Diagnosis of Type 2 Diabetes by HbA1c
An HbA1c level of 6.5% or higher is diagnostic of type 2 diabetes and should be confirmed with a repeat HbA1c test. 1
Diagnostic Thresholds
- HbA1c ≥6.5% establishes the diagnosis of diabetes and warrants initiation of treatment 1
- This diagnostic threshold should be confirmed with a repeat HbA1c measurement to avoid misdiagnosis from laboratory variability 2
- HbA1c 5.7-6.4% indicates prediabetes, requiring lifestyle interventions and counseling about increased risk for progression to diabetes and cardiovascular disease 1
- HbA1c <5.7% is considered normal 1
Important Diagnostic Considerations
When HbA1c Testing May Be Unreliable
- Certain patient factors can systematically raise or lower HbA1c values relative to true glycemia, creating potential for diagnostic error 2
- If HbA1c results are discordant with clinical presentation or equivocal, confirm diagnosis with fasting plasma glucose or oral glucose tolerance testing 2
- HbA1c testing is not recommended during pregnancy; use plasma glucose measurements instead 2
Diagnostic Accuracy Limitations
- Using HbA1c ≥6.5% as the sole diagnostic criterion misses a substantial number of people with type 2 diabetes, including some with fasting hyperglycemia, and misses most people with impaired glucose tolerance 2
- Combining HbA1c with plasma glucose measurements reduces the risk of systematic bias inherent in HbA1c testing alone 2
- HbA1c demonstrates strong predictive value for future diabetes development, with each 1% increase in HbA1c associated with a 4.5-fold increased odds of developing type 2 diabetes over 20 years 3
Clinical Action After Diagnosis
- Once diabetes is diagnosed (HbA1c ≥6.5%), immediately initiate lifestyle modifications and start pharmacologic therapy with metformin at or soon after diagnosis unless contraindicated 1
- For prediabetes (HbA1c 5.7-6.4%), focus on intensive lifestyle interventions including dietary modifications and at least 150 minutes weekly of moderate physical activity to prevent progression to diabetes 1