A1c and Blood Glucose Levels
Does your hemoglobin A1c level not appear to agree with the average meter readings you get at home? You’re not alone.
There are numerous reasons your A1c might appear to be higher or lower than what you were expecting. The most common reason is related to the fact that your A1c reflects an average blood glucose (BG) level. You can have a lot of highs but also a lot of lows and end up with a relatively normal A1c, the same as you’d have if you kept your BG levels normal all the time.
But this isn’t the only reason for variation.
The A1c depends on glycation of the hemoglobin in your red blood cells (RBCs). Glycation means adding glucose, and the higher your BGs are, the more glucose you’ll add to the hemoglobin.
Anything that affects the lifetime of your RBCs, which are assumed to live 120 days, will affect the A1c. If you give blood or have some kind of internal bleeding, or if you have a hemolytic anemia, you will lose some of the older RBCs cells with a lot of glycated hemoglobin, and your body will make new RBCs with unglycated hemoglobin, so the percentage of glycated hemoglobin will be lower, and your A1c will be lower.
Conversely, if you have spleen damage or no spleen at all, your body will take longer to remove old RBCs from your body, because the spleen is where this housekeeping chore normally happens.
In addition, individuals may have average RBC lifetimes that are different from normal.
Finally, different people may glycate hemoglobin at different rates because of individual variation in the enzymes involved.
Numerous studies throughout the years have investigated this problem and shown that the A1c does not always match average BG levels. Recently a report, titled Does A1c consistently reflect mean plasma glucose? was published in the Journal of Diabetes.
They had type 2 patients treated with insulin measure BG 7 times a day: before and 2 hours after breakfast, lunch, and dinner, and again at 3 a.m. They said previous studies showed that this was enough to determine average BG. The study would have been better if they’d wired the patients up with continuous monitors, but they didn’t do that.
What they found was that the average BGs in the groups matched the A1cs pretty well. (Formulas for converting BG and A1c differ slightly depending on the formula you use. There’s a handy converter here that will give you results on the basis of 2007 ADAG [A1c-Derived Average Glucose] study group recommendations.)
But there was a lot of variation among the patients. For example, in the group that had A1cs between 6.5 and 7.5, the average home-tested BG among 260 patients was 142. With the A1c calculator mentioned, the range should have been 140 to 169.
Patients in the 90th percentile of that group had an average BG of 171, which agrees well with the top of the range you get with the calculator. But patients in the 10th percentile had an average BG of only 115.
Conversely, among patients in the group with BGs between 110 and 140, the mean A1c was 6.92. But those in the 90th percentile had A1cs of 8.1 and those in the 10th percentile had A1cs of only 6. According to the calculator, the range should have been from 5.4 to 6.5. However, the results in the paper are presented as averages among that group. Individuals could have had higher or lower numbers.
And results in the groups above the 90th percentile had greatly increased numbers.
The actual numbers aren’t really that important for us. But what this means is that if you think your A1c doesn’t really reflect your average BG levels, and you know you haven’t been going very high and then very low, you might be correct. You might be one of the patients that just doesn’t fit the mold.
One question researchers haven’t yet answered is whether complications are more closely related to A1c or to average BG levels in people in whom they don’t match well. The Joslin Diabetes Center is currently studying why some people get complications and others don’t, but the results aren’t in yet.
If your BGs con’t seem to match your A1c, you can try to find out why. Do you have an abnormal hemoglobin type? The problem is, it’s expensive to determine this, and it’s not often done outside research settings.
Are you anemic? Have you lost blood lately, either intentionally at a blood draw or accidentally? Do you lack a spleen? Iron levels may also be involved. You can read about that here.
If you can’t figure out why, you should just accept that you don’t have average A1c-to-BG ratios, and the important thing is to compare one test to a previous test, rather than to someone else’s test.
Finally, has your hospital changed its lab methods lately? You can read my experiences with this here.
Gretchen wrote for HealthCentral as a patient expert for Diabetes.