Glucose Fasting (Fasting Plasma Glucose, FPG, Fasting Blood Glucose, FBG)

The fasting plasma glucose (FPG) test, also known as the fasting blood glucose test (FBG) or fasting blood sugar test, measures the levels of glucose (sugar) in the blood. Used to screen for diabetes, it is a relatively simple, accurate, and inexpensive test that exposes problems with insulin functioning. Too much or too little glucose in the blood can be a sign of a serious medical condition. High blood glucose levels (hyperglycemia) may be a sign of diabetes, a disorder that can cause heart disease, blindness, kidney failure, and other complications.

A blood sugar level less than 140 mg/dL (7.8 mmol/L) is normal. A reading between 140 and 199 mg/dL (7.8 mmol/L and 11.0 mmol/L) indicates prediabetes. A reading of more than 200 mg/dL (11.1 mmol/L) after two hours indicates diabetes.

When your blood sugar level gets too high — known as hyperglycemia or high blood glucose — the quickest way to reduce it is to take fast-acting insulin. Exercising is another fast, effective way to lower blood sugar. In some cases, you should go to the hospital instead of handling it at home. The highest blood sugar level that's considered safe will depend on the person and whether they have diabetes, but will typically be between 160 to 240 mg/dL.

What if fasting plasma glucose is low?

Hypoglycemia needs immediate treatment. For many people, a fasting blood sugar of 70 milligrams per deciliter (mg/dL), or 3.9 millimoles per liter (mmol/L), or below should serve as an alert for hypoglycemia. But your numbers might be different. Ask your health care provider.

Special Considerations for Prediabetes and Diabetes:

If a person has been diagnosed with prediabetes or diabetes, target blood glucose levels may differ from those of healthy individuals. In cases of diabetes, doctors often set higher target ranges to prevent hypoglycemia.

For people with diabetes, fasting glucose levels upon waking are often targeted to be between 80 and 130 mg/dL (4.4–7.2 mmol/L), rather than below 100 mg/dL as for healthy individuals.

Target ranges can vary based on factors such as age, overall health, and other risk factors. A healthcare provider helps establish individualized glucose goals that are safe and realistic for patients with diabetes or prediabetes.