Should I check the Hemoglobin A1C (HbA1C) level in a patient with consistently elevated fasting glucose levels?

A1C Testing for Patients with Consistently Elevated Fasting Glucose

Yes, you should check an A1C in a patient with consistently elevated fasting glucose in the 100's to confirm diagnosis and assess long-term glycemic control.

Rationale for A1C Testing

When fasting glucose levels are consistently elevated in the 100's mg/dL, this indicates potential prediabetes or diabetes that requires further assessment:

• Fasting glucose in the 100-125 mg/dL range meets criteria for impaired fasting glucose (IFG) or prediabetes 1
• A1C provides a complementary measure that reflects average glycemia over the preceding 2-3 months 1
• The American Diabetes Association recommends using A1C as one of the diagnostic tests for diabetes and prediabetes 1, 2

Diagnostic Criteria and Interpretation

A1C testing helps categorize the patient's glycemic status:

• Normal: A1C < 5.7%
• Prediabetes: A1C 5.7-6.4%
• Diabetes: A1C ≥ 6.5% 1, 2

The correlation between A1C and average plasma glucose levels is well-established:

• A1C of 6% correlates with mean plasma glucose of 126 mg/dL
• A1C of 7% correlates with mean plasma glucose of 154 mg/dL 1

Benefits of Adding A1C to Fasting Glucose Assessment

1. Provides complementary information: A1C and fasting glucose don't always identify the same individuals with dysglycemia. Only 7.7% of U.S. adults have prediabetes according to both definitions 3

2. Reduces day-to-day variability: A1C is not affected by acute changes in glucose levels due to stress or illness 1

3. Patient convenience: A1C doesn't require fasting, making it more convenient than fasting plasma glucose testing 1

4. Establishes baseline for monitoring: If treatment is initiated, A1C should be checked at least twice yearly in patients meeting treatment goals and quarterly in those not meeting goals or with therapy changes 1

Clinical Approach

For a patient with consistently elevated fasting glucose in the 100's:

1. Order A1C test to establish diagnosis and determine severity

   • If A1C < 5.7%: Normal glycemic status despite borderline fasting glucose
   • If A1C 5.7-6.4%: Confirms prediabetes diagnosis
   • If A1C ≥ 6.5%: Suggests diabetes (requires confirmation with repeat testing) 1

2. Confirm diagnosis if needed:

   • If A1C ≥ 6.5%, repeat the A1C or perform another test (such as fasting glucose) to confirm diagnosis 1
   • Two abnormal test results from the same sample or in two separate test samples are required for diagnosis 1

Important Caveats

• A1C limitations: A1C may be unreliable in conditions affecting red blood cell turnover (hemolysis, blood loss, transfusion, erythropoietin therapy) or in patients with hemoglobinopathies 1

• Discordant results: If A1C and fasting glucose results are discordant, consider the possibility of A1C assay interference or other factors affecting the relationship between A1C and glycemia 1

• Point-of-care testing: While point-of-care A1C testing provides opportunity for more timely treatment changes, it is not recommended for diagnostic purposes 1

By checking an A1C in patients with consistently elevated fasting glucose, you can more accurately assess their glycemic status, establish appropriate follow-up intervals, and determine if lifestyle interventions or pharmacologic therapy are warranted.