Hemoglobin A1c
(HbA1c)
What is HbA1c?
Hemoglobin is the oxygen-carrying
pigment that gives blood its red color and also the predominant protein in red blood cells. About 90% of hemoglobin is
hemoglobin A. (The "A" stands for
adult type.) Although one chemical component accounts for 92% of hemoglobin A,
approximately 8% of hemoglobin A is made up of minor components that are
chemically slightly different. These minor components include hemoglobin A1c, A1b, A1a1, and
A1a2. Hemoglobin A1c (HbA1c) is a minor component of hemoglobin to which glucose is bound.
HbA1c also is referred to as glycosylated or glucosylated hemoglobin.
How is HbA1c measured?
A chemical (electrical) charge is present on the molecule of
HbA1c, and the amount of the charge differs from the charges on the other
components of hemoglobin. The molecule of HbA1c also differs in size from the
other components. HbA1c may be separated by charge and size from the other
hemoglobin A components in blood by a procedure called high pressure (or
performance) liquid chromatography (HPLC). HPLC separates mixtures (e.g., blood)
into its various components by adding the mixtures to special liquids and
passing them under pressure through columns filled with a material that
separates the mixture into its different component molecules. Because HbA1c is
not affected by short-term fluctuations in blood glucose concentrations, for
example, due to meals, blood can be drawn for HbA1c testing without regard to
when food was eaten.
Why measure HbA1c?
HbA1c levels depend on the blood glucose concentration. That
is, the higher the glucose concentration in blood, the higher the level of
HbA1c. Levels of HbA1c are not influenced by daily fluctuations in the blood
glucose concentration but reflect the average glucose levels over the prior six
to eight weeks. Therefore, HbA1c is a useful indicator of how well the blood
glucose level has been controlled in the recent past and may be used to monitor
the effects of diet, exercise, and drug therapy on blood glucose in diabetic patients.
In healthy, non-diabetic patients the HbA1c level is less than 7% of
total hemoglobin. It has been demonstrated that the complications of diabetes can be delayed or
prevented if the HbA1c level can be kept close to 7%. In general, values should be kept below
8%.
What are the limitations to measuring HbA1c?
Since HbA1c is not influenced by daily fluctuations in blood glucose
concentration, it cannot be used to monitor day-to-day blood glucose concentrations and to
adjust insulin doses nor can it detect the day-to-day presence or absence of hyperglycemia or
hypoglycemia.
HbA1c may be increased falsely in certain medical conditions.
These conditions include uremia (kidney failure), chronic
excessive alcohol intake, and hypertriglyceridemia. Medical conditions that may
falsely decrease HbA1c include acute or
chronic blood loss, sickle cell disease or thalassemia. Diabetes during
pregnancy, commonly
referred to as gestational diabetes, may falsely increase or decrease HbA1c.
Last Editorial Review: 4/24/2002
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