#68 Hemoglobin A1c for the Diagnosis of Type 2 Diabetes
CLINICAL QUESTION
What are the advantages and disadvantages of using hemoglobin A1c (A1c) as a diagnostic test for Type 2 diabetes mellitus?
BOTTOM LINE
Hemoglobin A1c can be used to diagnose diabetes. Controversy remains around the best cut-off, but ≥6.5% is most commonly recommended. Different tests to diagnosis diabetes (A1c, fasting plasma glucose, and oral glucose tolerance tests) may give inconsistent results, so it is recommended the same test be used for retesting to confirm a diagnosis.
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EVIDENCE
No single test represents a gold-standard to diagnose diabetes.1
Agreement between A1c, fasting plasma glucose (FPG), and oral glucose tolerance testing (OGTT) is poor.2
- A1c >6.5% misses 47% of cases of diabetes diagnosed by FPG (>7 mmol/L) and 63% of diabetes by OGTT (>11.1 mmol/L).
- In some studies, A1c diagnosed more diabetes than OGTT.3-5
- Notably, FPG also misses 46% of diabetes diagnosed by OGTT.
- Microvascular: A1c as good as FPG or OGTT.6,7
- Macrovascular: A1c better than FPG,8,9 and similar to OGTT.8
- Best cut-off for prediction of complications varied from >5.8 to >7.3%.6-9
- Cut-off for black individuals (>5.5%)10 may be lower than Asian or white patients.
- Lower A1c improves sensitivity, but decreases specificity.
- Example: Sensitivity (compared to FPG) improved from 53% to 73% when decreasing the threshold from >6.5% to >6.1%.2
- All major guidelines11-13 now include A1c >6.5% in the diagnostic criteria for diabetes.
- Positive results (FPG, OGTT or A1c) should be confirmed by presence of symptomatic hyperglycemia, or by repeating the same test on a different day.11
- Although previously the preferred diagnostic test for diabetes, FPG:
- Requires patient compliance with fasting.
- Has high variability within the same individual.11,14
- Other considerations for A1c:
- Does not require fasting and has less variability in the same individual than FPG.14
- More expensive.
- Not reliable in certain medical conditions (e.g. anemia, hemoglobinopathies).15
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