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Results of an A1C test and a blood glucose check don't always match up. The A1C test measures your average blood sugar levels over a 120-day period (the lifespan of a red blood cell). But a blood glucose check measures your blood sugar at a single moment. If your blood sugar levels were high last week, and you adjusted your diabetes treatment plan so that your blood sugar returned to normal, the A1C result may still be high, because it includes the high blood sugar levels from the previous week.
The A1C test measures the percentage of glycated hemoglobin in your blood. Glycated hemoglobin is created when molecules of hemoglobin (the oxygen-carrying protein in your blood) attach to molecules of glucose (the sugar in your blood). The more sugar you have in your blood, the higher your percentage of glycated hemoglobin.
Your blood glucose levels are probably based on your premeal blood glucose levels. While this usually works well, it does not take into account the level of your blood glucose after you eat. It may be that your blood glucose is rising unexpectedly after your meal because you are either not taking enough insulin or not taking insulin far enough ahead of eating your meal. To see if this is the reason that your A1C is high, measure your blood glucose 2 hours after breakfast, lunch, and dinner for several days (in addition to your premeal blood glucose levels). Blood sugar that is more than 200 mg/dl 2 hours after a meal is too high. In addition, your blood glucose may be high throughout the night, when you are asleep. To find out, wake yourself up at 4:00 a.m. several times during the week to check your blood glucose. At 4:00 a.m., blood glucose that is higher than 150 mg/dl is too high. Continue to check your blood glucose 2 hours after meals and in the middle of the night once a month to be certain that you are not having unexpected high blood glucose levels at these times.
I want you to picture a freshly fried doughnut hole, just out of the deep fryer. Steam wafts from its surface. Still gleaming wet with oil, it’s sent spinning across a tray of powdered sugar. As it tumbles, sugar sticks to the wet oil, coating it, covering it as it rolls and bounces along.
Well, the same thing happens to your red blood cells. As they tumble and spin and roll through your blood vessels, sugar molecules stick to their skins. Or another way to think of it is to picture a windshield of a truck after driving through a swarm of mosquitoes. Splat-Splat-Splat-Splat-Splat! (OK, I just threw that in so you wouldn’t log off and drive straight to Dunkin Doughnuts.)
So an A1C test is a measure of how much of the skins of your red blood cells are splattered with sugar, which in turn gives as a picture of the blood sugar environment in which the cells lived, which in turn gives us a notion of your average blood sugar over the last several months.
So the A1C test looks at average blood sugar, while your fingersticks look at moments in time. The most common cause of a difference between A1C and fingersticks is that the fingersticks are not frequent enough, or are not at the right times to catch the elevated blood sugars that are jacking your A1C to higher levels.
For instance, if you are only testing in the morning, you are missing possible elevations after meals or overnight while you are sleeping. The A1C test is not perfect. For instance, if your sugar was ranging scary low to scary high you might get a number that looked pretty good. That said, most “false reads” are good looking numbers that mask troublesome variability. Additionally, if you are anemic, you can also get a false low, as your blood cells don’t last as long as typical.
But if your A1C is high, there are some high blood sugars lurking somewhere. And that is exactly why we do both fingerstick testing a A1Cs. They serve as checks and balances against each other.
Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.