Normal Fasting Blood Glucose for Adults
Normal fasting blood glucose for an adult is less than 100 mg/dL (5.6 mmol/L). 1, 2
Defining Normal Range
- A fasting glucose level below 100 mg/dL (5.6 mmol/L) is considered normal in adults according to established diagnostic criteria. 1, 2
- The American Diabetes Association uses this threshold to distinguish normal glucose metabolism from impaired fasting glucose. 2, 3
- After meals, blood glucose in healthy adults typically does not exceed 140 mg/dL two hours after eating, demonstrating effective insulin response. 3
Categories of Abnormal Fasting Glucose
Impaired Fasting Glucose (Prediabetes):
- Fasting glucose levels between 100-125 mg/dL (5.6-6.9 mmol/L) indicate impaired fasting glucose, representing an intermediate stage before diabetes. 1, 2, 4
- This range places individuals at significantly higher risk of progression to diabetes and warrants lifestyle intervention. 1
Diabetes Diagnosis:
- A fasting glucose level ≥126 mg/dL (7.0 mmol/L) on two separate occasions is diagnostic of diabetes mellitus. 1, 2
- This threshold was specifically chosen because it identifies populations at substantially increased risk for microvascular complications like retinopathy. 1
Clinical Context and Risk Stratification
Optimal Range for Cardiovascular Risk:
- Research indicates the lowest cardiovascular risk occurs with fasting glucose levels between 85-99 mg/dL, following a J-shaped curve. 5
- Even within the "normal" range, fasting glucose levels of 95-99 mg/dL show increased cardiovascular disease risk compared to levels <80 mg/dL. 6
- Fasting glucose levels ≥87 mg/dL demonstrate progressively increased risk of developing type 2 diabetes compared to lower levels, even though technically "normal." 7
Critical Thresholds to Remember:
- Hypoglycemia requiring immediate attention: <70 mg/dL (3.9 mmol/L). 1, 2
- Normal fasting: <100 mg/dL (5.6 mmol/L). 1, 2
- Impaired fasting glucose: 100-125 mg/dL (5.6-6.9 mmol/L). 1, 2
- Diabetes threshold: ≥126 mg/dL (7.0 mmol/L). 1, 2
Important Clinical Pitfalls
Unit Conversion Errors:
- When reviewing international literature, always verify whether glucose is reported in mg/dL (North America) or mmol/L (most other countries). 2
- To convert mg/dL to mmol/L, divide by 18; confusing this by multiplying instead is a common error. 2
Context-Dependent Interpretation:
- Random (non-fasting) glucose values require different interpretation and should be correlated with time since last meal. 1
- Stress hyperglycemia in hospitalized patients may cause transient elevations that do not reflect baseline glucose metabolism. 1
- Different sample types (plasma versus whole blood) may have different reference ranges even when using the same units. 2