Wednesday, October 6, 2010

Is HbA1C Really a "Diabetic Truth Detector?"

Millions of newly diagnosed type 2 diabetics are told that their doctors have a means of telling whether those blood sugar levels they so carefully test every day are truthfully recorded. Diabetics are threatened and cajoled with the promise of a "lie detector," HbA1C, which measures the long-term average of blood sugar levels over a period of about 120 days. But is this measure really useful in that way?

HbA1C is short for glycosylated hemoglobin. It's a measure of the percentage of red blood cells that are coated with sugars joined to their proteins. The higher blood glucose levels go, the theory says, the higher the HbA1C. Generally, doctors try to treat their patients in ways that keep the number  to 7.0% or lower, but there is considerable evidence that a 5.5% level would be associated with a really minimal long-term risk of developing complications.

There are several problems with using HbA1C as a "diabetic lie detector." For one thing, some diabetics have relatively low fasting blood glucose levels in the morning, but anunexpectedly high number. This is a sign of a problem with impaired phase 1 response, an inability of the beta cells of the pancreas to release insulin from its storage form, pro-insulin. Blood sugar levels soar after meals, but by the time dawn rolls around, the pancreas has finally been able to do its work. The numbers taken first thing in the morning won't make sense compared against the long-term measure of glycosylated hemoglobin, unless the patient or the doctor is aware that this kind of problem is possible. When it occurs, the answers are being careful to eat small meals, or taking insulin or insulin-stimulating medication.

Another problem with the use of this "diabetic lie detector" is that sometimes the long-term average is falsely low. That's because really high glucose levels kill off red blood cells. If a blood cell dies before it is measured for glycosylation, it doesn't contribute to the count. Really, really high blood sugars don't necessarily lead to really, really high long-term measures of glycosylation.

HbA1C is a great way to measure progress if you are clearly diagnosed as a diabetic, but it's an imprecise way to tell whether someone is diabetic or not. The problem with using a cutoff below 7.0% is that people who aren't diabetic get labeled as diabetics when the issue is really some variation in the testing process. The initial diagnosis of diabetes still has to be done on the basis of fasting and post-prandial (after-meal) blood sugar levels.

Just don't allow yourself to be browbeaten for a number that is higher than your home blood glucose testing would predict. That only means that there is some unusual pattern in how your pancreas responds to the sugars released by the digestion of a meal, that can be fixed if it is properly identified.

Selected Reference:

Piette JD, Milton EC, Aiello AE, Mendoza-Avelares MO, Herman WH.
Comparison of three methods for diabetes screening in a rural clinic in Honduras
Rev Panam Salud Publica. 2010 Jul;28(1):49-57.

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