What fasting plasma glucose range defines pre-diabetes?

Pre-Diabetes Fasting Glucose Range

Pre-diabetes is defined by a fasting plasma glucose (FPG) of 100-125 mg/dL (5.6-6.9 mmol/L). 1, 2

Diagnostic Criteria

The American Diabetes Association establishes three methods to diagnose pre-diabetes, any one of which is sufficient 1, 2:

• Fasting plasma glucose: 100-125 mg/dL (5.6-6.9 mmol/L) - termed Impaired Fasting Glucose (IFG) 1
• 2-hour oral glucose tolerance test (75g glucose load): 140-199 mg/dL (7.8-11.0 mmol/L) - termed Impaired Glucose Tolerance (IGT) 1
• Hemoglobin A1C: 5.7-6.4% (39-47 mmol/mol) 1, 2

Important Threshold Nuances

The 100 mg/dL Cutoff Controversy

The World Health Organization uses 110 mg/dL (6.1 mmol/L) as the lower threshold for IFG, not 100 mg/dL. 1, 3 This reflects ongoing debate about where to draw the diagnostic line, as the difference between 99 and 100 mg/dL is clinically arbitrary - risk increases continuously across the entire glucose spectrum rather than at a specific cutpoint 3.

Test Variability Considerations

• Fasting glucose has 12-15% day-to-day variability, meaning the same person could measure 99 mg/dL one day and 110 mg/dL the next without actual metabolic change 3
• Confirmation with repeat testing is essential unless unequivocal hyperglycemia or classic symptoms are present 1, 3
• Plasma samples must be separated immediately or kept on ice due to poor preanalytical stability 3

Clinical Application Algorithm

When to Screen for Pre-Diabetes

Test adults with overweight/obesity (BMI ≥25 kg/m² or ≥23 kg/m² in Asian Americans) who have additional risk factors 1, 3:

• First-degree relative with diabetes 1
• High-risk ethnicity (African American, Latino, Native American, Asian American, Pacific Islander) 1
• History of cardiovascular disease 1
• Hypertension (≥140/90 mmHg or on therapy) 1
• HDL cholesterol <35 mg/dL and/or triglycerides >250 mg/dL 1
• Polycystic ovary syndrome 1
• Physical inactivity 1
• Conditions associated with insulin resistance (severe obesity, acanthosis nigricans) 1

For adults without these risk factors, begin screening at age 45 years. 1

Repeat Testing Intervals

• If pre-diabetes is identified: annual screening 1, 3
• If tests are normal: repeat at minimum 3-year intervals 1

Critical Pitfalls to Avoid

Don't Panic at Borderline Values

A single fasting glucose of 100 mg/dL requires confirmation given the substantial test variability 3. The risk at 101 mg/dL versus 99 mg/dL is not meaningfully different - both warrant lifestyle counseling and follow-up 3.

Different Tests Identify Different Populations

The three diagnostic methods (FPG, OGTT, A1C) have incomplete concordance and identify partially overlapping but distinct at-risk populations 3, 4. A patient may have normal fasting glucose but abnormal glucose tolerance on OGTT, or vice versa 3.

Evidence Base Limitations

Intensive lifestyle interventions proven effective for diabetes prevention were primarily studied in people with IGT (abnormal OGTT), not isolated IFG or A1C-defined pre-diabetes. 3 This means the evidence is strongest for intervening when the 2-hour OGTT is 140-199 mg/dL, though lifestyle modification is still recommended for all pre-diabetes categories 1.

Clinical Significance

Pre-diabetes represents increased risk for both type 2 diabetes and cardiovascular disease, not a disease entity itself. 1, 2 Approximately 10% of people with pre-diabetes progress to diabetes annually, though risk varies substantially across the 100-125 mg/dL range 3.

When pre-diabetes is identified, assess and treat other cardiovascular risk factors including hypertension, dyslipidemia, and obesity 1, 2.