Normal Fasting Blood Sugar
A normal fasting blood sugar level is less than 100 mg/dL (5.6 mmol/L), with fasting defined as no caloric intake for at least 8 hours. 1
Diagnostic Thresholds for Fasting Glucose
The American Diabetes Association and American Heart Association define the following categories based on fasting plasma glucose (FPG):
- Normal: <100 mg/dL (<5.6 mmol/L) 1
- Impaired Fasting Glucose (Prediabetes): 100-125 mg/dL (5.6-6.9 mmol/L) 1
- Diabetes: ≥126 mg/dL (≥7.0 mmol/L) 1
The diagnosis of diabetes requires confirmation with repeat testing on a separate day unless the patient has unequivocal hyperglycemia with acute metabolic decompensation. 2
Important Clinical Context
The 100 mg/dL Threshold Is Somewhat Arbitrary
The difference between 99 and 100 mg/dL is clinically arbitrary, reflecting the inherent limitations of using dichotomous cut points for continuous biological variables. 3 The actual risk of progression to diabetes and cardiovascular disease increases gradually across the entire "normal" range. 3
- The World Health Organization uses 110 mg/dL as the lower threshold for impaired fasting glucose, highlighting that expert consensus varies. 3
- Fasting glucose measurements have inherent day-to-day variability of 12-15%, meaning the same person could measure 99 mg/dL one day and 110 mg/dL the next without any actual change in their metabolic state. 3
Risk Increases Even Within the "Normal" Range
Research demonstrates that higher fasting plasma glucose levels within the normoglycemic range (particularly ≥87 mg/dL) constitute an independent risk factor for future type 2 diabetes, especially when combined with elevated triglycerides or obesity. 4 However, very low fasting glucose levels (<70 mg/dL) are also associated with increased cardiovascular and all-cause mortality. 5
Practical Application
When to Screen
Testing should be considered in adults who are overweight (BMI ≥25 kg/m²) and have additional risk factors including physical inactivity, first-degree relative with diabetes, high-risk ethnicity, history of gestational diabetes, hypertension (≥140/90 mmHg), HDL cholesterol <35 mg/dL or triglycerides >250 mg/dL, polycystic ovary syndrome, or history of cardiovascular disease. 1
For those without risk factors, testing should begin at age 45 years (or age 35 per more recent guidance). 1, 3
Interpretation Pitfalls to Avoid
- Don't panic at 100 mg/dL: Confirm with repeat testing given the 12-15% day-to-day variability. 3
- Don't rely on fasting glucose alone: HbA1c (normal <5.7%) and 2-hour oral glucose tolerance testing identify different at-risk populations, and the tests have incomplete concordance. 3
- Don't skip confirmation testing: Any abnormal result requires confirmation unless the patient has unequivocal hyperglycemia with acute symptoms. 2
Follow-Up Based on Results
- If <100 mg/dL: Repeat screening at least every 3 years. 3
- If 100-125 mg/dL (prediabetes): Annual screening is recommended, and intensive lifestyle modification (targeting 5-7% weight loss and 150 minutes/week of moderate physical activity) reduces diabetes incidence. 3, 6
- If ≥126 mg/dL: Repeat testing on a separate day to confirm diabetes diagnosis. 2