While long considered the test of choice for the chronic management of diabetes, the American Diabetes Association with the International Expert Committee recently issued a report endorsing the use of HgbA1c level ≥6.5% as the favored test for the diagnosis of diabetes. [Diabetes Care 2009;32(7):1327-34] The following are highlights of this report:
- HgbA1c assays are accurate, precise measures of chronic glycemic levels and correlates well with the risk of diabetes complications (e.g. retinopathy). This is particularly true of modern assays that are standardized and aligned with the DCCT (Diabetes Control & Complications Trial) assay and traceable to NGSP (National Glycohemoglobin Standardization Program) reference material. The BioRad ion exchange HPLC primary method and the boronate affinity alternate method performed at MuirLab meet these criteria.
- HgbA1c has several technical advantages over laboratory measures of glucose. It is relatively stable after collection. In addition, the between- and within- subject coefficients of variation (CV) are substantially lower for HgbA1c than for glucose. For example, the day-to-day within person variance for HgbA1c is <2%, but 12-15% for fasting plasma glucose (FPG). Finally, the convenience for the patient supports the use of HgbA1c, which can be obtained at any time and without any specific patient preparation (e.g. carbohydrate diet, fasting, etc.).
- Diabetes should be diagnosed when HgbA1c is ≥6.5%. Diagnosis should be confirmed with a repeat HgbA1c test. Confirmation is not required in symptomatic patients with plasma glucose levels >200 mg/dL.
- If HgbA1c testing is not possible, previously recommended diagnostic methods (e.g. FPG or 2 hr plasma glucose during an OGTT) are acceptable for the diagnosis of diabetes.
- HgbA1c testing is indicated in children in whom diabetes is suspected but the classic symptoms and a casual plasma glucose >200 mg/dL are not found.
- The diagnosis of diabetes during pregnancy, when changes in red cell turnover make the HgbA1c assay interpretation problematic, will continue to require plasma glucose measurements.
- The risk of diabetes based on levels of glycemia is a continuum. Therefore, there is no lower glycemic threshold at which risk clearly begins.
- Individuals with HgbA1c value ≥6.0% but <6.5% are likely at high risk for progression to diabetes and should be considered for preventative measures.