What is the HbA1c (Hemoglobin A1c) level used to diagnose diabetes in an adult patient?

HbA1c Threshold for Diagnosing Diabetes

An HbA1c of ≥6.5% (48 mmol/mol) is the diagnostic threshold for diabetes in adults, and this test must be performed in a laboratory using a method that is NGSP certified and standardized to the DCCT assay. 1

Diagnostic Criteria and Confirmation Requirements

The American Diabetes Association establishes clear diagnostic thresholds for diabetes using HbA1c 1:

• HbA1c ≥6.5% (48 mmol/mol) confirms diabetes 1
• HbA1c 5.7-6.4% indicates prediabetes (risk of developing diabetes) 1
• HbA1c <5.7% is normal 1

Confirmation Testing

In the absence of unequivocal hyperglycemia (symptoms plus random glucose ≥200 mg/dL), a second confirmatory test is required. 1 This can be accomplished by:

• Repeating the same test (HbA1c) on a new blood sample 1
• Using a different diagnostic test (fasting plasma glucose ≥126 mg/dL or 2-hour OGTT ≥200 mg/dL) that also exceeds the diagnostic threshold 1

For example, if the initial HbA1c is 7.0% and the repeat result is 6.8%, diabetes is confirmed because both values exceed 6.5%. 1

Critical Limitations and When NOT to Use HbA1c

Do not use HbA1c for diagnosis in the following situations—use only plasma glucose criteria instead: 1

• Conditions with increased red blood cell turnover: sickle cell disease, pregnancy (second and third trimesters), hemodialysis, recent blood loss or transfusion, or erythropoietin therapy 1
• Hemoglobin variants: certain hemoglobinopathies can interfere with HbA1c measurement, though most U.S. assays are unaffected by common variants 1
• Children and adolescents: the epidemiological studies supporting HbA1c diagnostic thresholds included only adult populations 1

Important Clinical Pitfall

Marked discrepancies between measured HbA1c and plasma glucose levels should prompt consideration that the HbA1c assay may not be reliable for that individual. 1 Age and race/ethnicity can affect hemoglobin glycation independently of glycemia. 1

Comparison with Other Diagnostic Tests

While three tests can diagnose diabetes (HbA1c, fasting plasma glucose, and 2-hour post-load glucose), HbA1c offers greater convenience because fasting is not required, has less intraindividual variation, and is more reproducible than glucose-based tests. 1

However, research demonstrates that HbA1c has low sensitivity (25-35%) when compared against combined fasting and 2-hour glucose criteria, resulting in false-negative rates of 65-75%. 2 This means HbA1c <6.5% does not reliably exclude diabetes, particularly in older adults and certain racial/ethnic groups. 2

Practical Application in Acute Care Settings

HbA1c ≥6.5% can reliably diagnose diabetes in hospitalized patients with hyperglycemia, as acute illness does not substantially affect HbA1c values. 3, 4 In emergency department patients with random hyperglycemia, an HbA1c >6.0% was 100% specific for diabetes diagnosis. 3

The measurement reflects glycemic control over the previous 8-12 weeks, which distinguishes undiagnosed diabetes from stress hyperglycemia in acutely ill patients. 1

Who Should Be Screened

Testing should be considered in adults of any age who are overweight or obese (BMI ≥25 kg/m² or ≥23 kg/m² in Asian Americans) with one or more risk factors: 1

• First-degree relative with diabetes 1
• High-risk race/ethnicity (African American, Latino, Native American, Asian American, Pacific Islander) 1
• History of cardiovascular disease 1
• Hypertension (≥140/90 mmHg or on therapy) 1
• HDL cholesterol <35 mg/dL or triglycerides >250 mg/dL 1
• Women with polycystic ovary syndrome 1
• Physical inactivity 1
• Conditions associated with insulin resistance (severe obesity, acanthosis nigricans) 1

For all other patients without risk factors, screening should begin at age 45 years. 1