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Hemoglobin is the main functional substance
in our red blood cells.
It is a protein, meaning it is made up
of a string of amino acids,
and it also contains an atom of iron,
giving our blood its red color.
The main job of hemoglobin, and our red blood cells,
is to deliver oxygen from the lungs
to other tissues and organs in our bodies that will use it.
Hemoglobin A is the most common type
of hemoglobin found in adults.
But there are other forms.
For example, fetal hemoglobin is called hemoglobin F,
and is normally present in a fetus and newborn.
Or, people with sickle cell disease have hemoglobin S.
It is important to know
that individual red blood cells last
about 120 days in our circulation,
before their function declines and they are removed.
During the lifespan of a red blood cell,
the hemoglobin molecules inside accumulate a form
of modification called glycation.
Glycation is the unregulated attachment
of a sugar molecule, glucose, to amino acids in a protein.
For hemoglobin, there are some chemically preferred spots
where the glucose can attach.
And the most commonly form glycated hemoglobin
is known as hemoglobin A1C.
The more glucose that is available to react,
the more hemoglobin A1C will form.
In people with diabetes, their ability to remove glucose
from the blood is impaired,
leading to higher concentrations of blood glucose.
Since there is more glucose available for glycation,
people with diabetes generate more hemoglobin A1C
during the lifespan of a red blood cell,
versus people who do not have diabetes.
So, measuring the amount of hemoglobin A1C,
typically expressed in the U.S.
as a percentage of all hemoglobin,
gives an indication of the long term amount
of glucose in the patient's blood,
during the lifespan of their red cells.
In other tissues of the body,
the same type of glycation is happening on other proteins,
leading to various types of damage.
The damage elsewhere results in common complications found
in diabetes such as kidney disease
and visual problems like retinopathy.
Years ago, several large studies showed
that by controlling the amount of glucose,
and limiting the amount of glycation damage,
the onset of these complications
could be delayed or prevented.
A1C could be used to determine how well the glucose
was being managed.
A1C became a surrogate, or proxy,
for the amount of damage taking place in other tissues.
And groups such as the American Diabetes Association
were now able to make recommendations
to doctors and patients about A1C treatment goals
for adequately controlling diabetes,
and improving quality of life.
In addition, the test is now also used to diagnose diabetes,
which is often viewed as an improvement
over historical tests of glucose tolerance.
But A1C is not the right test for everyone.
Since the test is a substitute for measuring
how well someone is able to control their glucose,
there are conditions in which A1C
does not really indicate this.
For example, normal A1C values are based
on red blood cells circulating for a typical period
of about 120 days.
In some people, their red cells don't last that long,
due to things like anemia or sickle cell disease.
So their cells don't accumulate the same amount
of glycation, they would if they lasted longer.
And their percentage A1C may not really show
how poor their glucose control is.
Or, in patients who have received blood transfusions,
their A1C will indicate the glucose control
in the blood donor until those cells
are removed from circulation.
This is good to keep in mind.
A1C is a great test for most people,
but it is not a good indicator
of glucose control in everyone.
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