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Diabetes Testing

How Often to Check A1C

All of us who have diabetes regularly get our A1C checked. Few of us check it often enough.

The A1C is the best way we have to show how well we are managing our disease. It’s a simple test that we can perform at home or at a doctor’s office or lab.

Just like the fingerstick tests that we use for spot checking our blood sugar levels before or after meals, the A1C test uses a small drop of blood. But the A1C measures our sugar level over a lot longer time.

How much longer is the question. The answer to that question can tell us how often that we need to check our A1C level.

The A1C test measures the percentage of glycated hemoglobin, also known as HbA1C, in our blood. We need to check our A1C level “twice a year at a minimum or more frequently when necessary,” is the advice that the American Diabetes Association offers in its “A1C” article. That’s because “the A1C test measures your average blood glucose control for the past 2 or 3 months.”

For a long time I’ve wondered why the ADA isn’t more specific about the time period that the A1C test measures. When I started using the A1CNow SelfCheck that finally became available three years ago so we can now check our A1C at home, I started to research this question. The patient insert that comes with it led me to a big book generally considered to be the definitive reference for clinical chemistry, which explains what’s going on with the A1C.

The book is Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, 5th edition, 2011, edited by Carl A. Burtis et al. This massive volume has 2,238 pages and weighs 10 pounds. I could have bought it for about $250, but instead I borrowed it from my local library on interlibrary loan. In seven of those pages, 1141-1147, the book explains the A1C.

How much glycated hemoglobin we have in our blood depends first on the lifespan of our red blood cells. The “average lifespan is 120 days,” the book says, which is of course four months.

But the glycated hemoglobin in our blood is also “directly proportional to the concentration of glucose in our blood” and “represents integrated values for glucose over the preceding 8 to 12 weeks.” Most people seem to think that our levels at any time during this two to three month period are equal. But they aren’t.

“More recent values,” the book continues, “provid[e] a larger contribution than earlier values.” How much more?

“The plasma glucose in the preceding 1 month determines 50% of the HbA1C, whereas days 60 to 120 determine only 25%,” it says on page 1,442. This is the key statement and the one that prompted me to change my testing strategy.

Since half of the A1C level comes during the previous month, it makes sense to check our A1C much more often than the ADA’s recommendation. In fact, I now check my level every month.

Even though I keep my level in tight control, I have been surprised to see how much if varies. A couple of months ago my level was 5.1, but last month it had shot up to 5.5. This month it’s back down to 5.1.

I use the A1CNow device to check at home. As a result I don’t need to go to the trouble and expense of going to a doctor’s office or a lab for testing. I get 10 tests for $129 from A1CTest.com. That’s less than $13 per test.

Since I test my A1C often and regularly, I appreciate that measuring my level actually changes what I do. The “Observer Effect” show that when we watch something, we will also change something about it. By watching my A1C level more closely, I have therefore changed my health for the better.

This article is based on an earlier version of my article published by HealthCentral.

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  • Alex Miller at

    I am pretty impressed with the Bayer home test A1CNow. I had used it before when I was concerned a lab test was too high. (I am just borderline, but very much like lots of data and want to do everything I can to avoid getting not-borderline.) The home test was very close to the lab result. Recently, I saw a new doctor (the joys of relocating and beginning the trial and error of finding practitioners who aren’t old school dismissive [blanks]), and told him my whole history of borderline with ocassional serious spikes. He did the full panel of bloodwork. Despite a fasting BG of 96, his only followup comment via the nurse was “you are not diabetic” with no suggestion of intervention or preventitive behavior. This was probably based on the A1C of 4.8%, so he assumed I am just a hypochondriac. Well, I know that can’t be right, as my A1C has been over 5.5% in every other lab test and my eating and exercise lately has been worse, and I never get a BG result under 90 when I test at home and post prandial varies from 140 to 180, so estimating an average BG of 91 is absolutely invalid in my opinion. So I did the home test. 5.5%. That was almost expired, so I bought another one a week later (after getting back on low carb) and got 5.4%. So I am happy with the consistency and trust the accuracy. For me. Also, I am very good at lab type work and feel my execution of the test procedure is very good. Results may vary. (And yes, I know 5.5% is not very bad. But I only usually eat twice a day, so am at 90-100 most of the time, highest in the mornings fasting, and then may spike moderately or significantly post prandial but it comes back down quickly. So I am watching it.) What I am very confused about right now is the accuracy of the BG meters. Being so borderline, 15 to 20 points is ALL the difference in perfect and “hmmm better watch it.” After more research and comparing a couple of meters I am sort of freaking out that all my readings are biased, except the 96 from the lab was consistent with my own morning readings, so I have to have some trust in my meter. I was trying another brand because the Accu-Chek strips are so very expensive. New one was giving me reading in the 70’s. headdesk. headdesk. I do see that one may be biased low, although I see many complaing about the Accu-Chek being high, although the data I see looks like it doesn’t have much bias compared to all the others. Over all I am terribly upset about the lack of accuracy. For the point I am at, the lack of accuracy makes them almost useless. I am trying to test foods and exercise, and if there is bias I can’t know what brings me to normal.

    • David Mendosa at

      Dear Alex,

      Like you, I use the A1CNow Self-Check meter, although I probably use it more often than you do, since I test once a month. It is actually more accurate than most tests that you will get in a doctor’s office or a hospital lab, because it is standardized to the DCCT (the gold standard) and many other tests aren’t. I have found in my own experience that using this device is much more important than fingerstick devices, because I follow a very low-carb diet and don’t take any medications that would drive me to having a hypo. My blood glucose level hardly changes at all during the day. My guess is that yours doesn’t either when you stay very low carb.



  • John Top at


    I would love to use this test kit if it were accurate.

    It has mixed reviews.

    Do you find it accurate?

    • David Mendosa at

      Dear John,

      Everything has mixed reviews. Some people delight in spreading their negativity. In my research and personal experience it is more accurate than most A1C tests that you can get at your local hospital, lab, or doctor’s office. As I wrote at http://www.mendosa.com/blog/?p=400 , it is certified by the NGSP (formerly the National Glycohemoglobin Standardization Program) as having documented traceability to the Diabetes Control and Complications Trial reference method, which established relationships between A1C levels and risk for complications of diabetes. The DCCT method is the gold standard for reliable A1C testing. The complete list of A1C tests that the NGSP certified is online at http://www.ngsp.org/docs/labs.pdf

      Is your local hospital, lab, or doctor’s office on that list?