Prediabetes Diagnostic Cut-off Values
Prediabetes is diagnosed when fasting plasma glucose is 100-125 mg/dL (5.6-6.9 mmol/L) or 2-hour plasma glucose during a 75-g oral glucose tolerance test is 140-199 mg/dL (7.8-11.0 mmol/L), or hemoglobin A1C is 5.7-6.4%. 1
Specific Diagnostic Thresholds
Fasting Plasma Glucose (FPG)
- Impaired fasting glucose (IFG): 100-125 mg/dL (5.6-6.9 mmol/L) 1, 2
- The World Health Organization uses a higher lower threshold of 110 mg/dL (6.1 mmol/L), though the American Diabetes Association recommends 100 mg/dL 1
- Requires at least 8 hours of no caloric intake before testing 1
2-Hour Plasma Glucose During 75-g OGTT
- Impaired glucose tolerance (IGT): 140-199 mg/dL (7.8-11.0 mmol/L) 1, 2
- Patients must consume at least 150 grams of carbohydrates daily for 3 days prior to testing to avoid falsely elevated results 1, 3
Hemoglobin A1C
- Prediabetes range: 5.7-6.4% (39-47 mmol/mol) 1, 2
- Must be performed using an NGSP-certified laboratory method standardized to the DCCT assay 1, 2
- A1C between 5.5-6.0% carries a 5-year diabetes incidence of 9-25%, while 6.0-6.5% carries a 25-50% risk 1
Important Clinical Considerations
Test Selection
All three tests (FPG, 2-hour OGTT, and A1C) are equally appropriate for diagnosing prediabetes 1, 2. However, practical considerations differ:
- A1C advantages: No fasting required, greater preanalytical stability, more convenient for patients 2
- FPG advantages: Inexpensive, widely available, easier to perform than OGTT 1
- OGTT advantages: May identify individuals missed by FPG or A1C alone, particularly those with isolated postprandial hyperglycemia 4
When to Avoid A1C Testing
Use only plasma glucose criteria in conditions that alter the relationship between A1C and glycemia 1, 2:
- Hemoglobin variants (sickle cell trait, G6PD deficiency) 1
- Pregnancy (second and third trimesters and postpartum) 1
- Recent blood loss or transfusion 1
- Hemodialysis 1
- Erythropoietin therapy 1
Risk Stratification Within Prediabetes Range
The risk of progression to diabetes is continuous across the prediabetes spectrum, becoming disproportionately greater at the higher end 1:
- A1C 5.5-6.0%: 9-25% 5-year diabetes risk 1
- A1C 6.0-6.5%: 25-50% 5-year diabetes risk, with 20-fold higher relative risk compared to A1C 5.0% 1
- Combined IFG and IGT: 38% or higher 5-year diabetes risk 1
Common Pitfalls to Avoid
Sample Handling for Glucose Testing
Glucose concentrations will be falsely low if samples are not processed promptly or stored properly due to glycolysis 1. Plasma should be separated immediately or samples kept on ice 2.
Carbohydrate Restriction Before OGTT
Fasting and carbohydrate restriction can falsely elevate glucose levels during OGTT 1, 3. Ensure patients consume at least 150 grams of carbohydrates daily for 3 days before testing 1.
Acute Conditions Affecting Results
Recent physical activity, illness, or acute stress can affect glucose concentrations 1. Consider repeating tests when these factors are present.
Confirmation Requirements
If test results are near diagnostic margins, repeat testing in 3-6 months is recommended 1. Two abnormal test results (either the same test repeated or two different tests) are required for diagnosis in the absence of unequivocal hyperglycemia 1.