5 Things That Can Affect a Fasting Blood Sugar Test

Checking your blood sugar level is an important aspect of monitoring your health, especially if you have diabetes. 

One of the most common types of blood sugar tests is fasting blood sugar, which can provide a lot of insight into your blood sugar trends. 

Keep reading to learn more about preparation for the fasting blood sugar test and five things that can affect its results.

What is a fasting blood sugar test?

A fasting blood sugar test measures the amount of glucose (sugar) in your blood after you’ve been fasting for at least 8-12 hours. Fasting means you haven’t had anything to eat or drink besides water. A fasting blood sugar test is helpful because it can tell you if you have high blood sugar regardless of what you had to eat recently.

Healthcare providers can use a fasting blood sugar test to diagnose prediabetes, type 1 diabetes, and type 2 diabetes. The preferred test to diagnose gestational diabetes is a glucose tolerance test, but a fasting blood sugar test may also be ordered to diagnose gestational diabetes. When fasting blood sugar is high, it’s called impaired glucose tolerance.

A fasting blood sugar test is also useful to determine how well your diabetes care plan is working and if you need any medication adjustments.

A normal fasting blood sugar test result is below 100 mg/dL. If you have prediabetes, your fasting blood sugar will be between 100-125 mg/dL. Fasting glucose levels of 126 mg/dL and higher are indicative of diabetes. 

If you have existing diabetes, The American Diabetes Association recommends a fasting blood sugar range between 80-130 mg/dL.

5 things that can affect the results of a fasting blood sugar test

1. Steroid medications

Corticosteroids, most commonly referred to as steroids, are a class of medications used to treat a variety of different health problems, including skin rashes, asthma, COPD (chronic obstructive pulmonary disease), arthritis, and other inflammatory conditions.

You can apply steroids topically, such as hydrocortisone cream, inhale them or take them by nasal spray for respiratory conditions, take them orally or have them injected into joints, to name a few examples.

Steroids mimic cortisol, which is a stress hormone. Cortisol raises your blood sugar and blood pressure. 

Steroids can make your body more insulin resistant over time, meaning your cells don’t respond to the hormone insulin the way they should. As a result, insulin resistance can occur, which raises blood glucose levels.

2. Exercise

If you exercise before your blood sugar test, it can impact your fasting blood sugar level. Your blood glucose level rises shortly after exercising and then lowers it afterward. 

Depending on how soon you exercised before your fasting blood sugar test, your blood sugar result may not reflect your true fasting blood sugar level.

Try to avoid exercising the same morning of your fasting blood sugar test to help keep your results as accurate as possible. Exercising the day before shouldn’t impact your test results, and you can of course exercise after your test.

Exercising without being able to eat may result in hypoglycemia (low blood sugar), which is another good reason to avoid exercising before you head to the lab for your blood draw.

3. Illness

If you’re fighting an infection or another illness, your blood sugar levels might be higher than normal. Illness is a form of stress on your body, and stress raises blood sugar

Having your fasting blood sugar tested when you have a mild illness like a cold likely won’t impact your results too much. However, if you have a more serious illness like an infection or the flu, you should reschedule your blood test for a later time, especially since you don’t want to spread your illness to others.

4. Lack of sleep

Sleep deprivation can cause insulin resistance. When your body is insulin resistant, it means your fasting blood sugar can be higher. 

A night or two of poor sleep likely won’t impact your fasting blood sugar levels. However, if you’re chronically sleep-deprived, you’re at greater risk of developing type 2 diabetes and having a higher fasting blood glucose level.

5. Drinking alcohol

If you drank a large amount of alcohol the night before your fasting blood sugar test, it might be lower than normal. When your liver metabolizes alcohol it doesn’t make glucose like it normally does, which results in lower blood sugar levels.

Drinking alcohol before the test could potentially result in a falsely low result and delay the diagnosis of prediabetes or diabetes, so it’s best to abstain from heavy alcohol use before undergoing a fasting blood glucose test.

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How to prepare for a fasting blood sugar test

Preparation for a fasting blood sugar test at your healthcare provider’s office

If you’re going to have a fasting blood sugar test at your healthcare provider’s office:

  • Step 1: Refrain from eating or drinking anything besides water for 12 hours before your fasting blood glucose test. You can usually take your medications as normal, but check with your healthcare provider to be sure.
  • Step 2: Avoid exercising in the hours before your blood sugar test, if possible.
  • Step 3: Show up to your blood draw appointment and follow their instructions for collecting the blood sample. It will be done through a venous draw (collecting a vial of blood from a vein in your arm) or a finger stick test depending on the facility. 

Preparation for a fasting blood sugar test at home

If you’re going to do fasting glucose testing at home with a glucometer, here are the steps to prepare and perform the test:

  • Step 1: Refrain from eating or drinking anything besides water for 12 hours before your fasting blood glucose test.
  • Step 2: Wash your hands with soap and water and dry thoroughly. Checking your blood sugar using dirty hands can impact the accuracy of your reading. 
  • Step 3: Load one lancet into the lancing device. Only one lancet should be used for each blood sugar test. Reusing lancets multiple times can dull the end of the needle and make finger pricks more painful.
  • Step 4: Insert a test strip into the glucometer. This often turns the glucometer on, but if it doesn’t, turn the machine on.
  • Step 5: Use the lancing device to prick the side of one of your fingers. Avoid using the center of the pad of your finger which can cause pain when doing things like typing later on. Your fingers are the most accurate testing site, but alternative testing sites can be used.
  • Step 6: Gently apply pressure to the lanced finger until a small bead of blood appears. Apply the bead of blood to the end of the test strip.
  • Step 7: Wait while the glucometer obtains a reading, which usually takes less than 10 seconds. If an error message occurs, consult the user manual for the glucometer.
  • Step 8: Record the reading in a blood glucose log if desired. However, many glucometers store results that you can go back through later.


Monitoring your fasting blood glucose levels is important for people with and without diabetes to keep tabs on their blood sugar health and diabetes risk. 

You should ideally have a fasting blood sugar test performed at least annually to screen for diabetes and as often as your healthcare provider recommends if you have existing diabetes.

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  1. Grandner MA, Seixas A, Shetty S, Shenoy S. Sleep Duration and Diabetes Risk: Population Trends and Potential Mechanisms. Curr Diab Rep. 2016. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5070477/


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