A Fasting Blood Glucose test (also called fasting glucose or fasting blood sugar) is a simple blood test that measures the amount of glucose (sugar) in your bloodstream after you have not eaten or had caloric drinks for a set period—usually at least 8 hours. Because food can temporarily raise blood sugar, testing after fasting gives a clearer picture of your body’s baseline glucose level and how well insulin is working.

Glucose is your body’s main source of energy. The hormone insulin helps move glucose from the blood into your cells. If your body doesn’t make enough insulin or doesn’t respond to it effectively (insulin resistance), glucose can build up in the blood.

In plain language, results generally fall into these ranges (your lab may use slightly different cutoffs):

• Normal: about 70–99 mg/dL (3.9–5.5 mmol/L). This suggests your fasting blood sugar is in a healthy range.
• Prediabetes (impaired fasting glucose): about 100–125 mg/dL (5.6–6.9 mmol/L). This means blood sugar is higher than normal and signals increased risk of developing type 2 diabetes.
• Diabetes: typically 126 mg/dL or higher (≥7.0 mmol/L) on two separate tests, or along with classic symptoms. This suggests diabetes may be present and needs follow-up with your clinician.

A single fasting glucose result is one important data point. Your clinician may interpret it alongside other tests such as HbA1c (average blood sugar over ~3 months) or an oral glucose tolerance test, as well as your symptoms, medications, and overall health.

Doctors commonly order a fasting blood glucose test to screen for diabetes, identify prediabetes, or monitor known blood sugar problems. It may be recommended during routine health checks, especially if you have risk factors for abnormal blood sugar.

This test is often done if you have symptoms that could suggest high blood sugar, such as:

• Increased thirst and frequent urination
• Unexplained weight loss or increased hunger
• Fatigue, blurred vision, or slow-healing cuts/sores
• Frequent infections (such as yeast infections or skin infections)

It may also be ordered if you have risk factors linked to insulin resistance or type 2 diabetes, including:

• Overweight or obesity, especially increased waist circumference
• Family history of type 2 diabetes
• High blood pressure, abnormal cholesterol/triglycerides, or cardiovascular disease
• History of gestational diabetes or having delivered a large baby
• Polycystic ovary syndrome (PCOS)
• Physical inactivity
• Older age (risk increases over time)
• Use of certain medications that can raise blood sugar (your clinician will advise), such as some steroids

Because the result depends on fasting, follow the lab’s instructions carefully. In general, you should avoid food and caloric drinks for 8–12 hours beforehand (water is usually allowed). Tell your clinician about any medications, supplements, illness, stress, or recent changes in diet that could affect your blood sugar.

• Prediabetes (impaired fasting glucose)
• Type 2 diabetes mellitus
• Type 1 diabetes mellitus (can be detected, though other tests are often needed)
• Gestational diabetes (screening/monitoring during pregnancy, often with additional testing)
• Metabolic syndrome
• Insulin resistance
• Polycystic ovary syndrome (PCOS)
• Hypoglycemia (low blood sugar), when fasting glucose is unusually low and symptoms are present

• Diabetes screening and early detection of rising blood sugar
• Monitoring blood sugar control and response to lifestyle changes (diet, weight loss, physical activity)
• Reducing long-term risk of complications (heart disease, kidney disease, nerve damage, eye problems)
• Cardiometabolic health monitoring (alongside cholesterol, blood pressure, and waist/weight trends)
• Preventive care and general wellness checkups, especially for people with diabetes risk factors
• Supporting healthy pregnancy planning and prenatal monitoring when blood sugar risk is a concern