Why Hemoglobin A1c Is Not a Reliable Marker | Chris Kresser
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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.

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.

256 Comments

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  1. Do you think altitude effects HGA1C? I live at 5800 feet. 58 year old female. My A1C is 5.8 – 6.0. I have BMI of 21. Eat 45 grams carbs/meal of 400 calories/meal. Exercise daily, wt lifting and aerobic. No diabetes in the family. Wore a CGM for 1 week and BS consistently 65-105, even after meals. I work with several thin physically active middle aged women with similar HGa1C in prediabetes ranges yet BS always acceptable. Very perplexed…

  2. MCD:

    Curcumin has been shown to be more effective than Metformin and can lower your A1c. Apple cider vinegar is also reported to lower blood sugar levels. Since the medical establishment has little interest in finding real solutions, you need to find your own and treat yourself. There are also other supplements that you can take. Good luck. Lastly, what does the color of your attackers skin have to do with the attack. Your description of him as black only perpetuates stereotypes. If you want enlightenment in your life act with concern for humanity and sensitivity for others.

      • Some of the reading I have done, claim that certain viruses introduced into the body can precipitate the onset of type 2 diabetes. Additionally having my attacker stab me thru my spleen and attempting to gut me, cutting halfway thru a rib may have also created such stress that it precipitated type 2 diabetes. The 3 inch hole in my left side was allowed to granulate in, hoping not to trap infection. 9 months later I was rushes to the hospital with an appendix 2.5″ in diameter and 7.5″ long. We suspected that infection introduced from the stabbing had caused the appendix to be such a large size. The black surgeon took pictures to memorialize the massive appendix, and did a great job in bringing me back to health. This same surgeon has performed surgery on 3 occasions and is the only one who I will allow to perform surgery on myself.

      • The guy happen to be be black and he said it that way because it was a FACT in the story. It is you that are a thin colorless skin bigot. Only liberals and Democrats look for subtext in everything. You could have just let it go but you were compelled by your warped sense of justice to chime in. Stick with being sick instead of stupid.

    • Angela,

      Ironically enough, YOU are the racist! Your reply to MCD included the following: “…Good luck. Lastly, what does the color of your attackers skin have to do with the attack. Your description of him as black only perpetuates stereotypes…”

      The only mention of race that MCD wrote was this: “…The black surgeon took pictures to memorialize the massive appendix, and did a great job in bringing me back to health…”

      You read the word “black” as a reference to race, which it was, and ASSUMED he was referring to the attacker, which he wasn’t. He was referring to his surgeon being black.

      Going forward, please do not assume that every attacker in a crime is black. I will give you the same advice that you gave to MCD, although you provided it in a misdirected way to him:
      “…If you want enlightenment in your life act with concern for humanity and sensitivity for others…”

      Thanks,
      TJ

    • Angela and Kiwi – you failed to read his story correctly AND you made assumptions because you read the word black. Stop looking for racism. Geez.

      • The race of the surgeon doesn’t seem like a salient bit of information, and makes me wonder why it was included if not for a touch of racism, or a tendency to gauge others by race.

        If racial heritage is relevant, he should be grammatically consistent and identify his skin color as well:

        “…The black surgeon took pictures to memorialize the massive appendix, and did a great job in bringing caucasian me back to health…”

  3. Yesterday I checked my blood sugar and the reading was 170. Today I had my a1c checked and it’s at 5.4. I checked my blood sugar again and it’s at 170 (~2 hrs after lunch). My Dr says that it is probably just stress raising the glucose reading. Is that possible?

  4. Each situation is unique but have you tried a starch based whole plant food diet such as advocated by Dr Jihn McDougall and others ? Having been diabetic for 20 years and now off all meds since this way of eating , is amazing. My sugars are in good control . I do a 2 hour post pea dial after dinner and adjust my exercise to that number .. 30-60 minutes on a DeskCycle . Just an idea . I had followed low carb and every other variation . This program allows potatoes, corn, rice …. Lentils, beans ( wow) all fruits and veggies plus seeds and nuts . Check him out . Good luck

    • Thanks! I just found out I am a body type 3 while browsing through a book on enzymes. They suggested a no protein diet. You just confirmed what I read.

      • The starch based is not no protein .. Just no animal proteins . Aside from taking care of my diabetes .. My weight is finally dropping after years of being heavy . But the starch based will address so many issues .. High blood pressure , gout , my atrial- fibrillation.. You have to make a commitment but it is well worth it

    • You had me at Potato and Rice 🙂 carbs are my weakness. My a1c read 5.9 at my last lab test but my fasting glucose is 85. I will def look into this diet and also the cycle since i’m on a computer 40hrs a week.

  5. Thank you for the article. I have always had normal fasting glucose results. The last time I was in the emergency room, I had my blood sugar tested an hour after eating a restaurant meal that included bread and a baked potato. My blood sugar was still only 91. Imagine my surprise when I looked at my lab results just now and found out my A1C is 5.6 (the lab reference range tops out at 5.4).

    I have chronic kidney disease. I also have abnormally large red blood cells, so I am wondering if the elevated A1C is caused by either of those things. We’ll see what my doctor says!

    • Sweetie, I am diabetic and on no meds. My a1c was 5.8 and my dr is ecstatic. Normal range is under 5.7.

    • My A1c came back 5.6. I live in Canada. Anyway two hours after I eat my blood is usually 14-18. And when I wake up in the morning it’s usually over 8. I am so confused. My doctor just said cut out all carbs and sugars for three months and do the A1c again.

      • I just did my yearly check up . Fasting read 76 and A1C is 5.9. I have eaten a lot of sweet before I went to test. My fasting number 76 has been the same for 10 years but my A1C changes since 2014 from 5.5 to 5.8 2015 and 5.9 in 2016. Any ideas

        • Most doctors do not know anything about nutrition . After Dr Neal Bernards book on Reversing Diabetes, Dr John McDougall and his books on a starch based whole plant food diet … My A1C was 4.8 last week . I have excellent control with eating a vegan diet with potatoes, rice, corn, beans , fruits , and vegetables . It takes a commitment to give up dairy , eggs, and all meat but my health is worth it . I am off all diabetic, cholesterol, and gout meds so far!

    • For all us smokers or nicotine users. To further support the idea that a1c test is inaccurate, this is what I found. I have also been diagnosed as a type two diabetic, I have an ok to poor eating habit, but I’m on the skinnier side of the weight spectrum. My a1c results showed numbers as high as 13.9. I did what the doctors recommend and too my meds, ate right and exercised, still nothing. After I stopped smoking, my daily average blood sugar went from a 565 to a 147. SMOKING WILL MAKE BLOOD SUGAR RISE THAT MUCH, DON’T CARE WHAT ANY DOCTOR WILL OR WON’T TELL YOU.

  6. Thank you very much for writing this article DR. I have never had a fasting Glucose above 90 so I was stunned when my clinic notified me I had an a1c of 6.8. Luckily my husband has a glucose monitor since he is diabetic. I have been checking my Glucose 6 times a day and highest reading is 131 after eating. My non-fasting glucose the day of my test was 88. Thank you for putting my mind at ease.

      • I’ve used my friend’s meter, but with my own lancet in the little thing that stabs your finger, and of course you use a fresh test strip each time. Can’t see any problem with that! She’s not saving numbers in the meter, just writing them down. Maybe the original comment meant that she did something similar. I don’t know much about these things, but I think I’ve seen quite a variety of systems and I’m sure her husband would know what she shouldn’t do with his.

  7. I came to read this article because my son, a T1D, has really struggled with bringing his A1C into the single digits. He’s been double digits since he was diagnosed 4 years ago. His A1C was 11 if February, and he had such a bad hospital experience that he decided he was going to do better. He’s been trying so hard, but today he was hospitalized again with a better pH, but a way low bicarb number, and his A1C was 15.5!!! That’s higher than when he was diagnosed. So I was just wondering if other things could be affecting his A1C, but with a number that high, it’s probably not just that the A1C’s not the most reliable thing to look at, especially since A1C’s typically look lower for T1s. *sigh* Back tot he drawing board.

    • My son was diagnosed with type 1 when he was 11. We were told then that adolescent hormone levels would cause fluctuations . They encouraged us to do our best but that despite the swings 50% would come out of adolescent without any health repercussions . Not sure if that still is the current thinking.

  8. I am overweight but battling atypical anorexia. I restrict too much. I was told by my dietitian that eating too little can actually raise A1c. My A1c was 5.3 and my fasting BS was 99. I drink 2 bottles of Milk Chocolate Boost for breakfast. I can’t handle food that early. I often skip lunch. I eat as little as I can get away with. And before anyone says that you can’t have a restrictive eating disorder and be overweight, yes you can. I’ve been hospitalized several times and tube fed. I’ve also been in a residential treatment center. I’ve had s pulse in the low 50’s and very low BP. Also I’m cold all the time. Has anyone ever heard of syarvayion causing an elevated A1c?

    • if your body is stressed, you are stressed which could cause that. even though boost says it controls glucose the ingredients are not natural and straight up sugar. The top ingredients are: WATER, CORN SYRUP, AND SUGAR. That’s just plain false advertising. off limits: dairy, sodas, anything packaged, potatoes.Stick to salads no dressing or croutons, no wheat whatsoever even whole wheat or veggie pastas, low carb veggies like broccoli, celery, etc. take a natural multivitamin. start out with some slow walking, light weights. meditate, yoga. take acidophilus, ALA, NAC,. For protein eat low fat fish like cod, tuna, and flounder. lots of water no caffeine nothing from a can. get any stressful things or people out of your life. you should be good.

      • Your absolutist statements are absurd. No foods are “off limits.” That’s how food obsessions and eating disorders are created. What is the basis for what you are saying? Have you ever heard of “All things in moderation?”

        • Kelly is correct in what she’s saying about what foods to avoid. Diabetics can’t do “everything in moderation” Starches like pasta, grains, potatoes all raise blood sugar. Read the book the Diabetic Solution by Dr. Richard K Bernstein. He has a FB page as well as a website.

    • Get your thyroid tested by an Endocrinologist including an autoimmune test and all of the thyroid tests available, not just the TSH. Keep a record of morning temps to take to appt. Regular G.P.s are pretty clueless in treating low thyroid and HASHIMOTO’S, unless they themselves are thyroid patients.
      I know this because of 47 years worth of clueless treatment.

      • Some in this thread discussing thyroid and Hashimoto’s may be interested in this film which was finally released. I haven’t seen it yet but I joined the Kickstarter early and should receive a copy eventually. I hope it helps to get the word out about hopelessly inadequate understanding and treatment of thyroid disorders.

        http://sick2death.com/

    • I agree with Marilyn–get your thyroid tested thoroughly. However, from what I’ve read at Hashimoto’s 411 (a Facebook group), you’re not likely to get what you need from an endocrinologist. I’d say about 90% of the people in the group who see one come back bitterly disappointed or frustrated. Instead try to find a doctor practicing functional medicine. It could be an MD, DO, DC, or AP. Try googling “functional medicine doctor” plus the name of your town.

    • Check your ferritin levels. If your nutrition is severely impaired, as with anorexia, you could be anemic. Ferritin is far more accurate for testing iron anemia than hemoglobin or other tests. And as stated, your a1c can be elevated when you’re anemic.

  9. My name is Paresh. I am from India. On 1st June 2016 my HBA1c found to be 6.1. Next Day I went for for fasting & pp. Fasting was 79.2 and pp was 112.4. I am highly thankful to you. I am not panic. Thanks for a valuable article

  10. Chris, first of all I want yo thank you for this article. It saved me tons of money and interventions by my midwife and her consulting physician. Because of my age (39), they were concerned with diabetes in pregnancy. This is my fifth and I have no history or family history. But they did an A1c on me and it came out yo 5.7. When they said they would treat me as high risk and started telling me all that it entailed, I panicked and started reading everything I could on false tests. They wanted me to do the glucose tolerance test the following week. I am paleo, and everything I had read indicated that paleo moms fail the glucose tolerance test, too. Then I saw your article. I sighed a huge sigh of relief. I am super healthy and all my tests came back absolutely perfect. She even marveled at all the results except for the a1c. So I suggested a one week glucometer and food diary “test” and they agreed. I am happy to say I am off the high risk list. All levels of blood glucose were in the eighties with two exceptions, one when it dipped to 75 after I pretty much skipped a meal and once when it went just over 100 due to an unusually high carb/sugar meal I craved. Anyway, thank you!!!!!

  11. I’ve had type 2 for about 15 years. Over the past 2 years I can’t get my A1C down (average 9). My Dr keeps giving me more medication but it doesn’t seem to work. I’m not the best eater but with the amount of meds it should be lower.

    I’m taking 1500mg metformin, 10 mg Farxiga, 1.8 Victoza and recently Novolog at meal times 10, 10 and 15 mg

    Is there a type of blood test to ask my Dr to run besides the A1C?

    • Lizzie i had this problem for years them my doctor figured out a good regimine of medications for me. I take 500mg of metformin 3 times per day. 2-10mg of glipizide twice a day. I was also on actos 1 time a day but it caused me to gain a lot of weight in a short span of time, so my doctor discontinued that medication. She replaced it with Trajenta once per day. So you may say the medication, but i have had the best numbers of my life and I was diagnosed with diabetes almost 20 years ago.

      Hope this helps

      • According to recent benchmark studies, Curcumin is much more effective than Metformin in controlling blood sugar. Additionally, they were able to show that it prevented study participants from becoming diabetics. Also, try drinking apple cider vinegar (about 2 tablespoons in a half a glass of water) every day. It is also recommended that exercising right after you eat will minimize blood sugar spikes (at least 8 minutes). Try also reading Dr. Julian Whittaker’s book, reversing diabetes.

    • Please drink green smoothie including kale/spinach/ginger/cilantro/lemon/green apple in the morning and also eat less calories and burn more and get rid of diabetes forever.

      • I’ve been on the green smoothie kick for several years(including fish oil supplements), and I’ve been healthy, good weight and sharp. Six months ago, for the first time, I checked by A1C-result was 5.7. Last week, I checked it again; it was 5.5. All I did differently was stop eating as much and moved around more. My lesson learned is that the green diet is good, but insure that it doe’st give a false sense that you can still eat everything. Calories do matter.

    • Pls check out Dr. Jason Fung, he advises diet along with Intermittent fasting, and I have been following his advice and has helped me alot.

  12. Finally I get the A1C and how it measures 3 months, why it is unreliable. Simple and clear. Thanks Chris.

  13. I have had 2 a1c tests in 2 months. The first was done by a nuerologist because he says i have nueropathy. I was always diagnosed with fibromyalgia because i have had alot of hurting and numbing but over the past 3 years i went from alittle numbing to everyday pain and numbing i have gotten worse hence the reason for nuerologist. His test in another town came back 10.7 and told me this is the reason why im hurting and having all the issues im having. So i got a new family doctor and did as he said but this doctor in my town different lab did a test on me cause he wanted to double check well that a1c came back as a 5.4. So i have these 2 tests within a 2 month span. So now what im still getting worse with no answer so im going back to nuerologist cause thays what the family doctor said because we dont know why im having so many issues but he said thats where i need to be. So im confused on how 2 different labs can be so far apart on there numbers. My sugar is usually normal only have had a 148 a couple times other then that i have had 70 or 80 0r 90 or 100s…what do u think? Any advised would be great..

  14. Hello, thank you for posting this article – I found it very informative.

    I just received results that are confusing to me and was wondering if you could shed light on the situation.

    My glucose (mg/dl) was 66, my HbA1c was 5.7% and my estimated average glucose (mg/dl) was 116.9. My insulin was also a bit high at 17 ul/ml.

    Any advice you can give me would be much appreciated!

    • Hi Jess,
      Your fasting glucose is too low and your HbA1c is a little higher than normal. That means that you have spikes of low and high glucose during the day. I am not a doctor, but I know that lower than normal blood glucose can also lead to diabetes after some time. You should consult a diabetologist doctor to stay safe.

  15. My doctor said he thinks my borderline high A1C is caused by iron deficiency anemia. My fasting glucose was 76, and I passed the oral glucose tolerance test. UA was also negative for glucose. He said my iron levels were low, though.

  16. I don’t know a whole lot about the A1C numbers, but, I am inclined to agree with Chris Kresser. My A1C has been fairly steady at 6.2. My doctor put me down as pre-diabetic (this is before I had ever hear of A1C). I wondered, and still kind of do, what the heck he is talking about. My urine specimen has no sugar, and my highest blood sugar reading EVER, was a NON-fasting 99. How do these two numbers reconcile with each other?

    • Hi

      Thanks for posting this ! I was diagnosed with TYP2 in early NOV 2015. My Hb1ac was 12.3%..I was told to lose weight and exercise , put on 1000mg x2 Metaformin… I lost 19 KG and waist went down by 6 inches in 3 months… Daily BGL testing NEVER read over 5.9 mmol/l !!! Have no symptoms…in early Feb another test revealed HB1ac of 11% and BGL 4 mmol-l!!! I am now testing 4 times a day and highest reading is 5.3 one hour after Dinner!… I was confused and now I am quite clear that the Hb1ac test is not the only thing…after meal BGL is very important!

  17. Hello, I am 5’3″ and weigh 124. I walk every day for close to an hour, and eat well. I have genetically high cholesterol (240, ldl is 103). I had my a1c tested twice recently and it was 5.7 I was shocked. No history of diabetes in my family. I never drink fruit drinks or soda. Ever. Hardly eat candy (except for small amounts of dark chocolate.) Cannot figure out why this a1c is high. My fasting glucose was 70. Any thoughts, anyone?

    • I had an a1c jump from 4.9 to 5.4 and freaked. But it takes an average of your levels the past three months. The doctor asked if I did anything unusual recently. I said a 6-day cruise, all you can eat and drink, truly debaucherous behavior. He said that could have done it. So just think back. Did you have an unusually wild week, out of the norm eating, drinking?

      • Well the first time I was eating lots of bad stuff, but I’ve since cleaned up my diet tremendously and it was a straight 4 months of clean eating. So I was shocked to see it that high on the repeat test.

  18. Wildly interesting. I had sarcoidosis which left my spleen larger than normal. I have hemochromatosis so my iron rides high. My high iron damaged my pancreas and I didn’t know it. My fasting surgery were 15 but my a1c was 7.5. My GP kept telling me I was fine because as he said my a1c was not really high. I demanded to see an endocrinology specialist. Within 5 minutes I was placed on insulin. My fructosimine is now normal. My fasting surger is now 5.

    • I would like to know if low testosterone could be the cause of my fiance having glucose intolerence but his AIC was 5.7 & microalbumin was 3.5 & his SGOT was 60 & he was dehydrated so that could throw off the microalbumin & liver enzyme but if he is not diabetic than that would drive up his HAIC !!!???

  19. I am 44 male got physical results today.Fasting Glucose 93 and a1c 5.7 and Total Bilirubin 1.5. a1c and Total Bilirubin marked as above normal range. My doctor didn’t say any thing about these results. Also my total bilirubin is not constant for last 8 years. Now I am worried about a1c (5.7). And I am thyroid patient. And all other results came back normal.

  20. Do you know HOW much anemia affects an A1C?

    Just found out my iron saturation is at 7% and my A1C 5.4 (on Victoza) I am wondering how much mine could be off? I’ve also been dealing with neuropathy and wonder if that has more to do with the Anemia. This is the highest A1c I have ever had. My fasting sugars before I was on meds range between 110-130