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.