Ask the Doctor

Ask the Doctor

ASK
THE DOCTOR

by Wesley
W. Wilson, MD

NOTE: If you have any questions
for "Ask the Doctor," please send them to the VOICE editorial office.
The only questions Dr. Wilson will be able to answer are the ones used in this
column.

Wesley Wilson, MD is an Internal Medicine practitioner

at the Western Montana Clinic in Missoula, Montana. Dr.

Wilson was diagnosed with type I diabetes in 1956, during

his second year of medical school.

Q: I read in the diabetes journals about a test called

an A1C. What is it, and how is it different from my daily

glucose monitoring? My diabetes educator and my doctor

disagree about my need for this test. How often should I

have an A1C?

A: Blood sugar, which you measure daily, fluctuates a

great deal from test to test and from hour to hour. I am

sure you would agree it is very hard, if not near

impossible, to keep your blood sugars within your "target

range" at all times, but these measurements are essential,

particularly for insulin-using diabetics, as they reveal how

high or low you are after meals, after insulin injections,

and especially before meals. A blood sugar reading from

your glucose monitor is a "snapshot," of your diabetes

control. It is essential, but even if done several times

each day, it does not really show your average blood sugar

control during a given 24-hour period of time.

There is growing evidence that in type I (insulin

dependent, IDDM) and type II (non insulin dependent, NIDDM)

diabetes, complications correlate with the average blood

sugar level over time. The higher the average blood sugar,

the greater the likelihood you'll develop diabetic

complications. The precise mechanism of development for

these complications; eye disease, kidney disease, and nerve

conduction disease, remains to some extent unknown, but it

would seem the underlying process is somehow related to

abnormalities of the proteins making up these structures:

the eye, blood vessels, kidney and nerve fibers. Increased

attachment of sugar to protein, a process known as

glycosylation (due to sustained high blood glucose levels)

seems a likely culprit.

Sugar attaches to proteins in any solution. The

greater the amount of sugar in a given solution, the greater

the amount of sugar that attaches to nearby proteins. It

has been known for years that sugar in the blood attaches to

the protein hemoglobin, a component of red blood cells. The

sugar attaches to the hemoglobin, or glycosylates it, in

direct relationship to the level of sugar in the solution

(the blood). Since red blood cells stay in the bloodstream

for about 110 days, measurement of the percentage of

hemoglobin that is glycosylated (has sugar attached to it)

can be used to give an estimate of the average blood sugar

level during the preceding six to eight weeks before blood

was drawn for the test. One name for this is "glycosylated

hemoglobin test."

Things always seem to become more complicated in

medicine! It is now known that there are several

differences within the glycosylated hemoglobin family. The

"hemoglobin A1C" you mentioned describes a single protein

with glucose attached to it. It is the one used most

frequently in monitoring diabetes, but many labs also run a

"hemoglobin A1," which gives a different value since it is a

different protein. The theory is the same, however.

Since hemoglobin A1C level correlates with the risk of

development of diabetic complications over time, measuring

hemoglobin A1C every three months allows us to estimate

average blood sugar level quite well from year to year, and

helps us act to reduce the risk of ramifications.

Unfortunately, there are considerable differences between

labs in the technique used to measure hemoglobin A1C and its

normal range. (The American Diabetes Association is

currently making an effort to standardize hemoglobin A1C

measurement and reportage.)

There must be some warning in interpreting hemoglobin

A1C values. Certain blood diseases can affect the

hemoglobin A1C, and thalassemia, an inherited disorder of

red blood cells that affects many individuals, particularly

of Mediterranean ethnicity, often causes a falsely high

hemoglobin A1C reading, even in persons who have normal

blood sugar levels. Many individuals with other blood

disorders can have falsely high hemoglobin A1C values.

When reviewing test results, it is important to

remember that the A1C test measures the "average" blood

sugar level. If a person has a "normal" hemoglobin A1C,

this may perhaps mean they have periods of excessive high

blood sugar, balanced by periods of abnormally low blood

sugar levels. We see this in some individuals whose blood

sugars are a bit elevated during the day (as revealed by

glucose meter), but who may have undetected low blood sugars

at night, so they end up with a "quite normal" hemoglobin

A1C, even though their diabetic control would be classified

as very unsatisfactory. That is why you need both tests:

measuring blood sugar levels frequently during the day, and

at least occasionally at night, to discover the specific

sugar levels at various times, but also measuring hemoglobin

A1C values four or five times a year to ensure that the

average blood sugar levels indicate adequate control.