Fasting Glucose (FBS): What Your Results Mean

Fasting glucose measures blood sugar after not eating for at least 8 hours. A result of 70-99 mg/dL is typically considered normal, 100-125 mg/dL may suggest prediabetes, and 126 mg/dL or higher may suggest diabetes, according to the ADA. Always discuss results with a healthcare provider.

Glucose is the main sugar circulating in the blood and the body's primary source of energy. It comes mainly from carbohydrates in food and from the liver, which releases stored glucose between meals. After eating, the pancreas releases insulin, a hormone that helps cells take in glucose for energy or storage. A fasting glucose test measures how much glucose is in the blood after a period without food, usually at least 8 hours, so results are not affected by a recent meal. This gives a clearer picture of how the body manages blood sugar over time compared with a random glucose check taken at any time of day.

Fasting glucose is one of the most common blood tests because it helps evaluate how the body regulates blood sugar. It is included in both the basic metabolic panel (BMP) and comprehensive metabolic panel (CMP), which are often part of routine checkups. Clinicians may order it to screen for diabetes or prediabetes, to check on general metabolic health, or to monitor someone with a known blood sugar condition. It can also be checked before surgery or as part of an evaluation for symptoms like unusual thirst, frequent urination, blurred vision, or unexplained fatigue.

Fasting glucose is reported in mg/dL in the United States and in mmol/L in many other countries. Reference ranges for adults are shown below and can vary slightly between laboratories depending on the testing method used. A healthcare provider interprets the result alongside your history and other findings, not as a stand-alone number.

A fasting glucose result above the typical range may be described as impaired fasting glucose or prediabetes when it falls between 100 and 125 mg/dL, and as consistent with diabetes when it is 126 mg/dL or higher, according to the ADA. These categories are used for screening and are not a diagnosis on their own; a healthcare provider typically confirms results with a repeat test or Hemoglobin A1c.

Several factors besides diabetes can raise fasting glucose. Common ones include recent stress, illness, or infection, which can temporarily raise blood sugar through stress hormones. Certain medications, such as steroids or some blood pressure or psychiatric medications, may also increase glucose. Not truly fasting before the test, drinking sugary beverages, or eating close to the draw time can affect results. Conditions like Cushing syndrome, pancreatitis, or hormonal disorders can also play a role. Family history, excess body weight, limited physical activity, and diets high in refined carbohydrates are commonly linked with higher fasting glucose over time. Because so many factors can affect this result, a single high reading is usually followed up rather than acted on alone.

A fasting glucose result below the typical range may be called hypoglycemia. Mild low blood sugar can cause shakiness, sweating, dizziness, irritability, or difficulty concentrating, though some people have few symptoms.

Common contributors to low fasting glucose include taking too much insulin or certain diabetes medications relative to food intake, skipping meals, prolonged fasting, or intense exercise without enough food afterward. Drinking alcohol, especially without eating, can also lower blood sugar. Some hormonal conditions, certain tumors that affect insulin production, and rare metabolic disorders can play a role as well. In some cases, no clear cause is found. Because very low blood sugar can be dangerous, especially if it causes confusion, seizures, or loss of consciousness, any low or unexplained fasting glucose reading should be discussed with a healthcare provider promptly rather than managed on your own.

Fasting glucose is often reviewed together with Hemoglobin A1c, which reflects average blood sugar over a longer period, and with Triglycerides, since blood sugar and fat metabolism are closely linked. Kidney markers such as Creatinine and electrolytes like Sodium may also be checked as part of the same metabolic panel, since long-term high blood sugar can affect the kidneys. Lifestyle factors such as recent meals, stress, sleep, illness, and physical activity can all shift a single fasting glucose reading. Reference ranges can differ slightly between laboratories, so it is best to compare results using the range printed on your own lab report rather than relying only on general numbers found online.

If your fasting glucose falls outside the typical range, consider discussing the result with a healthcare provider, especially if it is repeated on more than one occasion. They may recommend a follow-up test, an A1c, or additional evaluation depending on your health history and symptoms. Seek prompt medical care if you experience symptoms of very low or very high blood sugar, such as confusion, fainting, severe thirst, or rapid breathing. You can upload your lab report to MediWing for a plain-language explanation of your results and to track how your fasting glucose changes over time.