Fasting Blood Glucose

Fasting blood glucose: an introduction

Fasting blood glucose is the fastest and simplest test used to diagnose diabetes. It also plays a role in ongoing monitoring of blood glucose control if you have type 2 diabetes. The test measures the level of glucose (a type of sugar) in your blood. Having high blood glucose can be a sign that your body is not functioning normally and that you may have diabetes. If you have high blood glucose and it is not treated, it can lead to a range of dangerous health complications. However, finding out that you have elevated blood glucose is powerful information that you can use to keep yourself healthy. If you know that your blood glucose is high, you can take steps to lower it, by losing weight (if you are overweight or obese), getting regular moderate physical activity, and taking a medication that lowers blood glucose.1

Why measuring blood glucose is important in diagnosing type 2 diabetes

Our bodies require energy to function properly and we get that energy from the foods we eat. Our diet (everything we eat and drink) includes three main sources of energy (also known as calories): protein, fat, and carbohydrates (sugars, starches, and fibers).When the body digests most sources of carbohydrates, they are transformed through digestion into a very important source of instant energy, a form of sugar called glucose. Our bodies depend on the action of a number of different natural body chemicals called hormones, including insulin, amylin, incretins, and glucagon, working together in conjunction, to control how we use glucose. In type 2 diabetes, these hormones no longer work in the way they should and this results in elevated blood glucose.2-6

How is a fasting blood glucose test done?

A fasting blood glucose test requires taking a sample of blood. Before the blood sample is taken, you must be fasting (not eating or drinking) for 8 to 12 hours (typically you fast overnight and have blood drawn in the morning).7

Someone at your healthcare provider’s office or the clinic where you are having the test done will take a sample of your blood. The test no different from any other simple blood test. It is quick and there should be minimal discomfort. Normally your fasting blood glucose should be less than 100 mg/dL. If your fasting blood glucose level is 126 mg/dL or higher you may have diabetes. If your test result is in the 100 mg/dL to 125 mg/dL range, you have impaired fasting glucose and may have prediabetes. Typically, for the diagnosis to be made the test must be repeated to confirm the result.8,9

Diagnosing type 2 diabetes: fasting blood glucose test results*




Less than 100 mg/dL 100 to 125 mg/dL 126 mg/dL or higher

*For diagnosis of diabetes, any test requires confirmation using a repeat measurement, unless symptoms of diabetes are clear.

Are there other causes of high blood glucose?

There are other health conditions that can cause high blood sugar. These include overactive thyroid gland, inflammation of the pancreas (pancreatitis), pancreatic cancer, and several rare tumors.8

How accurate is a fasting blood glucose test?

Every laboratory test method used in medicine is subject to a certain level of error. The margin of error for a fasting blood glucose test is about 16 mg/dL lower or higher than the test result. For instance, if your fasting blood glucose measurement is 126 mg/dL, your actual fasting blood glucose level could be anywhere from 110 mg/dL to 142 mg/dL. The margin of error for fasting blood glucose can be even greater depending on how quickly the blood sample was processed (if the sample is not placed on ice promptly after being taken, it can cause the glucose level to decrease).10

How often should I have a fasting blood glucose test?

Fasting blood glucose testing is useful in screening and diagnosis of type 2 diabetes, as well as in management of type 2 diabetes (monitoring how well your blood glucose is being controlled).
Screening and diagnosis. The American Diabetes Association (ADA) recommends regular screening for type 2 diabetes in all adults age 45 and older who do not have prediabetes and/or other risk factors for type 2 diabetes every 3 years. Fasting blood glucose is one tool that can be used to do this screening. For individuals who are overweight (body mass index [BMI] of 25 kg/m2 or greater) and have one additional risk factors that put them at increased risk for developing type 2 diabetes, testing should be done more frequently, for instance yearly for individuals with prediabetes. The list of risk factors in addition to being overweight that warrants more frequent screening includes9:

  • Physical inactivity. Being physically inactive.
  • Family history. Having a close family history of type 2 diabetes (a first degree relative with the disease).
  • At risk ethnic group. Being in a high-risk ethnic group (eg, African American, Latino, Native American, Asian American, Pacific Islander).
  • Gestational diabetes or baby with high birth weight. Being a women who delivered a high birth weight baby (more than 9 lbs) or was diagnosed with gestational diabetes.
  • High blood pressure. Having high blood pressure (140/90 mmHg or greater) or being treated for high blood pressure.
  • Abnormal lipids. Having an high-density lipoprotein (HDL) cholesterol level of 35 mg/dL or less or having a triglyceride level of greater than 250 mg/dL.
  • Polycystic ovarian syndrome. Having polycystic ovarian syndrome.
  • Prediabetes. Having prediabetes (impaired fasting glucose or impaired glucose tolerance or A1C of 5.7% to 6.4%).
  • Cardiovascular disease. Having a history of cardiovascular disease.
  • Insulin resistance. Other health conditions associated with insulin resistance (e.g., severe obesity).

Remember, if you have prediabetes (impaired fasting glucose or impaired glucose tolerance or A1C of 5.7% to 6.4%), this is a wake-up call to make healthy life changes to decrease your risk for developing type 2 diabetes, including losing weight (if you are overweight or obese) and getting regular moderate physical activity. Your healthcare provider may also decide that you might benefit from treatment with an oral diabetes medication that has been shown to prevent or delay the development of type 2 diabetes in people with prediabetes.9

Learn more about how to lower your risk for developing type 2 diabetes, if you have prediabetes.

Fasting blood glucose testing may also play a role in your care plan if you have type 2 diabetes. If you do self-monitoring of your blood glucose, your healthcare provider may ask you to take readings of your blood glucose in the morning before you eat or your provider may check your blood glucose level in the office before you have eaten. This will give you a fasting blood glucose level. For many non pregnant adults with type 2 diabetes, the target range for fasting blood glucose is 70 mg/dL to 130 mg/dL, with specific goals tailored for the individual based on factors including your age, duration of diabetes, other health conditions, and level of awareness of hypoglycemia.9

Written by: Jonathan Simmons | Last reviewed: May 2014.
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