Prediabetic Range: Diagnostic Laboratory Criteria
Prediabetes is diagnosed when any one of three laboratory tests shows intermediate hyperglycemia: fasting plasma glucose (FPG) 100-125 mg/dL (5.6-6.9 mmol/L), 2-hour plasma glucose during 75-g oral glucose tolerance test (OGTT) 140-199 mg/dL (7.8-11.0 mmol/L), or hemoglobin A1C 5.7-6.4% (39-47 mmol/mol). 1, 2
Diagnostic Criteria
The three equivalent tests for diagnosing prediabetes are:
- Impaired Fasting Glucose (IFG): FPG 100-125 mg/dL (5.6-6.9 mmol/L)
- Impaired Glucose Tolerance (IGT): 2-hour plasma glucose 140-199 mg/dL (7.8-11.0 mmol/L) during 75-g OGTT
- Elevated A1C: 5.7-6.4% (39-47 mmol/mol)
Any single abnormal test result from these three options establishes the diagnosis of prediabetes 1, 3, 2. You do not need multiple abnormal tests or confirmation testing for prediabetes diagnosis, unlike diabetes where confirmation is typically required.
Important Testing Considerations
For OGTT preparation: Ensure adequate carbohydrate intake of at least 150 grams per day for 3 days prior to testing to avoid false results 4.
A1C limitations: Use only plasma glucose criteria (not A1C) in conditions with increased red blood cell turnover: sickle cell disease, pregnancy (second and third trimesters), hemodialysis, recent blood loss or transfusion, or erythropoietin therapy 5. A1C may also be affected by hemoglobin variants, though most U.S. assays are unaffected by common variants 5.
Risk Stratification by Criteria Level
The risk of progression is continuous across the prediabetic range, becoming disproportionately greater at higher values 1, 2. Individuals with A1C 6.0-6.5% have substantially higher diabetes risk (25-50% over 5 years) compared to those with A1C 5.5-6.0% (9-25% over 5 years) 6, 7.
Controversy: ADA vs WHO/IEC Criteria
A critical caveat: The World Health Organization (WHO) and International Expert Committee (IEC) use more stringent criteria—IFG starting at 110 mg/dL (6.1 mmol/L) rather than 100 mg/dL, and A1C 6.0-6.4% rather than 5.7-6.4% 8, 6, 9. Recent evidence suggests WHO/IEC criteria may better identify individuals at truly elevated cardiovascular risk 10, 11. With ADA criteria, approximately 50% of people test normal on repeat testing, one-third spontaneously revert to normal, and two-thirds never develop diabetes in their lifetime 9.
For clinical practice: Use the ADA criteria (FPG ≥100 mg/dL, A1C ≥5.7%) as these are the established U.S. standards 1, 3, 2, but recognize that individuals at the lower end of these ranges (FPG 100-109 mg/dL, A1C 5.7-5.9%) have lower absolute risk and may benefit primarily from lifestyle intervention rather than pharmacotherapy.
Who Should Be Tested
Screen adults with overweight/obesity (BMI ≥25 kg/m² or ≥23 kg/m² in Asian Americans) who have additional risk factors, or begin screening all adults at age 35 years 4, 2. If normal, repeat testing at minimum 3-year intervals 2.
High-risk groups requiring annual testing: Those already diagnosed with prediabetes, women with prior gestational diabetes, and individuals on high-risk medications (glucocorticoids, statins, thiazide diuretics, certain HIV medications, second-generation antipsychotics) 4, 2.