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Why Hemoglobin A1c Is Not a Reliable Marker


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Over the last few years doctors are increasingly relying on a test called hemoglobin A1c to screen for insulin resistance and diabetes. It’s more practical (and significantly cheaper) than post-meal glucose testing, and it’s less likely to be skewed by day-to-day changes than fasting blood glucose.

What is hemoglobin A1c?

Sugar has a tendency to stick to stuff. Anyone that has cooked with sugar can tell you that. In our bodies, sugar also sticks – especially to proteins.

The theory behind the A1c test is that our red blood cells live an average of three months, so if we measure the amount of sugar stuck to these cells (which is what the hemoglobin A1c test does), it will give us an idea of how much sugar has been in the blood over the previous three months.

The number reported in the A1c test result (i.e. 5.2) indicates the percentage of hemoglobin that has become glycated (stuck to sugar).

Why is hemoglobin A1c unreliable?

While this sounds good in theory, the reality is not so black and white. The main problem is that there is actually a wide variation in how long red blood cells survive in different people. This study, for example, shows that red blood cells live longer than average at normal blood sugars. Researchers found that the lifetime of hemoglobin cells of diabetics turned over in as few as 81 days, while they lived as long as 146 days in non-diabetics.

This proves that the assumption that everyone’s red blood cells live for three months is false, and that hemoglobin A1c can’t be relied upon as a blood sugar marker. In a person with normal blood sugar, hemoglobin will be around for a lot longer, which means it will accumulate more sugar. This will drive up the A1c test result – but it doesn’t mean that person had too much sugar in their blood. It just means their hemoglobin lived longer and thus accumulated more sugar. The result is that people with normal blood sugar often test with unexpectedly high A1c levels.

This confused me early in my practice. I was testing blood sugar in three different ways for all new patients: fasting blood glucose, post-meal blood sugar (with a glucometer) and A1c. And I was surprised to see people with completely normal fasting and post-meal blood sugars, and A1c levels of >5.4%.

In fact this is not abnormal, when we understand that people with normal blood sugar often have longer-lived red blood cells – which gives those cells time to accumulate more sugar.

On the other hand, if someone is diabetic, their red blood cells live shorter lives than non-diabetics. This means diabetics and those with high blood sugar will test with falsely low A1c levels. And we already know that fasting blood glucose is the least sensitive marker for predicting future diabetes and heart disease. This is a serious problem, because fasting blood glucose and hemoglobin A1c are almost always the only tests doctors run to screen for diabetes and blood sugar issues.

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Another condition that affects hemoglobin A1c levels is anemia. People who are anemic have short-lived red blood cells, so like diabetics, they will test with falsely low A1c levels. In my practice, about 30-40% of my patients have some degree of anemia, so this is not an uncommon problem.

What blood sugar markers are reliable?

Testing accurately for blood sugar is like putting pieces of a puzzle together. Fasting blood glucose, A1c and post-meal blood sugar are all pieces of the puzzle. But post-meal blood glucose testing is by far the most reliable and accurate way to determine what’s happening with blood sugar, and the most sensitive way of predicting future diabetic complications and heart disease.

For more on why post-meal blood sugar is a superior marker, read my article When Your Normal Blood Sugar Isn’t Normal (Part 2). To learn how to test your post-meal blood sugars at home, and what healthy targets should be, read my article How to Prevent Diabetes and Heart Disease for $16.

Another useful – but underused – blood sugar marker is fructosamine. Fructosamine is a compound that results from a reaction between fructose and ammonia or an amine. Like A1c, it’s a measure of average blood sugar concentrations. But instead of measuring the previous 12 weeks like A1c, fructosamine measures the previous 2-3 weeks. And unlike A1c, fructosamine is not affected by the varying length of red blood cell lifespans in different individuals. Fructosamine is especially useful in people who are anemic, or during pregnancy, when hormonal changes cause greater short-term fluctuations in blood glucose levels.

To put the most accurate picture together, I like to have all four: fasting blood glucose, A1c, post-meal glucose and fructosamine. But if I only had to choose one, it would definitely be post-meal glucose.

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  1. Time for Dr.s to start looking outside the box. My A1C is always normal, but my sugars are super erratic. Long story short I was going from Dr. to Dr. with no one finding this problem. At 5′ 3″, 100 lb woman that doesn’t drink, smoke and exercised. Fasting sugars were fine, A1C was fine. Problem was I was passing out, having heart palpitations, thirst and a boat load of issues. Finally I used my mother’s sugar tester. (she was pre diabetic at that time) We shared the same Dr. and I tested my sugar an hour after us eating. My mother’s was normal mine was still over 200. My Dr. felt horrible because she missed it. Its way too long of a story but my Dr.s are calling me rare. I am not sure that I am. My sugars spike and drop so quickly making daily life exhausting. Thankfully now I know why and do not feel crazy any longer. I get a lot of reactive hypos ( will get a 50 reading after eating 20 minutes earlier). Its so unpredictable that I can’t use insulin for my highs. Just throwing this out there if it can help someone. No, there is no cure for this yet, but at least I know that I am not panicking, that this isn’t in my head. My body is in fight or flight response when this happens. I think its time for Drs. to send blood sugar testers home with patients with symptoms. At the very least, I know there are some days I should not be on the road. There are some days that exercise is something I have to avoid. Again, its about getting to know your condition. It is my belief that the pancreas and autoimmune issues need to be studied. Anyway, thanks for the article!!

    • lori, i think i’m very similar to you. i was diagnosed as prediabetic years ago thanks to a doctor who actually ordered the OGTT for me instead of just using FBG or A1C (this was before A1C was tested, really, thank god — my doc now thinks the A1C is the be-all and end-all of diabetes) — on which my BG spiked at 198. 2 points below diabetic! anyway — i’m pretty sure i’m like you — that my blood sugars don’t just go high, they also go low. when i had the hyperglycemic episode that led to my doc diagnosing pre-d, i also had a hypoglycemic episode after where i almost fainted in my car in the parking lot of a drugstore, though i managed to get myself into the store and buy and drink a bottle of OJ…

      anyway, i had a question for you, because i feel like i’m not able to find ANY info on my condition and symptoms, and i’m wondering if you’ve experienced something similar. (and my current doc is of no help, as he looks at my A1C, says it’s fine, and refuses to send me to an endocrinologist.) i’m wondering, do you have any problems with extreme “dawn phenomenon”? i wake up, without fail, after about 4-5 hours of going to sleep with racing heart, dry mouth, and the need to pee. (and my BG is normal, and i’ve been eating keto). everything i find says to deal with this by increasing carbs at mealtime, which i did, so now i’m just low carb, not keto. but it doesn’t seem to help much — my guess is because my body just deals with it as quickly as possible, as yours does. i’m afraid to increase my carbs further because i don’t want my BG to spike. i’m not sure WHAT to do. and, i’m pretty sure this happens to me during the day, too — i get pretty cranky before dinnertime, but if i check my BG, it’s fine (80s) or even a bit on the high end of normal (low 90s). my guess is that just before that happens, i’m having a hypoglycemic episode, and my body is then releasing cortisol to increase my BG, resulting in my heart rate increase, crankiness etc…

      is this similar to anything you’ve experienced? how do you deal with your body’s tendency to veer wildly between high and low?

      • Yes, I have experienced all of that. There is more going on with you.
        What I would do is get a hold of a sugar tester. Drink a coke and test your sugar in 10 minutes, then 10 minutes later, then another 10. You should not have a reading of like you had. When I was sent to my endo, she said ” can you raise your blood sugar for me?” I said yes and chewed a bunch of bubble gum and drank some OJ. It went over 200. She was baffled because of my other tests. When I had the blood glucose 4 hour test at the hospital the highs were not as bad as my lows – but still too high for a functioning pancreas. Best Dr. told me my pancreas was like a wire shorting out. Anyway, the endocrinologist told me within 5 years I would be insulin dependent. I was a type 1 diabetic. 10 years later I am exactly the same. She is blown away and has contacted universities. What have I learned since? I am certain I have issues with the a region of brain called the hypothalumus which coordinates both the autonomic nervous system and the activity of the pituitary, controlling body temperature, thirst, hunger, and other homeostatic systems, and involved in sleep and emotional activity. When you have time take a look at this. I also have the coldest hand, feet that turn blue, my sugars tank if I am heat.
        I could go on and on. A saliva test showed my cortisol levels were out of control among many other things.
        I feel terrible for you. Why? Because after 20 years of me dealing with this there has been no help. At least the Dr.s not only believe me, they now can see it on me. At least I have some vindication.
        I do take my diet seriously and see that I have gluten sensitivities that worsen with “flares”. That being said its still like trying to kill a swarm of fruit flies.
        I now take Ambien to sleep or I would never sleep through the night. I know its not a great solution, but my body needs a break from the fight or flight response it goes through. I don’t abuse it. I can’t take beta blockers for the tachyardia ( racing heart) because we do need to know when our sugars are getting too low. No Dr. would prescribe them to me. I know its a scary feeling and I am so sorry you are going through this. Please get the sugar tester and start testing for a week straight. Take pics of your results and find a new Dr. Tell them you are suffering. This takes its toll on the body.
        Also, if it helps I had other tests that were very abnormal. I had the highest homocystein count my hemotologist had ever seen. My ferritin level bottoms out, and I have chronic anemia. I have to get infusions. I have chronic low B12, and D levels. I have the mthfr gene mutation. Sorry this is so long. I know I am not alone and if I can help one person by seeing this, it is worth my time. I will be praying for you. Please ask me any questions!!

  2. Yesterday I went for a blood test. My fasting blood sugar measured 110 and my A1c measured 4.6. I’ve been on a high-fat paleo/primal diet for the past four years and practice daily intermittent fasting with bulletproof coffee. My diet tends to fluctuate between low carb and moderate carb — definitely not keto. I don’t seem to have blood sugar issues, as I am typically able to go a long time between meals when necessary without mood/energy swings.

    Can I trust the 4.6 A1c reading and write off the 110 FBS as a result of intermittent fasting? Or is 110 a high enough number to cause my red blood cells to turn over at an increased rate, thus cancelling out my low A1c numbers? I know I should start monitoring my post-meal blood sugar (I’m setting that up), but for now I’m trying to make sense of these numbers.

  3. I was very disturbed by my Hba1c levels. Last year i weighed 160lbs at 5’3 with a bf of 13-15% and A1C was 5.5 and fasting glucose at 91. This past week my A1C was 5.7 and fasting glucose was 91. Keep in mind that i was dieting for 4+ months and am down to 147 lbs with 10% bf. I am an athlete and dont understand how this is possible??? a) did they mix up the labs? b) was glucagon releasing glucose in to the blood as a result of caloric restriction? would love to know!

    • Do you eat a lot of sugar heavy foods? Granola bars, tomato sauce, salad dressings, rice, flavored drinks etc?

      • Yes, However; wouldn’t it be lower regardless of choice of food because my body was in a calorie deficit and i was losing weight?

        • no, it wouldn’t be lower regardless of food. if you eat foods high in carbs, those carbs are converted to glucose and enter your bloodstream in order to make it either to 1) your muscles for usage or 2) to your liver for storage as glucagon or conversion to fatty acids (in your case, it’s the former, but still, either way, the glucose gets there via your blood). while in the bloodstream, some of that glucose gets stuck to your red blood cells. doesn’t matter that you later burned them in your muscles — your RBCs and therefore your A1C is affected.

  4. Thanks for the article. I am also trying to make sense of my annual check results. Fasting blood sugar was 89, but A1C was 6.1! Not sure what that means, or if i should be concerned. I thought those two stats would be in synch. As someone else mentioned, may be the A1C result was wrong by lab? Years ago, before i started exercising regularly, my fasting sugar was higher (low 100s), and at that time my A1C was around 5.6 – so both stats were consistent to show a slight elevation. So this times, these numbers make no sense…

  5. Hello. My A1C has been between 5.2 and 5.5 for the past several years. Which I understand is ‘normal’ being ‘not diabetic’. However, my fasting blood sugar has been over 100 for the past several years as well. the highest was 109. Last month A1C was 5.4 and fasting was 107. So my doctors is saying I’m ‘pre diabetic’.
    I bought a blood test meter and started testing last night. 2 hours after eating it was 122. Then two hours later before bed it was 97. This morning, 10 hours of fasting, it was 112. Higher than before I went to bed. I went to the gym and swam 500 meters and came home and retested and it was 98.
    What is going on that it was higher this morning then last night? And do you think there is really anything I need to be concerned with?

    • Sorry – I should also say that for the past 4 years I have been mostly vegetarian. I never drink sodas or eat sweets, cookies. Never any bread. Pasta maybe once a month. Raw vegetable smoothie most days for breakfast. Organic when possible. No processed foods. Fried food maybe once a month. Little alcohol. When I eat meat it’s wild fish or organic chicken. I eat avocados, coconut oil, nuts, cheese, kale, spinach, tons of other greens, oranges, bananas, apples…..

      • Hi Jeff, you are smart for researching your question. I think Chris Kresser is a very informed person, read all his articles and it should help. The other thing you should be aware of, is not all people are the same, some may have elevated glucose levels where others are lower and be perfectly fine for their entire life. There is a lot to be weary of with conventional medicine too. I could elaborate, but I think you would be better off looking into it yourself as you already are. Look into google searches about facts about Diabetes and other big health issues, but don’t only look at main stream conventional medicine, but alternative views such as Chris Kresser, Dr Julian Whitaker and Dr H. Gilbert Welch. Hope this helps

    • Jeff,
      In some of the research I have been doing it is not uncommon for everyone to have a slightly higher reading in the morning when you first get out of bed. A few hours before rising your body begins to prepare for activity by putting energy (glucose) into the system, so until you burn it off first thing and test, you have a higher reading. Seen that on several sites which makes sense but also points to how incredible the body is to prepare us for things.

      Here is a link from ADA with some other possibilities:


    • Your body releases sugar every morning to prepare for the day. You can look up “dawning effect”.

  6. Does donating blood regularly affect the A1C levels (lower them)? Could that account for normal A1C with slightly elevated fasting glucose levels?

  7. I’m 29 years old male from India. I recently went for a regular heath check up that my company organizes with a couple of labs. My fasting BS was 103 and post meal BS was 127. The doctor at the centre asked me to go for an HbA1c test. And I got 7.2! I have never suffered from any sort of diabetes related issues in my life so far. The doctor said that I have diabetes. I couldn’t believe her. I think I’m going in depression by thinking about this from the last 2 days. I don’t know what to do. Should I go for another test at a better lab? Please help.

    • You would not have any symptoms until diabetic for years . By then the damage would have set in. Read Dr Neal Barnards book on Reversing Diabetes .. It works . Your numbers are on the edge .. A diet change is needed . I made those changes and I have successfully reversed my diabetes .. My last HGA1C was 4.8 after 20 years on the medication . My fastings are in the 70s and 80s and my 2 h post prandial now runs 70- 80s now . Do not get depressed .. Read.. And Act . I follow a starch based whole plant food diet now Dr McDougall , Dr Esselstyn , and Forks Over Knives as well as Chef AJ. Stop it now and be healthy . Diabetes , even borderline will ruin your health if not managed and it can be done with diet change .

      • Actually you should get it checked at another lab. My A1C came back at 6.8 even though all my fasting sugars were below 100, and after testing my blood glucose around the clock for 3 weeks I averaged around 110. Finally I went to another lab and my a1c came back as 5.3. If you are in doubt definitely get it re-checked at another lab. Good Luck!

    • Hi, Anoop,
      Did you get a chance to get your HbA1c checked again from a NGSP (National Glycohemoglobin Standardization Program) approved lab? There are some in Mumbai and I think in Delhi.

    • Dear Anoop
      Read the book” Over-Diagnosed, making people sick in the pursuit of Health”, by Dr H. Gilbert Welch,MD.

      Everyone else, read it too!
      Don’t believe everything conventional medicine claims.

      You’re welcome

    • The normal blood sugar is 69-99 mg/dl & yours is above !!! My glucose was fasting 132 mg/dl for years & nobody told me !!!
      By the time I caught it years later it was 286 mg/dl after an glass of orange juice, tang & lemonade !!! I was diabetic !!! I lost 40lbs in a year & went to diabetes school @ my local hospital !!! You can do it too !!!

      • Melissa, you are following the conventional thinking of what they call Diabetes! Not all fit into that low or what you and conventional medicine believe. The fact is, over the years the numbers have been lowered by “experts” that have had ties with big Pharma, simply stated, they gain in their pockets the money that poor unsuspecting folks as yourself and far too many others believe. So often lab and home testing is not accurate anyhow. Most people with so called elevated sugar levels live perfectly healthy lives and usually die from something else. Big Pharma have so many dangerous side effects, even death! If you are terrified of having high numbers, then use natural remedies that so many educated people people such as Chris Kresser, Josh Axe, Julian Whitaker and on and on, have info on. I suggest you read the book, “Over- Diagnosed, Making people sick in the pursuit of Health”, by Dr H. Gilbert Welch MD. Anxiety is so much worse and that is exactly what is caused by persons that don’t have the FACTS!!! Feel free to discuss this with me, AFTER YOU READ THE BOOK.

      • Melissa, another thing, you state your sugar level is 286 after having OJ, Tang and lemon aid, omg, anyone is going to have a higher level, 286 is actually low after basically drinking a glass of high sugar drink.

        Sure hope you read that book and relax and enjoy life

        • Actually that isn’t correct. I have a healthy skepticism of doctors as they almost killed me on more than one occasion. But advising people to ignore something isn’t smart either.

          We tested my boyfriend after eating a carb heavy meal. His blood sugar spiked to 123. That would be more normal. A healthy person would never have a blood sugar of 268. Insulin would be released and would be effective in removing the extra sugar from the blood.

  8. Setting aside the cost and inconvenience, what are your thoughts on CGM (continuous glucose monitoring) as a reliable and accurate marker for type 2 diabetes in comparison to FPG, PPG, and A1C?

  9. DR. Kresser, is there a test out there that would confirm someone has a slightly elevated A1C due to their “long living” red blood cells. That is, that I can know for sure that is the cause and not elevated blood gloucose levels?

    • Hi, I have a very similar question! what would be the reason behind good average blood glucose readings but having a high hba1c? I would be very grateful for a reaponse