Ordering HbA1c for Elevated Blood Glucose on CMP
Yes, you should order a Hemoglobin A1c (HbA1c) test when a patient has hyperglycemia on a Comprehensive Metabolic Panel (CMP). 1 This is the appropriate next step to assess the patient's average glycemic control over the past 2-3 months.
Rationale for HbA1c Testing
HbA1c testing provides several advantages over a single glucose measurement:
- Reflects average glycemia over approximately 2-3 months 1
- Does not require fasting 2
- Has strong predictive value for diabetes complications 1, 2
- Provides a quantitative and reliable measure of glycemia over an extended period 1
- Helps determine if hyperglycemia is persistent or transient 3
Interpretation of Elevated Glucose on CMP
When hyperglycemia is detected on a CMP:
A single elevated glucose value may represent:
- Undiagnosed diabetes 3
- Stress hyperglycemia (due to acute illness)
- Prediabetes
- Random variation
Research shows that among ED patients with glucose ≥110 mg/dL and no prior diabetes history, 22.4% had elevated HbA1c levels, suggesting undiagnosed diabetes 3
HbA1c Testing Protocol
The American Diabetes Association recommends:
- Order HbA1c for all patients with hyperglycemia to document the degree of glycemic control 1
- Use the results to determine if the patient:
- Has diabetes (HbA1c ≥6.5%)
- Has prediabetes (HbA1c 5.7-6.4%)
- Has normal glycemic control (HbA1c <5.7%) 1
Correlation Between HbA1c and Mean Glucose
HbA1c values correlate with mean plasma glucose levels:
| HbA1c (%) | Mean Plasma Glucose (mg/dL) |
|---|---|
| 6 | 126 |
| 7 | 154 |
| 8 | 183 |
| 9 | 212 |
| 10 | 240 |
Source: American Diabetes Association 1
Important Limitations and Caveats
Be aware of conditions that may affect HbA1c accuracy:
- Conditions affecting red blood cell turnover (hemolysis, blood loss) 1
- Hemoglobin variants (sickle cell anemia) 1, 4
- Recent blood transfusions 1
- End-stage kidney disease 1
- Pregnancy 1
- Use of drugs that stimulate erythropoiesis 1
In these cases, consider alternative testing methods:
- Fasting and 2-hour postprandial glucose measurements 5
- Glycated serum protein (GSP) or fructosamine testing 1, 2
Follow-up Testing Frequency
After initial HbA1c testing:
- For patients meeting treatment goals (stable glycemic control): Test at least twice per year 1
- For patients not meeting glycemic goals or with therapy changes: Test quarterly 1
Conclusion
Ordering an HbA1c test is the appropriate next step for a patient with hyperglycemia on a CMP. This test will help determine if the elevated glucose represents a chronic condition requiring treatment or was simply an acute finding. The results will guide treatment decisions and establish baseline glycemic control for future monitoring.