Blood Glucose Numbers And Acronyms Demystified

Diabetics know that their blood glucose numbers are important, since all forms of diabetes present as some amount of difficulty in maintaining proper glycemic control i.e. maintaining the blood glucose level. Shamefully, those involved with the relevant clinical assays have not made matters easy to understand. Indeed, many physicians are unclear on the details.

The “junk” aspect of this is the way the numbers are manipulated—often to promote drug sales—and the free-wheeling manner in which such concepts as eAG (explained later) can be introduced as valid, or even necessary. This little article is my attempt to elucidate the important parameters, and some of the science behind them.

Starting with the basics, as in blood glucose concentration, two units of measurement are utilized: milligrams per deciliter (mg/dL) and millimoles per liter (mmol/L). There is simply no need to have two units extant, considering that both of them are metric. I suspect the reason that many countries prefer the cumbersome SI units (mmole/L) is because clinical labs in the US tend to employ the conventional (mg/dL) units.

To convert from mg/dL to mmol/L, divide the former by one-tenth the molecular weight of the compound of interest. (For glucose, that would be 18.0155899.)

Depending upon the source cited, normal fasting blood glucose (rigorously plasma glucose) falls in the range 70-110 mg/dL (3.89-6.11 mmol/L). A blood glucose level of 126 mg/dL (6.99 mmol/L) or higher after an 8-hour fast, if repeated on a different day, is a definitive diagnosis of diabetes.

However, the blood glucose reading is only a snapshot, reflecting the situation at the precise time the blood is drawn (or the finger pricked and tested with a home meter). Absent outfitting the patient with a continuous glucose monitoring device, the standard method for determining a nominal “average” blood glucose takes advantage of the fact that glucose binds slowly to blood hemoglobin (Hb), producing glycated Hb.

Chromatography of normal adult blood separates two main classes of Hb: HbA (92-94%) and HbA1 (6-8%) where the beta chain of the hemoglobin has an additional glucose group. As it happens, the HbA1c subgroup is the most useful for clinical analysis. With higher blood glucose, more HbA1c will be formed. Red blood cells circulate for 60-120 days, and the HbA1c level is in part affected by blood glucose levels over a three-month period. However, it is heavily weighted to levels over the past 45-60 days.

HbA1c values are reported as the ratio of this particular form of glycated Hb (HbA1c) to total Hb—expressed as a percentage. Depending on the source cited, an HbA1c of 6% or less is normal. Even though you will tend to see these percent values, there is a trend to report HbA1c as mmol HbA1c/mol total Hb. If any readers understand the point of this beyond generating more confusion, please let me know.

By the way, if you would like to perform the conversion, take your old-fashioned HbA1c expressed as a percent, and subtract 2.5. Then take that number and multiply it by 10.929. For example, a 7.0% HbA1c converts to 53.01 mmol/mol.

Inasmuch as HbA1c is weighted to more recent blood glucose levels, it does not represent a true running average, even if studies have shown that broad brush, it is not terribly far off. Still, relating HbA1c values to those readings that are obtained with home meters is not intuitive, say diabetes educators. That’s where the A1C-Derived Average Glucose (ADAG) Study comes in.

The objective of the ADAG Study (2006-2008) was to define the mathematical relationship between HbA1c and estimated average glucose (eAG) and determine if HbA1c could be reliably reported as eAG, which would be in the same units as daily self-monitoring. HbA1c was compared with a combination of continuous glucose monitoring and frequent finger stick glucose measurements. Investigators were able to derive an equation so that HbA1c levels can be interpreted accurately as an average glucose level or eAG.

A formula was derived such that 28.7 times HbA1c expressed as a percentage minus 46.7 = eAG expressed in mg/dL.

It is noted that many assumptions were made to create the eAG formula, and some in the diabetes community are not convinced of its accuracy or utility. Professor Ian S. Young, MD FRCP, of Queen’s University Belfast argues that since no single relationship exists between average glucose and HbA1c, it is not possible to have a single equation that expresses this be applicable to all patients. It is well known, he says, that two individuals with identical glucose profiles over a period of months will not have the same HbA1c.

He also takes issues with the relatively small sample size and the limited numbers of non-Caucasians.

From my vantage point, there does not seem to be a whole lot wrong with conventional HbA1c numbers, used in conjunction with self-monitoring of blood glucose (SMBG)—given the availability of easy-to-use continuous glucose monitors, for those who need them.

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21 responses to “Blood Glucose Numbers And Acronyms Demystified

  1. I stopped having A1C’s done when my physician could not tell me what value the test had. Meter companies will tell you that finger stick tests and A1C’s will not correlate and are not supposed to. Doctors often will not believe this even if the company calls the doctor. Then there is the “magic number” component–having “6” or below. I have read of diabetics who avoid all carbohydrates and will even pass out trying to get blood sugars low enough to reach the magic number.

    Originally, the A1C did serve the purpose of flushing out diabetics who only tested when they knew their blood sugars were near normal but avoided it after eating donuts or drinking soda. With today’s intensive testing and meters with memory, it’s not really necessary.

    Also, my doctor gave me the printout showing the “averages” after the last A1C I actually did. I used a home test. The interesting thing was the test had a listing of averages, too, and they did not match what my doctor showed me.

    I have fought for years with doctors and medical persons over the accuracy and usefulness of the A1C and all diabetic testing. Glucose meters can be 20% off “true” and that is an acceptable error according to the FDA. However, with a blood sugar reading of 300, that gives a range of 240 to 360. More accurate meters would be nice….

    • @Reality–

      Driving this whole mess is drug sales. The magic 125 mg/dL number was lowered from 140 in 1997, and *that* was lowered from 160 some years before. Of course, the lower this gets, the more diabetics and “Pre-diabetics” there are.

      The bellwether papers supposedly proving the dire consequences of poor glycemic control show nothing of the sort. Don’t take my word for it, look them up. Differences between normal and type 2’s on various neuropathies are maybe 5-6%.

      In fact the ACCORD study, that was going to show how important tight glycemic control is, had to be STOPPED, since far too many of those doing tight control (mostly type 1’s) were dying.

      HbA1c is reasonably accurate (probably within that 20 percent figure), but may well not agree with your averages of home testing, unless you are testing a lot—say 10 times per day.

      More accurate meters *would* be nice, but no one would want to pay the price. The test strips, made by the billions in China, are good enough for most purposes. You can also look into the continuous glucose monitors, which are kinda cool.

      Finally, no one dies of hyperglycemia per se, but many folks die when they go hypo. I think this happens *way* more than is acknowledged, and the cause of death is written down to be something else.

      I knew a guy who (foolishly) wore a MedAlert bracelet saying he was a type 2. EMT’s were called to his house for some complaint, and (not surprisingly) under stress his glucose went high.

      The brain-trust paramedics shot him with insulin–and he died.

      • Very true on low blood sugars and causing death, especially in type 1. Plus, doctors don’t always tell type 2 diabetics they can get hypoglycemic on oral medication or insulin.

        I really agree with you on the “moving the goalpost” with blood glucose. The diet industry also went nuts with the “glycemic index” and trying to tell people your blood sugar should never exceed 80 no matter what. No one seems to understand the natural variation in both diabetics and non-diabetics.

        Actually, you can die of hyperglycemia if it gets high enough. I found out I was diabetic when I went into a coma and woke up in intensive care. My blood sugar was 850. It is much more rare today than in the past. Usually it is the complications that kill, unlike with hypoglycemia.

        • “Actually, you can die of hyperglycemia if it gets high enough. ”

          Are you saying a non-diabetic can die of hyperglycemia?

          • No, I did not mean a non-diabetic can die of hyperglycemia. I read Mr. Shaw’s statement to mean diabetics. As far as I know, only if one’s pancreas stops working (islets of langerhans actually) can one’s blood sugar rise to levels that are lethal. I don’t know that non-diabetics can die of low blood sugar either. Sorry for the confusion.

            • @Reality and Gamecock–

              Be careful of the word games. Either a hyper or hypo could likely be diabetic, since a normal would never have his glucose go below 70 after missing meals or above 250 even right after eating tons of carbs.

              That 850 must be some kind of record! But, even then, not sure that being that hyper—all by itself—would have killed you.

              Also, don’t forget “liver dump,” whereby type 2’s will crank the equivalent of 45-60 grams of carbs into their blood (via glycogen breakdown) if they have not eaten in 4.5 – 5 hours. BTW–Preventing liver dump is mostly all that metformin does. You’re better off eating regularly!

            • I am trying to avoid word games, but this whole mess has so many words and terms, it’s proving difficult! There are people who claim to be “hypoglycemic”–though when I have used my meter to check their blood sugar, it really does not show low blood sugar. It was kind of a “disease de jour”. Also, in a non-diabetic, the pancreas stops kicking out insulin when blood sugar normalizes, unlike diabetics whose insulin works at a specific rate independent of blood sugar.

              The 850 actually is not a record–I was told of a drug addict who had a blood sugar of 1300. Also, a diabetic who reached 900 right after taking prednisone and one who reached 900 who had failed to check glucose levels and his pump had plugged off. I don’t know if the hyper alone would have killed me, but that’s what my doctor seemed to think. Perhaps it’s best not know!

              Yes, the glucagon does come into play. It happens in type 1 also and is associate with the “Somogyi effect” where an undetected overnight low blood sugar triggers glucagon and other hormones and pushes the blood sugar up. You are definitely right on no skipping meals–it only makes it worse.

            • K. Understood now.

            • @Reality–

              Sorry. I meant be careful not to get caught up in the terminology rabbit hole.

              Yes. I remember the hypo fad of the 1970s. It disappeared when the personal meters were introduced, for the reasons that you already mentioned. These pseudo-hypo morons weren’t hypo at all. Rather they were tired.

              You probably remember “tired blood”—an earlier fad. Take Geritol for iron-deficiency anemia.

              As to the 1300—yeah, it must have been an addict in crisis. Numbers can go way hyper under stress. Even “mild” stress can crank them 30 points–easily. (Probably less in a non-diabetic.)

            • “Be careful of the word games. Either a hyper or hypo could likely be diabetic,”

              10-4, I know.

              Just yesterday I helped a diabetic in borderline hypoglycemic shock. Happens once or twice a month. My golf buddy’s wife is insulin dependent, and she SCREWS up the balance between insulin and carb intake TOO frequently.

              I did have a girlfriend in high school who was hypoglycemic. Took gelatin pills for it, as I recall. I thought it was a disease for the uppity, like gout used to be thought of. Anywho, if real, as I assume it was, she was hypo without being diabetic.

  2. Now, this stirs a question that I’ve been pondering for a long time: are those home finger-prick tests really useful? I’ve been told that sugar-free, zero-calorie soda causes the home meters to show an increase in blood sugar, but that makes no sense to me, absent a significant explanation of how non-sugar intake becomes sugar in the bloodstream. So, which is it? Are the blood sugar testing meters used “at home” measuring actual sugar levels, or measuring a proxy that works under “average” or “most of the population” conditions, or is the non-caloric compound somehow being converted to sugars in the bloodstream, or are those home tests just plain bogus?

    • As I mentioned above, the results have a +/- accuracy of 20%. The cheapest strips are worse yet. I spent a month comparing cheap generic to name brand strips to get my insurance to cover the name brands. The cheap ones can be off by 40 to 50%.
      Not only do the test results not match A1C’s, they also don’t match lab tests much of time. The meter people will again explain that the measuring techniques are different. My labs can be 100 points different from the test I did 5 minutes before the blood draw. Strips measure the change in electrical resistance, I believe. Originally they were colorimeters that could be very far off–if you did not calibrate the meter daily, you could be hundreds of points off.
      The tests are not bogus. Part of the problem is that testing in the lab is different from home testing and A1C’s are totally different. My glucose meter tests did average out close to what my A1C’s were when I actually had the tests done. Not precisely, but within an acceptable range (as noted, this is not always the case). This is a quantum leap over peeing on testape and checking urine sugars, especially when I did not start to spill sugar into my urine until my blood sugars approached 200. Those were really, really inaccurate, but still somewhat better than guessing. I find myself comparing tests done in two different meters if I am not sure of the results. Also, you do have to follow proper testing procedures–dirty hand, etc, can affect results. If you have a background in science, as I do, it certainly helps.
      The diet soda question–in my experience, if my blood sugars go up due to “diet” soda, it may not be diet soda, but regular instead. This happens most often in soda fountains in convenience stores, etc, where someone has hooked up the wrong syrup. There was a product that could test for urine sugar that also would tell me if the soda was real. I haven’t seen it for a while. Mostly, I just steer clear of anything but canned soda. So far as I can tell, diet soda does not raise blood sugar and I do drink a fair quantity of the stuff.

      • Good post, Mr. Check.

        Perhaps you, or Mr. Shaw, or others, could answer a question that perplexes me.

        My older brother has Type 2. He is on medication, but he is not insulin dependent. Every morning, he does the finger-prick blood sugar test. What I can’t figure out is why. If it is high, it’s because he did something stupid the day before (ate too much of the wrong thing). He would likely know he did the something stupid. And even if his blood sugar level was elevated, he wouldn’t do anything. So why test? The test, from my view, is onerous. Does the joy of seeing an acceptable number exceed the pain of the prick?

        • To determine a trend, specifically, to determine if his condition is deteriorating. If it starts reading consistently high or wildly fluxuating no matter what his actions, that indicates that he might need to start taking insulin.

          In environmental, we have lots of tests (especially water tests) that are promptly put into a folder and forgotten. However, they are all pulled out next permit renewal so that the state can determine if there are any additional requirements that they want to stick on us.

          Still, I think that if he’s stable it might not be worthwhile to do daily. That level of testing seems excessive. However, that’s between your brother and his doctor.

        • First, the finger pricks are not as bad as they used to be. Type 2 can often use their forearm rather than finger. They also make gentler, thinner lancets.

          As benofhouston noted, it determines a trend. Some Type 2 diabetics will modify their diet, ie leave out some carbohydrates, if their blood sugar is high. Since type 2 is often controlled solely by diet, modifying the diet is the response to the test.

          Also, as noted in my first response, diabetics often just test and ignore. That was one of the reasons the A1C came into use. Some diabetics test and then their doctor determines what response is needed. I had an employer who was type 1, who never adjusted his medication or diet, only his doctor did. So the tests may just stay recorded in a person’s meter until a medical professional looks at them (much as benofhouston notes).

        • Thanks, guys. Makes sense. I had the perspective of it being an acute test, which seemed useless. I see the long term value.


          “In environmental, we have lots of tests (especially water tests) that are promptly put into a folder and forgotten.”

          I worked for the SC Pollution Control Authority in 1971. We took water samples, tested them, then sent the results to Columbia, where they were ostensibly filed. We got enough phone calls from Columbia asking us to check our records when there was a pollution event that we realized Columbia just put them in the round file.

      • Westchester Bill

        I am Mr. Diabetes and have been for two to three years. My experience with finger pricks is that the readings make sense and conform to my A1c readings. At the doctor’s office last Tuesday my finger prick reading was 398 and my A1c was 12.7.

        What was of interest to me and might be to somebody else was that I hated diet soda until about a year ago, and then my taste changed completely where full sugar soda is too sweet and diet soda tastes good. I use unsweetened ice tea as my major thirst quencher.

        My beloved Uncle Roy was a many beers a day alcoholic until about age 62 when something clicked. I was astounded when I saw him throw away part of his first can of beer.

        The instant article was terrific. My thanks to Mr. Milloy for passing it along to us.

        • It’s actually nice to read someone say their glucose was 398, though not because that’s a good number, but rather the glucose monitor commercials show 108 and the like, which is not realistic at all! If every diabetic only saw 100’s on the meter, they probably buy a new meter and see what the deal was!

          I guess I just got used to diet soda (it had saccharin, then cyclamates, in it when I started drinking it.). The soda industry has been trying to make diet soda more palatable in a effort to reach more consumers. Interesting observation, though.

      • Thanks for your reply. To clarify: this is only with regard to reasonably trustworthy sources of zero-calorie soda: cans & bottles. My wife has been at war with diabetes for many years, and relayed the suggestion that blood sugar levels are affected by the zero-calorie sweeteners.

        That’s why I asked the question: do the home meters actually measure blood sugar directly, or are they using a proxy that may be affected by other things (like blood resistance, or a more readily detected non-sugar molecule, etc)?

        Anybody have an answer for that?


          It’s more or less direct. I did just realize that some sodas may be sweetened with sugar alcohols (though these are not zero calorie). Most of what I drink is sweetened with aspartame. Sugar alcohols theoretically could raise blood sugar.

          That being said, every person is different. If your wife has a blood sugar of 100, drinks diet soda and tests and hour later and the blood sugar consistently increases, that would certainly indicate the soda is the source. I usually try the test with two different meters and multiple times just to be sure. No one knows for sure how each diabetic will react to any food so yes, it’s possible. I know that allergies can raise my blood sugar, even food ones. I would never tell anyone it’s impossible something raises blood sugar–it may be unique to your wife. And there is always the possibility of other factors affecting any medical test.

          I have the same questions on the A1C–what other factors besides blood sugar affect it? It’s not as straightforward like doctors think.

          • @Reality–

            Although there are rare conditions that can render the HbA1c not indicative of average blood glucose, for the most part, it’s really not too bad.

            I suppose one could sabotage his HbA1c by switching to a very high (or zero-low) carb diet for 30 days before he would be tested.

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