Background

A1C, also known as glycated hemoglobin or glycosylated hemoglobin, indicates a patient's blood sugar control over the last 2-3 months. A1C is formed when glucose in the blood binds irreversibly to hemoglobin to form a stable glycated hemoglobin complex. Since the normal life span of red blood cells is 90-120 days, the A1C will only be eliminated when the red cells are replaced; A1C values are directly proportional to the concentration of glucose in the blood over the full life span of the red blood cells. A1C values are not subject to the fluctuations that are seen with daily blood glucose monitoring.

The A1C value is an index of mean blood glucose over the past 2-3 months but is weighted to the most recent glucose values. Values show the past 30 days as ~50% of the A1C, the preceding 60 days giving ~25% of the value and the preceding 90 days giving ~25% of the value. This bias is due to the body's natural destruction and replacement of red blood cells. Because red cells are constantly being destroyed and replaced, it does not take 120 days to detect a clinically meaningful change in A1C following a significant change in mean blood glucose. (1)

Clinical Utility

The American Diabetes Association (ADA) recommends A1C as the best test to find out if a patient's blood sugar is under control over time. The test should be performed every 3 months for insulin-treated patients, during treatment changes, or when blood glucose is elevated. For stable patients on oral agents the recommended frequency is at least twice per year.

The Diabetes Control and Complications Trial (DCCT) and the United Kingdom Prospective Diabetes Study (UKPDS) studies showed that the lower the A1C number, the greater the chances to slow or prevent the development of serious eye, kidney and nerve disease. The studies also showed that any improvement in A1C levels can potentially reduce complications.

The ADA recommends that action be taken when A1C results are over 8%, and considers the diabetes to be under control when the A1C result is 7% or less. The Helpful Links page of this site contains several resources to assist you in managing your patients.

Relationship of A1C to Average Whole Blood and Plasma Glucose Levels (2)

1. web.missouri.edu/~diabetes/ngsp/ghbmbg/ghbmbg.htm

2. Diabetes Care 2002;25:275-8