Fasting Blood Glucose
FBG· also: FPG, FBS, Fasting Glucose, Blood Sugar
Clinical Overview
Fasting blood glucose (FBG) measures blood sugar after at least 8 hours without food or caloric drink. It is the primary test for diagnosing diabetes mellitus, prediabetes, and hypoglycemia. Together with HbA1c, it forms the cornerstone of diabetes screening and management.
Why This Test Matters
FBG above 126 mg/dL on two separate occasions confirms diabetes mellitus. FBG of 100–125 mg/dL indicates impaired fasting glucose (prediabetes), which carries a significant risk of progression to diabetes and cardiovascular disease. Hypoglycemia (FBG <70 mg/dL) causes neurological symptoms and is a medical emergency at levels below 50 mg/dL.
Reference RangesWHO/IFCC standards
| Reference Range | Unit | Notes |
|---|---|---|
| 70 – 99 | mg/dL | Fasting normal |
| 100 – 125 | mg/dL | Prediabetes (impaired fasting glucose) |
Also reported in: mmol/L.Conversion factor: 1 mmol/L = 0.056 mg/dL.
Critical (Panic) Values
Critical Low: < 50 mg/dL. Critical High: > 500 mg/dL. Values outside these limits require immediate clinical attention.
What Causes Abnormal Results?
High FBG Causes
- Type 1 diabetes mellitus (autoimmune, absolute insulin deficiency)
- Type 2 diabetes mellitus (insulin resistance and relative deficiency)
- Gestational diabetes
- Corticosteroid-induced hyperglycemia
- Cushing's syndrome
- Pancreatitis or pancreatectomy
- Acromegaly (growth hormone excess)
Low FBG Causes
- Insulin overdose (most common cause of severe hypoglycemia)
- Oral hypoglycemic agents (sulfonylureas — glibenclamide, glipizide)
- Prolonged fasting or starvation
- Insulinoma (insulin-secreting tumor)
- Adrenal insufficiency
- Liver failure
- Reactive hypoglycemia (postprandial)
Signs & Symptoms to Watch For
How to Prepare for This Test
Fast for 8–12 hours before the test. Only water is allowed during fasting. Do not exercise vigorously before the test as exercise can lower blood glucose. Take morning diabetes medications only after the blood draw unless instructed otherwise.
Factors That Can Affect Results
- Non-fasting state (most important — eating raises blood glucose)
- Stress, illness, or surgery (causes stress hyperglycemia)
- Medications: corticosteroids (raise), insulin and sulfonylureas (lower)
- Sample processing delay (glucose decreases in unprocessed tubes due to glycolysis)
- Fluoride tube required if processing is delayed beyond 30 minutes
Related Topics
Frequently Asked Questions
What is the difference between fasting glucose and HbA1c for diabetes diagnosis?
Fasting glucose gives a snapshot of blood sugar at a single moment. HbA1c reflects the average blood glucose over the past 2–3 months (the lifespan of red blood cells). Both can diagnose diabetes: fasting glucose ≥126 mg/dL or HbA1c ≥6.5% on two separate occasions. HbA1c is more convenient (no fasting required) but can be unreliable in conditions affecting red blood cell turnover (anemia, hemolysis, kidney disease).
What fasting glucose is considered prediabetes?
Prediabetes (impaired fasting glucose) is defined as fasting glucose of 100–125 mg/dL (5.6–6.9 mmol/L) by the American Diabetes Association. Prediabetes is not a disease but a high-risk state — approximately 15–30% of people with prediabetes will develop type 2 diabetes within 5 years without lifestyle intervention. Weight loss of 5–7% body weight and increased physical activity can prevent or delay progression.