often ordered as part of a regular physical exam, and it can indicate prediabetes, blood sugar levels that are elevated and linked to increased risk for diabetes, or diabetes, which is a chronic disease that affects how your body uses energy.

Blood Cholesterol Tests

This test is done to measure cholesterol, a waxy substance in your body which is important for proper functioning of all your cells. Too much cholesterol can be a sign of increased risk for heart disease. Generally, a blood cholesterol test, also called a lipid panel, will include:

  • Total cholesterol - the total amount of cholesterol in your blood including high-density lipoprotein (HDL) and low-density lipoprotein (LDL).
  • HDL - this is referred to as the "good cholesterol," which can help remove cholesterol from your arteries.
  • LDL - often called the "bad cholesterol," this is the main type of cholesterol that leads to buildup or blockage in arteries.
  • Triglycerides - this is the most common form of fat in the body, and high levels can raise your risk for heart disease.

If your blood cholesterol levels are not within a healthy range for your gender and age, your doctor might order a repeat fasting blood test to verify blood cholesterol levels. Certain medications such as birth control pills, antipsychotic and anticonvulsant meds can affect blood cholestrol levels, so be sure to discuss this with your doctor.

Some research supports the use of non-fasting blood tests for blood cholesterol levels as a way to improve the likelihood that people take advantage of regular blood cholesterol testing. When non-fasting tests are used, it's possible that limits on blood cholesterol cut-off points are raised slightly, and these abnormal levels are then flagged for referral for treatment with a lipid clinic or doctor.

You might have heard that what you eat in the days prior to a blood test will affect your blood cholesterol results, but this turns out to be an urban legend. On the other hand, if you're trying to improve blood cholesterol levels, research shows that you can start to see improvements in three months of diet and lifestyle changes.

Are Fasting Blood Tests Necessary?

Having your fasting blood tests done regularly can help you know what's happening with your health. Once you schedule your test, plan ahead to keep yourself as comfortable as possible that day. If your healthcare provider has ordered a test, they probably have a good reason for it, but you can communicate with them directly to find out why the test is needed.

But even when you know that the test is necessary, we understand that preparing for a blood tests is hard for some people. This angst might make it hard to even schedule the test. Other folks might not understand what is meant by fasting, and show up unprepared. When testing is delayed, treatments are also delayed. Talk to your doctor about your concerns, and work out a plan to keep your medical care on track. Before you know it, it will all be over.

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What Are Fasting Blood Tests? Preparation Tips