What are the diagnostic fasting plasma glucose, oral glucose tolerance test, hemoglobin A1c, and random plasma glucose thresholds that define prediabetes and diabetes?

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Last updated: September 22, 2026 • View editorial policy

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Diagnostic Thresholds for Prediabetes and Diabetes

Based on the 2025 American Diabetes Association Standards of Care, prediabetes is defined as A1C 5.7-6.4%, fasting plasma glucose 100-125 mg/dL, or 2-hour OGTT 140-199 mg/dL; diabetes is diagnosed when A1C ≥6.5%, fasting plasma glucose ≥126 mg/dL, 2-hour OGTT ≥200 mg/dL, or random glucose ≥200 mg/dL with hyperglycemic symptoms. 1

Prediabetes Diagnostic Criteria

The following thresholds define prediabetes in nonpregnant adults 1:

  • A1C: 5.7-6.4% (39-47 mmol/mol)
  • Fasting Plasma Glucose (FPG): 100-125 mg/dL (5.6-6.9 mmol/L) — termed impaired fasting glucose (IFG)
  • 2-Hour Post-OGTT Glucose: 140-199 mg/dL (7.8-11.0 mmol/L) — termed impaired glucose tolerance (IGT)

Important caveat: Risk is continuous across these ranges, becoming disproportionately greater at the higher end. A person with A1C 6.3% carries substantially more risk than someone at 5.7% 1.

Diabetes Diagnostic Criteria

Diabetes is diagnosed when any of the following criteria are met 1:

  • A1C ≥6.5% (≥48 mmol/mol) using NGSP-certified laboratory method
  • Fasting Plasma Glucose ≥126 mg/dL (≥7.0 mmol/L) after at least 8 hours without caloric intake
  • 2-Hour OGTT ≥200 mg/dL (≥11.1 mmol/L) using 75g glucose load
  • Random Plasma Glucose ≥200 mg/dL (≥11.1 mmol/L) in individuals with classic hyperglycemic symptoms (polyuria, polydipsia, unexplained weight loss) or hyperglycemic crisis

Critical requirement: In the absence of unequivocal hyperglycemia (such as hyperglycemic crisis), diagnosis requires confirmatory testing—either two abnormal results from different tests obtained simultaneously (e.g., A1C and FPG) or the same test repeated at two different time points 1.

Practical Considerations for Test Selection

The 2-hour OGTT identifies more people with prediabetes and diabetes compared to FPG or A1C alone 1. This matters clinically because interventions proven to prevent progression to diabetes (lifestyle modification, metformin) were primarily studied in individuals with impaired glucose tolerance, not isolated impaired fasting glucose or A1C-defined prediabetes 1.

When to Use Plasma Glucose Instead of A1C

Use plasma glucose criteria (not A1C) for diagnosis in these conditions 2:

  • Hemoglobin variants (e.g., sickle cell trait)
  • Pregnancy (second/third trimesters and postpartum)
  • Glucose-6-phosphate dehydrogenase deficiency
  • HIV infection
  • Hemodialysis
  • Recent blood loss or transfusion
  • Erythropoietin therapy

Common pitfall: The X-linked G6PD G202A variant, carried by 11% of African Americans, decreases A1C by approximately 0.8% in homozygous men and 0.7% in homozygous women, potentially masking diabetes 2.

Test Concordance Issues

The three diagnostic tests reflect different aspects of glucose metabolism and identify partially overlapping populations 1. FPG shows the lowest diagnostic performance for detecting both prediabetes and diabetes compared to 2-hour OGTT and A1C 3. When substantial discordance exists between glucose and A1C results, investigate the underlying cause—it may have clinical implications for that individual 2.

No Role for Random Glucose in Prediabetes

There is no random plasma glucose threshold for diagnosing prediabetes—random glucose ≥200 mg/dL is used only for diabetes diagnosis and only when accompanied by classic hyperglycemic symptoms or crisis 1.

Clinical Algorithm

For screening and diagnosis:

  1. Initial testing: Use A1C, FPG, or 2-hour OGTT based on availability and patient factors
  2. If results near diagnostic margins: Educate about hyperglycemic symptoms and retest in 3-6 months 2
  3. If prediabetes identified: Consider 2-hour OGTT if not already performed, as IGT identifies those most likely to benefit from prevention interventions 1
  4. If conditions affecting A1C present: Use plasma glucose criteria exclusively 2
  5. For confirmation: Obtain second abnormal test result (same or different test) unless unequivocal hyperglycemia present 1

References

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