How Should Hemoglobin A1c (HbA1c) Results Be Interpreted in Your Patients with Diabetes?

Ellie S. Strock, RN, CS, ANP, CDE

Disclosures

February 22, 2001

Question

I know that hemoglobin A1c (HbA1c) represents an average blood glucose level. I occasionally see patients whose blood sugars oscillate between extreme high and extreme low levels, yet their HbA1c is within normal range. What is HbA1c and how should it be interpreted?

Response from Ellie S. Strock, RN, CS, ANP, CDE

Hemoglobin A1c (HbA1c), also referred to as glycohemoglobin, is a relative measure used to evaluate overall blood glucose average over the previous 6-8 weeks. The test measures glycosylation of hemoglobin in the red cells over their lifetime of 90-120 days. Hemoglobin A1c reflects both fasting glucose and postprandial glucose. Fasting blood glucose is influenced by hepatic glucose production and hepatic sensitivity to insulin. Postprandial glucose is influenced by preprandial glucose, insulin secretion, and glucose load from meals and insulin sensitivity in the peripheral tissues.

Postprandial glucose monitoring is not commonly recommended, but is becoming more important as a tool for managing diabetes. Initial data suggest that postprandial glucose may have a stronger correlation to cardiovascular disease than fasting glucose or HbA1c.[1] Postprandial glucose should be measured 2 hours from the start of the meal with ideal value of less than 160 mg/dL; 180 mg/dL is acceptable.

Individuals who do have wide variations in blood glucose may have a normal HbA1c. Usually they have a history of frequent, if not significant, hypoglycemia. Self-monitored blood glucose data are necessary to correlate HbA1c in such situations. A comprehensive assessment including a review of the food plan, exercise, and diabetes therapy along with verification of self-monitored blood glucose data using a meter with a memory should be completed. Appropriate changes in therapy should be made to reduce the fluctuations in blood glucose.

Underlying pathologic processes can affect HbA1c such as hemolytic diseases or other conditions with shortened erythrocyte survival.[2] In such cases (albeit rare), HbA1c may be falsely low. If you are confident that the self-monitored blood glucose data are correct and do not correlate with HbA1c, further evaluation may be indicated.

Hemoglobin A1c is an excellent measure to evaluate the efficacy of diabetes therapy; the lower the HbA1c, the less risk of complications in both type 1 and type 2 diabetes as shown in the Diabetes Control and Complications Trial (DCCT) and United Kingdom Prospective Diabetes Study (UKPDS). The recommended target for HbA1c is less than 7% for most individuals with diabetes; this may be adjusted for young children or the elderly in whom there is a concern for hypoglycemia.

HbA1c should be measured every 3-4 months with a minimum of twice per year. Ideally, the result should be available at the time of the office visit to compare with self-monitored blood glucose (SMBG) data so appropriate modifications in therapy can be made. Recent studies have shown that having this data available while using in-office, rapid HbA1c devices results in significant improvement in glycemic control at 6 months and 12 months.[3,4] New technologies are now available for home testing of HbA1c using a finger-stick method.

It is important to realize that there are still several methods of measuring glycohemoglobin and their results vary significantly. Some methods measure total glycosylated hemoglobin (HbA1a, HbA1b,and HbA1c). Standardization of measurement has not occurred, although there is a national ongoing effort, the National Glycohemoglobin Standardization Program (NGSP). The current "gold standard" for HbA1c is to use a method certified as traceable to the DDCT HbA1c reference method. According to the NGSP, about 75% of all testing is now using a DCCT-certified assay method. If there is a question about the particular method used in your laboratory, ask if they are using a DCCT-certified assay method.

Hemoglobin A1c is an effective tool for assessing overall control of diabetes and risk of complications. Use in conjunction with self-monitored blood glucose data is necessary to make appropriate therapeutic adjustments.

Correlation of HbA1c and Average Glucose*
(Assumes normal range of 4% to 6%)

HbA1c Value Average Glucose
7% ~150 mg/dL (8.3 mmol/L)
8% ~180 mg/dL (10.0 mmol/L)
9% ~210 mg/dL (11.7 mmol/L)
10% ~245 mg/dL (13.6 mmol/L)
11% ~280 mg/dL (15.6 mmol/L)
12% ~310 mg/dL (17.2 mmol/L)
13% ~345 mg/dL (19.2 mmol/L)
14% ~380 mg/dL (21.1 mmol/L)

*Nathan DM, et al. N Engl J Med. 1984;310:341-346.

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