When to Advance from Random Blood Glucose to A1C Testing
A random blood glucose level of ≥200 mg/dL (11.1 mmol/L) should prompt A1C testing for confirmation of diabetes diagnosis, especially when accompanied by classic symptoms of hyperglycemia. 1
Diagnostic Pathway for Diabetes Using Random Blood Glucose
Initial Random Blood Glucose Interpretation
- ≥200 mg/dL (11.1 mmol/L) with symptoms: Diagnostic of diabetes when accompanied by classic symptoms (polyuria, polydipsia, weight loss, blurred vision, fatigue) 1
- ≥200 mg/dL without symptoms: Requires confirmation with A1C or another glucose test 1
- 140-199 mg/dL (7.8-11.0 mmol/L): Warrants further testing with A1C due to high specificity (92-98%) for potential diabetes 1
- <140 mg/dL: Lower risk, but consider A1C testing based on risk factors
When to Advance to A1C Testing
- After finding a random blood glucose ≥140 mg/dL (especially ≥180 mg/dL which has 98% specificity) 1
- In patients with risk factors for diabetes even with normal random glucose:
- Age ≥45 years
- BMI ≥25 kg/m² (≥23 kg/m² in Asian Americans)
- Family history of diabetes
- High-risk ethnicity
- History of gestational diabetes or cardiovascular disease
Advantages of A1C Testing
- No fasting required (greater convenience)
- Greater preanalytical stability
- Less day-to-day perturbations during stress and illness 1
- Reflects average glycemia over previous 2-3 months
Important Considerations When Using A1C
Diagnostic Criteria
Limitations of A1C Testing
- Less reliable in conditions with altered red blood cell turnover:
- Hemoglobinopathies (sickle cell disease/trait)
- Pregnancy (second and third trimesters)
- Recent blood loss or transfusion
- Hemodialysis
- Erythropoietin therapy
- Iron-deficient anemia 1
Confirming the Diagnosis
- In the absence of unequivocal hyperglycemia with classic symptoms, diagnosis requires two abnormal test results:
- Two abnormal results from the same test on different days, or
- Two different tests (e.g., A1C and FPG) both showing abnormal results 1
Clinical Pearls
- A1C testing should be performed using a method certified by the NGSP and standardized to the DCCT assay 1
- Point-of-care A1C testing is not recommended for diagnostic purposes due to lack of standardization 1
- Marked discrepancies between A1C and plasma glucose levels should raise suspicion of A1C assay interference 1
- Random blood glucose values between 140-180 mg/dL have high specificity (92-98%) but limited sensitivity (39-55%) 1
- A random glucose ≥200 mg/dL with classic symptoms doesn't require confirmation with A1C 1
By following this systematic approach to advancing from random blood glucose to A1C testing, clinicians can effectively identify patients with diabetes while minimizing unnecessary testing and false diagnoses.