What are the normal, prediabetic, and diabetic fasting blood glucose ranges and the target fasting glucose values for management, including special populations?

Fasting Blood Sugar Ranges

Normal fasting blood glucose is less than 100 mg/dL (5.6 mmol/L), prediabetes is 100-125 mg/dL (5.6-6.9 mmol/L), and diabetes is diagnosed at ≥126 mg/dL (7.0 mmol/L) on two separate occasions. 1, 2

Diagnostic Categories

Normal Range

• Fasting glucose <100 mg/dL (5.6 mmol/L) is considered normal for adults without diabetes 1, 3
• The optimal range for healthy individuals is 70-99 mg/dL (3.9-5.5 mmol/L) 3

Prediabetes (Increased Risk for Diabetes)

• Fasting glucose 100-125 mg/dL (5.6-6.9 mmol/L) defines impaired fasting glucose (IFG) 1, 2, 4
• Alternative prediabetes criteria include 2-hour glucose of 140-199 mg/dL during oral glucose tolerance test or HbA1c of 5.7-6.4% 1, 4
• Approximately 10% of people with prediabetes progress to diabetes annually 4

Diabetes

• Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) after at least 8 hours of fasting confirms diabetes 1, 2, 5
• This threshold must be confirmed by repeat testing on a separate day unless the patient has unequivocal hyperglycemia with classic symptoms 1, 5
• Alternative diagnostic criteria include HbA1c ≥6.5%, 2-hour glucose ≥200 mg/dL during OGTT, or random glucose ≥200 mg/dL with symptoms 1, 2

Management Targets for Established Diabetes

General Adult Population

• Target fasting/preprandial glucose: 80-130 mg/dL (4.4-7.2 mmol/L) for most non-pregnant adults with diabetes 1, 2, 3
• Postprandial glucose should be <180 mg/dL (10.0 mmol/L) 1, 2

Older Adults (Age-Stratified Targets)

The American Diabetes Association recommends individualized targets based on health status 2:

• Healthy older adults: 80-130 mg/dL fasting 2
• Complex/intermediate health: 90-150 mg/dL fasting 2
• Very complex/poor health: 100-180 mg/dL fasting 2

Hospitalized Patients

• Target glucose: 140-180 mg/dL (7.8-10.0 mmol/L) for both critically ill and non-critically ill patients 2, 3
• Initiate insulin therapy when glucose persistently exceeds 180 mg/dL 2

Gestational Diabetes

• Fasting plasma glucose <95 mg/dL (<5.3 mmol/L) 1
• 1-hour postprandial <140 mg/dL (<7.8 mmol/L) or 2-hour postprandial <120 mg/dL (<6.7 mmol/L) 1

Critical Pitfalls and Caveats

Biological Variability

• Day-to-day fasting glucose variability ranges from 4.8% to 6.1% within the same person 3, 6
• For a true fasting glucose of 100 mg/dL, normal biological variation means values could range from 87-113 mg/dL on different days 3
• Recent CGM data shows mean intraperson standard deviation of 7.52 mg/dL, meaning 40% of individuals with initially normal fasting glucose would be reclassified as prediabetic on subsequent measurements 6

Confirmation Requirements

• Always confirm abnormal results with repeat testing on a different day unless unequivocal hyperglycemia is present 1, 5
• When a test above the diagnostic threshold is repeated, the second value may fall below the cutpoint due to biological variation, laboratory error, or other factors 5
• If repeat testing is discordant, follow the patient closely and retest in 3-6 months 1, 5

Testing Conditions

• Ensure proper 8-hour fasting before the test to avoid inaccurate results 2, 3
• Use laboratory-based testing, not point-of-care glucose meters, for diagnosis 1, 3
• Factors affecting fasting glucose include stress, illness, medications (especially glucocorticoids), and time of day 3

Hypoglycemia Thresholds

• Glucose <70 mg/dL (3.9 mmol/L) requires immediate treatment with 15-20g fast-acting carbohydrate 2
• Glucose <54 mg/dL (3.0 mmol/L) represents clinically significant hypoglycemia with neuroglycopenic symptoms requiring urgent intervention 2