#68 Hemoglobin A1c for the Diagnosis of Type 2 Diabetes
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- 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