Fasting Blood Glucose (FBS)
Fasting blood glucose is measured after an overnight fast of 8–12 hours (water is allowed). It provides a baseline blood sugar reading without the influence of recent meals. It is one of the primary tests used to diagnose diabetes and prediabetes, along with HbA1c and the oral glucose tolerance test (OGTT).
Reference Ranges
Male
70 – 100
mg/dL
Female
70 – 100
mg/dL
Child
70 – 100
mg/dL
Ages 2–18
Reference ranges may vary by laboratory. Always compare with the range printed on your lab report.
What Low Levels Mean
Fasting glucose below 70 mg/dL indicates hypoglycemia. In non-diabetics, this may suggest reactive hypoglycemia, adrenal insufficiency, or liver disease. In diabetics, it often results from excess medication or skipped meals.
What High Levels Mean
Fasting glucose between 100–125 mg/dL indicates prediabetes (impaired fasting glucose). Values of 126 mg/dL or above on two separate tests confirm diabetes. Early detection of prediabetes allows lifestyle interventions that can prevent or delay diabetes.
Frequently Asked Questions
What is the difference between prediabetes and diabetes?
Prediabetes (fasting glucose 100–125 mg/dL) means your blood sugar is higher than normal but not yet diabetic. With lifestyle changes — diet, exercise, weight loss — many people can reverse prediabetes. Diabetes (≥126 mg/dL) requires more intensive management.
Can I drink water before a fasting glucose test?
Yes, plain water is allowed and even encouraged to prevent dehydration. Avoid tea, coffee, juice, or any caloric beverages as they can affect the result.
How often should I check fasting glucose?
If you are over 45, overweight, or have risk factors (family history, PCOS, gestational diabetes), check annually. If you have prediabetes, test every 6–12 months. People with diabetes test as frequently as their doctor recommends.
This information is for educational purposes only and should not replace professional medical advice. Always consult your doctor for interpretation of your test results.
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