Why 140 mg/dL is the Cut-off for 2-Hour Postprandial Glucose
The 140 mg/dL threshold for 2-hour postprandial glucose represents the evidence-based boundary between normal glucose tolerance and impaired glucose tolerance (prediabetes), established by identifying the approximate glucose level above which there is substantially increased risk for microvascular complications, particularly retinopathy, and cardiovascular disease. 1
The Scientific Rationale: Microvascular Risk Threshold
The Expert Committee on the Diagnosis and Classification of Diabetes established this cut-off based on identifying an approximate threshold that separates individuals at substantially increased risk for adverse diabetes outcomes from those who are not. 1
Key evidence supporting the 140 mg/dL threshold:
Population studies in Pima Indians, Egyptians, and NHANES III participants demonstrated that retinopathy prevalence increases significantly at 2-hour postload glucose levels above 140 mg/dL (7.8 mmol/L). 1
The diagnostic criteria were developed by examining the distribution of plasma glucose concentrations across populations and identifying where microvascular complications, especially retinopathy, begin to increase substantially. 1
This threshold represents a balance between the medical, social, and economic costs of diagnosing someone not truly at substantial risk versus failing to diagnose someone who is at risk. 1
The Diagnostic Framework
The 140 mg/dL cut-off creates three clinically meaningful categories: 1
- 2-hour postload glucose <140 mg/dL (7.8 mmol/L) = Normal glucose tolerance
- 2-hour postload glucose 140-199 mg/dL (7.8-11.0 mmol/L) = Impaired glucose tolerance (IGT/prediabetes)
- 2-hour postload glucose ≥200 mg/dL (11.1 mmol/L) = Provisional diagnosis of diabetes
This classification system allows clinicians to identify not just diabetes, but also the intermediate state of impaired glucose tolerance, which carries independent cardiovascular risk. 2, 3
Cardiovascular Disease Risk Considerations
Beyond microvascular complications, the 140 mg/dL threshold has significance for cardiovascular disease risk:
Isolated postprandial hyperglycemia (2-hour glucose >140 mg/dL) in the face of normal fasting glucose and normal HbA1c is associated with a 2-fold increased risk of death from cardiovascular disease. 4
Meta-analyses demonstrate that both fasting and 2-hour postload glucose are significantly associated with future cardiovascular events in a continuous graded fashion, beginning at levels consistent with impaired glucose tolerance. 1
Subjects with impaired glucose tolerance (2-hour glucose 140-199 mg/dL) have a 3-fold higher risk of carotid stenosis compared to those with normal glucose tolerance, even after adjusting for confounders. 5
Clinical Implications and Monitoring
The 140 mg/dL threshold serves different purposes depending on the clinical context:
For Diagnosis (OGTT):
- Values <140 mg/dL indicate normal glucose tolerance 1
- Values 140-199 mg/dL warrant lifestyle intervention and monitoring for progression to diabetes 2, 3
- Values ≥200 mg/dL require confirmation and diabetes management 1
For Diabetes Management:
- The American Diabetes Association recommends postprandial glucose targets <180 mg/dL for most nonpregnant adults with diabetes, which is more lenient than the diagnostic threshold because it balances glycemic control with hypoglycemia risk. 6, 7
For Pregnancy:
- Stricter targets apply: 1-hour postprandial <140 mg/dL and 2-hour postprandial <120 mg/dL for gestational diabetes, reflecting the heightened sensitivity to hyperglycemia during fetal development. 1, 8
Important Caveats
The 140 mg/dL cut-off has limitations that clinicians should recognize:
Plasma glucose exists on a continuum, and the threshold represents an approximation rather than an absolute dividing line between health and disease. 1
The 2-hour OGTT identifies more people with impaired glucose homeostasis than fasting glucose alone, which is why it's essential to document which test was used. 1
Time to peak glucose varies substantially (45-120 minutes) even in normal individuals, so the standardized 2-hour measurement may not capture the true peak in all patients. 6
For patients with HbA1c near 7%, postprandial hyperglycemia contributes more to overall glycemic burden than fasting hyperglycemia, making the postprandial threshold particularly relevant. 9, 5
The 140 mg/dL threshold represents decades of epidemiological research correlating glucose levels with clinical outcomes, particularly microvascular disease, and provides a practical, evidence-based cut-point for identifying individuals who would benefit from intervention to prevent progression to diabetes and reduce cardiovascular risk.