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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. That’s a great relief. I lost my peace of mind with this A1C diab thing. I got 5.7 and I was told the same, insulin resistance issue could be there. Oh God, I’m trying for baby I do not want to get onto more pills n troubles.

    • on the positive side, if you did end up on metformin… it can raise your fertility level :). But still i wouldnt worry too much, but if you want to be ultra healthy, ditch the carbs.

  2. So i went to er tuesday with chest pain and palpitations. Found out i was having psvt. Thry found out from blood work that my blood sugar was 209 at the time. So i went to my dr the next morning for blood work. My a1c was 5.8 and bs was 100. He didnt seem to concerned about that. But i have been checking my bs at home, got 130 fasting and today i got 239 2 hrs after i ate a bowl of oatmeal with sweetner and a cup of decaf coffee with sweetner and cream.

    • Angela, honey, please stop eating like that. Please. I dont care what your doc says, that blood sugar is dangerous, it wont kill you quickly, but it will kill you. Research a high fat, low carb diet (not a low fat, low carb diet, not a low fat, high carb diet (which is what you just described) but a high fat, low carb diet. Please.

  3. My sister is 35 and a survivor of leukemia when she was a child. She has been diabetic for over 10yrs as a result of the chemo and radiation used for her treatment. Her daily readings are about 90-120. However, her A1C continues to skyrocket. A little over a year ago it was around 7.5 and is now up to 8.5. She’s barely 5ft and weighs only 90lbs. She’s not your standard patient since she suffers from several other late effects of the radiation, including osteoporosis. And she is very regimented regarding her diet. Any suggestions on why the A1C is so high?

  4. hey have you ever read Dr. walech “dead doctors dont lie” and his reomendation of cromium and vanadium for diabetes. is this a real deal you have heard about or is it a scam?

    • I have read an article about supplementing with Chrmium piconolite. It was grest article. I took the supplement and it increased my energy level noticably. It helps maintain sugar level. Worth a try.

  5. Hello Doc,

    I had a recent blood work come back with my Glucose serum at 92 and my A1c Hemoglobin at 6.1. I have made changes in my diet and excerise now. Am I at risk for diabities at 10 to 15 years from now or should I be fine? I also lost 8 pounds in the last three weeks from eating better.


  6. Just got back from the docs after 2nd blood test in 6 months, started high fat, low carb (under 50 grams) and med protein in feb this year, first blood test was normal back then according to doc. Today he said you are pre diabetic and have hyperlipidemia. TC was 291 and TG 205!! HBAc1 was 5.7 (NG SP), the blood test was taken first thing 8AM after a short fast of 12 to 14 hours since night before. For 2 months have had no starches, only veggie greens and protein and fats. Now my family wants me to stop eating this way ASAP! Don’t know what to do? 46 yrs, 82kg and 12% body fat. Lost 10kg and halved the body fat in 6 months, so why am I now pre diabetic according to my doc? And must eat normally or I might kill myself….

    • Simon,
      I realize your post was a while ago yet would share what I was told.
      It may take a body a while to acclimate to a low carb diet.
      Richard Bernstein, MD, is a Type one diabetic who is close to 80 years old and still practicing medicine in 2014. He went low carb after he was almost killed by what he was told to do initially after diagnosis.

      Hope you were tested again before freaking out!!

  7. My first a1c test was 5.9. My doctor ordered a glucose tolerence test .My fbg was 94 and my 2hr post after 75 grams of glucose was 105. My a1c at that time was 6.2. I weight 128 lbs . I have lost about 7 lbs from when the first a1c was done until my the second one. Do I have something to be concerned about? There is diabeties in my family.

  8. What is the quickest way to lower my HB AIC I’m age 70 and my AIC is 6.6

    Thank you


  9. I had gestational diabetes with my last two kids. I have been eating healthy and walking five days a week for one hour. In January I had a A1C done,I had a 107 glucose and my A1C was 5.6. I had it done again two weeks ago now my glucose is 79 and my A1c is 5.7. I lost 45 lbs. Why did my a1c go up? My doctor said I am fine. He also did other lab work and everything came out fine. I dont want to become a diabetic thats why I am taking care of myself.I dont have high cholesterol. How often should I check myself?

    • A1c isn’t a particularly reliable marker for individuals as I’ve pointed out in this article. I’d go by your fasting glucose and, more importantly, your post-meal glucose. If you’re staying below 140 mg/dL one-hour after meals and 120 mg/dL 2-hours after meals, that’s a good sign.

      • My understanding is that a 2 hour window and a 3 hour window will give more accurate readings, especially for someone up in years and/or with some type of digestive difficulties.

  10. BTW I’m not a big Wal-Mart guy either but they do have home a1c tests that are about $9. You send your sample to a lab and get the results on-line. I did one the same time I had one from Quest Laboratories and it came within 0.1.

  11. Thanks for the info explaining long-lived RBC. This would explain the extended lag time for lowering my a1c, which continues to get lower each time I test it but not quite as quickly as I expected based on the assumption of an “average of the last 2-3 months”.

    6 months ago I was over 13, I’m just above 6 now, trying to get under 6.

    I do test regularly an hour after meals, and it is never over 130, as a result primarily of low carb eating.

  12. “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.”

    FYI Iron deficiency anemia actually causes an elevated A1c. This is a common misconception. Providers sometimes believe pts with microcytosis or a low MCV anemia, that the binding of sugar to the heme group is lesser than normal MCV levels or that the smaller the RBC some how reduced the A1c % concentration. Research shows the opposite is true.

    • I am a 73 year old male with early stage MDS. My RBC is 11.4, fasting BG is 133, and A1C is 7.9. My doctor says I need to start medication to manage my BG. What do you think? Do you think I can manage my BG levels with diet and exercise? I currently am on a diabetic diet and ride a bicycle about 30 minutes per day. I also walk about 30 minutes a day. My weight is 200 and I am 6 feet tall. My cholesterol and triglycerides are normal.

      • *I* think that a “diabetic diet” if you mean the ADA diet, is the worst possible thing for diabetics. Try low carb high fat, and keep your carbs down to around 30 a DAY, with no bread, sugar, pasta, or simple carbs, choose low glycemic veggies and a few berries for your carb count and you WILL reduce your blood sugar. Go to diabetesforum for support and tons and tons of advice and recipes, tell em I sent ya 🙂

  13. I had my AC1 test without fasting and the reading was 6.8 my doctor already wants me on metformin, can I try diet and exercise first or should I not wait and get on this medication. My glucose in my urine is at normal levels (84) Please advise

    • you can ALWAYS try diet and exercize first!! Look up Low Carb High Fat and diabetes and do what it tells you, guaranteed your a1c AND your real blood sugar will drop 🙂

  14. Hi Chris,

    Thanks for the great article. I am 34 yrs old and recently was diagnosed with type-II. My first A1C was 7.3 and the latest A1C i.e. my second was 5.5. However, I used to check my sugar level using glucometer during last three months – each day one reading – 1 hr before and 1 hr after (fasting, lunch, supper and dinner) and it consistently averaged 170-180. But suprisingly, my A1C was only 5.5. I also tried my glucometer with one other glucometer couple of times and it seems to be same. Do you think this is normal and I should rely A1C result (irrespective of the fact of RBC’s life). Do you think fructosamine test would be more reliable for me?

  15. I just got back an A1C of 5.8, which shocked me. I spent a few weeks testing my fasting blood sugar, and it ranged from 75- low 90’s. I also have tested my postprandial levels at 1 hr and 2hrs after purposely high carb meals (for testing purposes only). Prior to eating, I had an 84 mg/dl, then I ingested a bagel, 2 cookies, and a tall glass of orange juice. One hour after, I was at 123 mg/dl, and 2 hours after, I was at 98 mg/dl, and 3 hours after, I was back in the 80’s.
    My question is, my prior A1C was done during a routine physical in 2009 , and resulted in 5.2 . The blood work was done by Lab Corp, which stated a reference range of 4.0-6.0. This time, my doctor’s office ran the blood work themselves, and I resulted in a 5.8, and their reference range is ” 5.5% – 7.0%”. Could this mean that if my blood work were tested at Lab Corp again instead, I may have had a lower A1C?

    • A re-test wouldn’t hurt, but as I said in this article it’s not an especially reliable marker in individuals. If your fasting and post-meal blood sugars are consistently low throughout the day, I wouldn’t worry about a slightly elevated A1c.

  16. I have anemia and not much appetite. I recently had an A1C test and my Dr. said my A1C was 6.3. I am freaking out because he also said I am pre diabetic. I test my blood glucose 3x a day and it is always in the high 80’s in the morning, 90’s at lunch and night.

    • You need to test your blood sugar just before a meal, and then at 1- and 2-hours after. If you’re below 140 mg/dl at one hour and 120 mg/dl at two hours, and your fasting glucose is normally in the 80s, I don’t think you need to worry much about your A1c. As I said in the article, it can be affected by red blood cell turnover, which would be altered in anemia.

      • Hi dr. Kresser
        I have a number of autoimmune diseases. 12 in fact. Today I saw a new specialist for my diabetes. I have had 3 blood transfusion in the last six month. I have ever scope you can think of each time. I even swallowed the capsule camera and there is no sign of bleeding anywhere. I have celiac and biopsies were taken during the scopes and the results turned out to be normal. My blood sugars are uncontrollable at times If stressed they rise to the high 20s and if I take insulin before I get home I am having a low at 1.9. I will get up, blood sugars at 6, eat breakfast and take 5 units of humalog and get to the doctors and my blood sugar all have risen to the high 20s. Today the doctor mentioned in passing that it could be my body is rejecting the insulin. Can you explain this.

        • Hi dr. Kresser
          I have a number of autoimmune diseases. 12 in fact. Today I saw a new specialist for my diabetes. I have had 3 blood transfusion in the last six month. I have ever scope you can think of each time. I even swallowed the capsule camera and there is no sign of bleeding anywhere. I have celiac and biopsies were taken during the scopes and the results turned out to be normal. My blood sugars are uncontrollable at times If stressed they rise to the high 20s and if I take insulin before I get home I am having a low at 1.9. I will get up, blood sugars at 6, eat breakfast and take 5 units of humalog and get to the doctors and my blood sugar all have risen to the high 20s. Today the doctor mentioned in passing that it could be my body is rejecting the insulin. Can you explain this.

  17. Great article Chris! Thyroid can also skew the results due to its effects on erythropoiesis. I have a few hypothyroid patients with normal fasting and post prandials whose a1c’s were elevated. After getting them from hypo to euthyroid, A1c came down! Just another interesting tid bit!

  18. hi – I am like others and have been freaking out b/c my practitioner told me I was pre-diabetic with a HgA1c of 6.0 but my triglycerides are only 55 and HDL is 68, over all chol 198. I am 5’4″ and 125 lbs. Yes, I eat sugar and wheat but also eat veggies, good fats, etc. The 6.0 just seems so out of whack compared to my other numbers which have actually improved over the last year. Should I investigate more or just have another test in 3 mos. (I am making sure to exercise at least 30 min 5 days/week)? Thank you!

  19. Hi Chris, Interesting article you have. I have a lot of interest in this subject because I was concerned with my blood test that I had this past October. I am a 22 year old male in good standing health. I was shocked to see that my hemoglobin a1c level was 5.3%. The fasting glucose was 83 and I started to test my blood glucose at home with a home tester, and the highest 1 hour glucose reading I had ever had post-pranial was 134, and most often my one hour reading is between 90-120 depending on which type of carbohydrates I had eaten. Those seem to me based on my research as a person with a normal range. The main reason for my post is I was hypothesizing that maybe the fact is that I have high blood hemoglobin levels, and I am a competitive athlete, that is it possible that I have red blood cells that live much longer than 120 days? Do athletes that do a lot of altitude training in order to increase their oxygen uptake increase the lifespan of their red blood cells, thus verifying why my hemoglobin a1c was over 5%? My next blood test I am going to do a fructosamine to have a comparison. From the best of my knowledge, the highest fasting blood glucose I have ever recorded was 85 when looking at all my blood tests from age 15.

    • An A1c of 5.3 is perfectly normal. That’s nothing to be concerned about. Your other blood sugars are normal as well.

      • April 2012 A1c was 4.1
        August 2012 A1c was less than 4.1

        I’ve lowered my Lantus dose and will get lab test in 90 days.

        Is Is there anything else I should be doing?

        What are problems/risks with this A1c level?

  20. Hello I am awake and can’t sleep because my Dr. thinks I may be prediabetic! My finger pricks taken over two weeks show after a full meal at the end of the day 80 or 85 . But when I had a fasting (2x’s) Blood work my Blood Glucose is 6.3 . I am home and had to retire due to a back injury fom work and this has limited my activity levels so much. I am slightly overweight , my chol is a little high 230 my trigl are good and my good chol is good . Can I really be pre diabetic or can it be from the pain meds and other meds I take fo asthma ,that I got when I was 40 from severe re-flux(had surgery fo reflux in 2005) Last I took advir disk 250/50 just before fasting Blood work . No family member ever had diabeties, high blood pressure ,high chol ,yes! I’m scared. Thanks so much