What is fasting blood sugar (FBS) and how is it used?

Fasting blood sugar (FBS) is the level of glucose in your blood after at least 8 hours of fasting, or not having anything to eat or drink (except water).
Normal FBS is less than 100 mg/dL, prediabetes ranges from 100 to 125 mg/dL, and diabetes starts at 126 mg/dL or higher.
FBS can be used with HbA1C, an average of blood glucose over 2-3months, and oral glucose tolerance test (OGTT), to diagnose diabetes and prediabetes. Multiple tests may be needed for diagnosis, as HbA1C can indicate prediabetes even when FBS is normal, especially if blood sugar levels spike after meals.
Diagnosis Criteria for FBS & HbA1C*
Less than 100mg/dL
≥ 100 mg/dL
≥ 126 mg/dL
Less than 5.7%
5.7 – 6.4%
≥ 6.5%
*Diabetes may be diagnosed based on A1C criteria or plasma glucose criteria, either the fasting plasma glucose (FPG) value, 2-h glucose (2-h PG) value during a 75-g oral glucose tolerance test (OGTT), or random glucose value accompanied by classic hyperglycemic symptoms (e.g. extreme thirst, extreme hunger, and unexplained weight loss) or hyperglycemic crises.


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