Why HbA1c of 5.8% is Below the Diagnostic Threshold for Type 2 Diabetes
An HbA1c of 5.8% falls below the diagnostic threshold of 6.5% for type 2 diabetes and instead places this patient in the prediabetes category (5.7-6.4%), which indicates increased risk for developing diabetes but does not meet criteria for a diabetes diagnosis. 1
Established Diagnostic Thresholds
The American Diabetes Association clearly defines the following HbA1c categories for diagnosis 1:
- Diabetes diagnosis: HbA1c ≥6.5% (≥48 mmol/mol)
- Prediabetes (high risk for diabetes): HbA1c 5.7-6.4% (39-46 mmol/mol)
- Normal: HbA1c <5.7%
This patient's value of 5.8% objectively falls within the prediabetes range, not the diabetic range. 1
Why This Threshold Was Established
The 6.5% cutoff for diabetes diagnosis was adopted in 2009 based on 1:
- Improved assay standardization through the National Glycohemoglobin Standardization Program (NGSP), ensuring reliable and reproducible measurements across laboratories
- Association with retinopathy risk: New data demonstrated that HbA1c concentrations ≥6.5% correlate with increased risk for diabetic retinopathy, the key microvascular complication used to define diabetes thresholds 1
- Technical advantages: HbA1c has better preanalytic stability and decreased biological variability compared to glucose testing 1
Clinical Significance of This Patient's Value
Despite this 21-year-old having obesity (BMI 33 kg/m²), which increases diabetes risk, his HbA1c of 5.8% means 1:
- He does not meet diagnostic criteria for type 2 diabetes
- He does meet criteria for prediabetes, indicating elevated risk for progression to diabetes 1
- He requires risk reduction interventions but not diabetes treatment
Prediabetes as a Continuum of Risk
The prediabetes category (5.7-6.4%) represents a continuum where higher values within this range confer progressively higher risk for future diabetes development. 1 Research demonstrates that overweight/obese adolescents with HbA1c in the prediabetes range already show impaired β-cell function relative to insulin sensitivity, a metabolic marker for heightened diabetes risk. 2
Important Caveats for This Young Patient
Given this patient's young age (21 years) and obesity, several considerations apply:
- Confirmation recommended: Guidelines suggest confirming with a repeated HbA1c test or glucose-based testing, though this applies more to diabetes diagnosis than prediabetes identification 1
- OGTT may detect additional cases: Studies show that 27% of youth with normal HbA1c and 41% with prediabetes HbA1c have dysglycemia (impaired fasting glucose or impaired glucose tolerance) by oral glucose tolerance test 2
- Population-specific considerations: Some evidence suggests optimal HbA1c cutoffs may vary by ethnicity, age, and population, though the ADA maintains universal thresholds 3, 4
Why Not Use a Lower Threshold
While some have proposed lower diagnostic thresholds, the 6.5% cutoff provides 1:
- High specificity (98.7%) for diabetes diagnosis, minimizing false positives 5
- Clear association with microvascular complications, particularly retinopathy 1
- Standardized approach that balances sensitivity and specificity across diverse populations
The prediabetes range (5.7-6.4%) was specifically created to identify individuals at high risk who require preventive interventions without inappropriately labeling them as having diabetes. 1