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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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Join the conversation

  1. I was diagnosed in 1986 with gestational diabetes with my second pregnancy, and took insulin twice daily. As expected, after I had my daughter, my sugar levels were normal. But I was told to expect to go diabetic when I hit about 40. I had normal BG levels into my early 40’s. At 42, I was diagnosed with Hep C (much to my surprise), and did a 6-month ribavirin and interferon therapy. It wreaked havoc on my body, including my red blood cell count. Shortly after treatment commenced, my doctor called to tell me that he wanted to put me on metformin. I was taken off of it about 6 months later because he said that my a1c was normal. I was about 108 lbs at the time. I gained about 16 lbs about a year later, so I expected to be diabetic again. I asked to be tested, and again was told my a1c was normal. I tested myself with my glucometer, and had high fasting sugars, and high PB’s. I went back to my doctor and told him about my readings. He brushed it off, siting that the a1c is normal.

    It has now been 7 years since I was taken off metformin, and I KNOW I am diabetic It is in my family history; mother, grandmother, sister. I don’t know how to convince my doctor otherwise.

  2. Hi There.

    I am 44 years old and female. 45 in May, 2014.

    I am 5 feet five and currently 202 lbs.

    I was 213 lbs in December 2013 but am losing weight following a semi Paleo diet. I am not eating grains, beans or added sugars.

    I take that back. I have had some agave, and organic peanut butter/chocolate with very low added natural sugar. I do eat fruit.

    I still have some dairy. Not a lot. No ice cream or cheese.

    I have coffee everyday with organic cream. I have had plain, whole fat organic yogurt.

    I have been doing this for nearly a month now.

    Before I started my AIC was 6.3 and my fasting blood sugar was 102. (January 2014)

    My primary care physician and endocrinologist said I was pre diabetic but not diabetic.

    I didn’t have any blood sugar issues until I was 42.

    (As far as I know.) Last time I was checked I think I was 34 or 35.

    In my late 30’s (36-38) I exercised a lot and ran races and 2 full marathons. I didn’t really watch what I ate per se but my weight was around 170. Still overweight but physically spry.

    Then between 38-42 all that went by the wayside.

    My weight went over 200 lbs and I just ate whatever.

    Then I lost 40 lbs at 42 yrs of age but then gained it all back.

    It was at 42 I found out my blood sugar was 102 fasting. It had (as far as I know) never been over 100 before.

    While I was working out and watching what I ate in my early 40’s my fasting blood glucose came down to 94-96. My AIC was 5.8 – 5.9.

    I have always struggled with weight since I was a teen. I was diagnosed with PCOS at the age of 29.

    I am a sugar addict big time. Intellectually, I KNOW what to eat.

    I know when I am eating junk I know how dreadful it is health wise.

    I do have high cholesterol and Triglycerides.

    It was 311. LDL over 200.

    After 3 weeks of semi Paleo my AIC came down to 5.7. But my fasting glucose was 104.

    My cholesterol came down to 295. LDL is still 205.

    (My blood was drawn February 10, 2014)

    Here’s the thing. How do I get a Fructosamine test done?

    My Endocrinologist said it was the same thing as AIC.

    I want to get a blood glucose test done as well.

    I have no clue what my post meal blood sugars are.

    How accurate would they be now since I am following a much healthier food plan?

    What would they have been when I was eating cake and ice cream?

    Maybe I was diabetic post meal and fructosamine wise but not AIC and fasting blood sugar wise three to 4 wks ago?

    I am confused and scared.

    Will a potato and rice post meal test be as accurate as a sugar drink test at the docs office?

    I don’t know if my Endo will perform a blood glucose post meal test?

    What do you do? Hang out at the docs office all day?

    Ok thank you!

    I know this is long!



  3. I have been on metformin ( 4- 500mg/day) for 1 year. My fasting BG is between 95-106. 1 hour after meal its 125-130. My A1c was 6.9 3 months ago and recently it rose to 7.4.
    My triglycerides are 120; LDL 90, HDL 45.
    Not sure what changed. I have lost 10 lbs since last year as well. Any suggestions as to what I can do to lower my BG and A1c?

  4. What about people like myself?
    My A1C number didn’t change much at
    all after my distal pancreatectomy.
    But aren’t I now at a bigger risk of
    developing diabetes post-operatively,
    in the here and now since I can’t replace
    the pancreatic tail?

  5. Chris,
    Just had a checkup and my fasting glucose was 110, last year it was 120. Both years my a1c was well within normal levels. My dr wants me to diet and exercise and recheck blood in 3 months. I am a very anxious person and the dr feels this can raise the fasting glucose. What would you suggest?

  6. From what I’ve read the studies on anemia are conflicting. It’s been found that anemia raises a1c, lowers a1c, and has no effect on it. Here is information from a study in India (reported 2 years ago) that said anemia falsely lowered a1c. However they said prior results were conflicting so they did their own study, and the conclusions were not supported elsewhere. They said further study is needed.

    I don’t know anything about other research, but they differentiated between mild, moderate, and severe anemia. It appears that all of the patients had either moderate or severe anemia. I have mild anemia and it appears none of the patients had this. At any rate the doctor told me not to worry about it. I think just saying anemia may not be sufficient; also this study only considered iron-deficiency anemia.

    But I’m a diabetic with an a1c in the normal range without meds (5.4%) but it’s a good reminder not to get complacent and test blood sugar. b/c if I slack off things can go the wrong way. My hemoglobin is 11.3.

  7. My last A1C test was 9.5. When I check my blood sugars morning and evening they rarely go over 200 mostly around 150 and I am on 40 units am, 50 units pm before meals. Two shots per day. Many times I hit 120 to 125 or down to 65 if I get too low. Another diabetic says I will go on dialysis and loose my legs but I don’t believe that. I am 72 and still doing well. I have my doubts about this test. My body always has been in the 8+ A1C range. My doctor said to increase my morning shots. Well I did and had a very low blood sugar early afternoon. I just don’t see how why I strike that high on the AIC tests? My friend is severe diabetic and goes anywhere from 50 to 350 all the time yet he comes out with a 7 on his A1C. I seem to be much healthier than he is. This is why I looked up this article. I don’t think the A1C test is at all accurate.

  8. Dec. 2011 I had a complete physical. My A1C was 6.2. I lost 20 lbs. and cut out sweets and limited my carbs. I was retested three months later and my A1C was 6.1. I read that broccolli reduces glucose. Well, I have been eating broccolli for almost two years with dinner … close to everyday. I went to my doctor Oct. 2013 for retesting. My A1C results were 5.6 and I eat rice, pasta, and bread whenever I desire ( in moderation). I also started taking Bilberry capsules once a day for about
    three months now. It is my opinion “BROCCOLLI WORKS! “

  9. I have daily after meal readings of 200- 365 and fastings of 180-199 but my A1c is only 8.1 I am on oral meds but the more they increase the higher my daily sugars. I also have antiphospholipid syndrome.

  10. Thanks for the article clarifying the diagnostic parameters in assessing when a person is ‘pre-diabetic’ or ‘has type Two Diabetes’, after a naughty, naughty ratio with the hemoglobin A1C. I feel fine, and am a bit overweight. My doctor telling me with a ratio of 6.9 with the hemoglobin A1C, now I have type two diabetes. Well, maybe not, since like I say, I feel fine. I am a persistent walker, mover, and shaker, pardon the cliches, and used to be a long distance runner. guess my red blood cells like my Swiss ancestors, may live quite awhile. giggle, huh, a bit. I think I will lose the twenty pounds afterall, and give the ratio what not, another spin, with less gin. uhhh, sugary lifestyle. lol. John. Thanks again.

  11. HI Chris,

    Just read your article and wonder if i may be a person whose hemoglobin is around a lot longer. 3 years ago i was told i was in the prediabetic range.i forget the number sorry! Anyway, i had a baby 2 years ago and did not have gestational diabetes. Is that uncommon to have a high ac1 and no gestational diabetes? (although i did have a 10lb baby). today i weigh 123lbs and am 5feet 2inches and in the past 6 months my ac1 has been tested twice and came back at 6.3 both times. Should i have my dr. give me the post meal glucose test?


  12. Hi,
    I’ve got some problems with the study you posted. So, the researchers took data from 23 subjects, who were diabetic and compared this to the normal range in the population. 23 is kind of a low number, but worse is that there was no control group. I know this is a little nit picky, but I have never been to their lab, I don’t know if the equipment they are using to determine hemoglobin lifespan is accurate. I feel that more research needs to be done before we can discount HbA1c. Specifically a larger study with a control group.
    I’m glad you are getting a conversation going about the topic, and I was not aware that this was a possibility.

  13. My blood sugar levels are great when I’m not pregnant, but at the end of my pregnancies (2 so far), I have glucose in my urine. My postprandial numbers are normal, but my A1c was a 6. My healthcare provider was very concerned. Here I am now pregnant with an unexpected bonus… I confess, the thought of going through so much testing again is so stressful. I feel great, and we are far from a sedentary life style (my boys and I walk 4 miles every morning). And we eat a balanced wholistic diet.

    So really what I’m saying is thank you for helping me feel normal, even though my healthcare providers insist I’m high risk. What a joke.

  14. I want to thank you for sharing your research, experience and knowledge. I have been diagnosed with gestational diabetes and based on how my endocrinologist approached my care, I feel pretty strongly, she was more interested in writing a prescription than providing the appropriate guidance and strategy to managing my GD.

    Also gauging from how my Mom was treated for her pre-diabetes, it seems to me that *some* of today’s doctors are motivated by external sources to diagnose and treat diabetes with specific drugs (pills or insulin). I am appalled that by the one hour test, this dr preferred to put me on a mid-range dose of Gliburide which is not FDA approved for pregnancy and only has a 10 year history of use during pregnancy. Thankfully, I am educated and confident enough to take some control in my own care and opted to attempt to control my blood sugar with better diet and exercise. In 12 weeks, I have only had 3 “high” readings and those 3 were after splurging on a refined sugar product and after 2 hours, my level was 107 or under.

    I think it’s dangerous and irresponsible for doctors to diagnose based on single test results which, as you state, do not mirror real life and so logically, cannot possibly tell the whole story. I believe if more doctors subscribed to your theory of the variety of tests to show the entire picture, we might get a better handle on diabetes, gestational or not.

    The bottom line is that the way this doctor treated me has damaged the trust I have for the care I am receiving. I encourage everyone to ask questions and do what feels right for themselves as, unfortunately, some doctors don’t have the aptitude or make the time to care enough about the individual to make the right choices for care.

  15. I am glad to finally find some possible answer to my puzzle. For I may have the exact opposite and potentially harmful circumstances. My fasting and post meal glucose puts me in the diabetic range and has for the last 5 or more years (age 55). Almost always it’s at least 120+ fasting overnight. However my A1C has consistently fallen in the under 5 range every six months since then as well. And neither the doctor nor anyone else I asked could explain that. So I’ve grown comfortable thinking I wasn’t really a true diabetic.

    Seeing a logical explanation now is going to get me motivated and back on track with diet and monitoring.

  16. Hi Chris, Just curious, do you use fasting or non-fasting insulin testing when working with blood sugar cases? Thank you~

  17. How is anemia defined? I have normal hemoglobin and hematocrit but my serum iron and ferritin are very low (about half of the bottom value of the normal range). In the past, hemoglobin and/or hematocrit have been slightly low.
    According to Mayo Clinic, low hemoglobin will give false low results on hA1c whereas low serum iron will give falsely high results. Based on that I would assume my 5.2 is higher than the result would normally be because of the low iron. If I am classified as anemic based on ferritin and serum iron, then presumably my red blood cells are short-lived and my hA1c should actually be higher. Based on hemoglobin and hamatocrit, I wouldn’t be anemic and the test would be fairly accurate.
    I am not sure how to interpret the results.

  18. Hi there. New to type two. Well- I was on file I was 10yrs ago but lost 33kg and it wasn’t issue so they re considered it (but u don’t get rid of it- you just manage it well). Well GD turned into type two. But 2yrs of regular hypos (daily)- I got first a1C and its 4.9%. I was anaemic after son was born born but not been tested. Iv been testing FBG, pre and post meals. Fastings 5-5.8 so not bad. Pre and post meals are my issue more so post… 8-10mmol. Hypos are happening less
    Now and I can only assume the test in June will be raised? Or is that assumption not valid? Just started metformin. 2 days ago and numbers are up! And then I feel low but I check and its up!

  19. My A1C was low the last time it was taken. My doctor suggested I eat more sugar, but I already have quite a bit in my diet already. I’m a little overweight, too. Could this number be an error, and should I look into other options of raising my A1C or checking it another way?

    • i would look into finding a new doc. Seriously! That is the most asinine advice ive ever heard!! If you are seriously hypoglycemic (low blood sugar) which is VERY hard to see on an a1c test, the LAST thing you want to do is eat more sugar!!! Hypoglycemia is often a precursor to HIGH blood sugar, aka diabetes! Eat a normal, healthy diet, with good fats and protein, but for heavens sake get a new doc.