Glycosylated Hemoglobin

GLYCOSYLATED HEMOGLOBIN

The red blood cells of all individuals contain hemoglobin, which is responsible for carrying oxygen through the bloodstream. When hemoglobin combines with glucose (sugar), a molecule called glycosylated hemoglobin, or Hemoglobin A1c (HgbA1c), is formed. Since everyone has glucose in their blood, all individuals also have glycosylated hemoglobin in their blood (usually between 3 and 5 percent of blood).

The amount of A1c in red blood cells is proportional to the amount or concentration of glucose in the blood, and to the age of the red blood cells. (The average red blood cell lives approximately 120 days, with new ones replacing dying red blood cells continuously. Accordingly, healthy individuals have a mixture of "young" and "old" red blood cells at all times.)

If an individual has high blood-sugar levels, such as exists in poorly controlled or untreated diabetes mellitus, the glycosylated hemoglobin percentage will be elevated. Since it is also related to the "age" of the red blood cells, the glycosylated hemoglobin percentage will correspond to the average glucose level over the previous two to three months. This average level is in contrast to a routine measurement of the blood-sugar level, which reflects any food intake over the previous twelve hours.

CLINICAL USEFULNESS

In diabetes mellitus, the blood sugar is elevated in the fasting state, with levels exceeding 126 milligrams per deciliter (mg/dL). Persistently elevated blood-glucose levels result in many chronic complications, such as kidney failure, blindness, and poor circulation in the legs, which can result in amputation. Accordingly, it is recommended that diabetics keep their blood-sugar level in the normal range whenever possible. This can be achieved through diet, exercise, and medications. The American Diabetes Association (ADA), recommends that all health care clinicians routinely obtain glycosylated hemoglobin levels in all diabetics. In 2001, the ADA recommended that this testing be performed at least twice annually for those diabetics whose blood sugar is well-controlled, and more frequently in those with persistently elevated blood sugars. Just as cholesterol levels are used to predict the risk of developing heart disease, the glycosylated hemoglobin value can predict the risk of developing many of the chronic complications associated with diabetes.

OTHER USES

Researchers collecting information on large numbers of individuals can assist scientists in determining whether relationships exist between glycosylated hemoglobin levels and age, gender, ethnicity, or socioeconomic status.

Research directed at the relationship between high glycosylated hemoglobin levels and various disease states could assist health care providers to predict the presence or absence of disease. For public health officials, the determination of glucosylated hemoglobin levels for community areas could provide them with information to develop community-based interventions to improve the control of blood-sugar levels in communities and neighborhoods with elevated average glycosylated levels.

Patrick Dowling

Michelle Anne Bholat

(see also: Diabetis Mellitus; Screening )

Bibliography

American Diabetes Association (2001). "Clinical Practice Recommendations 2001." Diabetes Care 24(Supp.1):1–7.