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Most physicians have embraced the hemoglobin A1c blood test to diagnose diabetes or track treatment success. It is also known as HbA1c (A1C for short), glycosylated hemoglobin or glycated hemoglobin. The A1C test is supposed to provide health professionals with a quick and easy assessment of average blood sugar levels over the past two to three months. All it requires is a blood draw and a lab analysis. The OGTT stands for oral glucose tolerance test. It provides an assessment of how the body responds to sugar over the course of three hours. Which is the best test to diagnose diabetes? Surprisingly, a new study suggests that the old-fashioned OGTT could be more accurate than the A1C (presentation at the annual Endocrine Society Meeting, March 25, 2019).

What’s the Difference Between the A1C and the OGTT?

Hemoglobin A1c:

The A1C blood test measures the amount of sugar attached to your blood hemoglobin. Your red blood cells contain hemoglobin, a protein that carries oxygen to cells throughout your body. Your hemoglobin proteins are also coated with sugar. The amount of sugar on your hemoglobin represents a kind of snapshot of your blood glucose over several weeks to three months.

By measuring the amount of sugar that is attached to hemoglobin the laboratory comes up with a percentage. A “normal” A1C level would be considered 5.7% or lower. If your percentage of sugar bound to hemoglobin is between 5.7% and 6.4% many doctors would consider your prediabetic. When the sugar-coated hemoglobin percentage goes over 6.5%, most health professionals would diagnose diabetes. Diabetologists encourage their patients to keep HbA1c levels at 7% or lower.

OGTT (Oral Glucose Tolerance Test)

This is a screening test to determine how your body reacts to a big dose of sugar. People are told to fast for eight hours before going to the lab. You will likely be told to avoid liquids as well. At the lab you will have your blood drawn to get a baseline blood glucose level. Then you will be given a gloppy drink containing a big slug of glucose (usually about 75 grams). A blood sample will be drawn every 30 to 60 minutes after you drink the syrup for up to three hours.

In a healthy person the glucose should be cleared by the body fairly quickly. Someone with prediabetes has “impaired glucose tolerance” if he has a value between 140 and 200 mg/dL after two hours. If at two hours a person has a glucose level of 200 mg/dL or above, the diagnosis will come back “diabetes.”

Why Has the A1C Test Become Popular?

There is a reason why clinicians frequently request an HbA1c blood test. It does not require that people fast or avoid liquids prior to the test. People do not have to drink a challenging sugary beverage and then hang around for hours while blood is drawn periodically. Instead, they give a little blood once and are done and gone. It’s fast and convenient. But is it as good as the OGTT test?

The New Study Suggests OGTT is the Best Test to Diagnose Diabetes:

A study presented at the annual meeting of The Endocrine Society (March 23-26, 2019 in New Orleans) has challenged standard medical practice when it comes to the diagnosis of diabetes. When researchers compared HbA1c values to the results of oral glucose tolerance testing, HbA1c was much less sensitive. In essence, the OGTT test was the best test to diagnose diabetes.

The data came from 9,000 adults who had not been diagnosed with diabetes. In a surprisingly large number of cases, people with normal HbA1c values reacted to sugar as people with diabetes do on the oral glucose tolerance test.

The lead author stated (March 23, 2019):

“Based on our findings, A1c should not be solely used to determine the prevalence of diabetes. It should be used in conjunction with the oral glucose test for increased accuracy.

“Our results indicated that the prevalence of diabetes and normal glucose tolerance defined solely by A1c is highly unreliable, with a significant tendency for underestimation of the prevalence of diabetes and overestimation of normal glucose tolerance.”

The researchers went so far as to suggest that the A1C test may miss up to three-quarters of people with diabetes. They maintain that the oral glucose tolerance test provides a more accurate reflection of how the body responds to a whopping dose of glucose. The OGTT is therefore more likely to detect an early problem with blood sugar control. Relying exclusively on A1C may delay diagnosis and early intervention. The authors caution their colleagues not to rely exclusively on the HbA1c test to diagnose diabetes.

Not the First Cautionary Tale About A1C Reliability:

A study published in the journal Family Practice (Oct. 14, 2015) compared the A1C test to the oral glucose tolerance test (OGTT). Over 1,100 people underwent both tests.  The conclusion:

“HbA1c may have a role as the initial test to exclude DM [diabetes mellitus], but it may not have sufficient accuracy to be used as the sole diagnostic test for DM. Our results showed that the ADA [American Diabetes Association]-recommended HbA1c cut-off of ≥6.5% missed 66.8% of the diagnosis of DM…”

Missing two-thirds of the people with diabetes is not very reassuring. The results of this study mirror those of the study reported at the Endocrine Society Meetings in New Orleans this week.

A Finnish study published in Diabetes Medicine (Jan. 2011) came to a somewhat similar conclusion: 

“Of those with diabetes diagnosis based on two oral glucose tolerance tests during the Diabetes Prevention Study follow-up, 60% would have remained undiagnosed if diagnosis had been based on HbA(1c) ≥ 6.5% (≥ 48 mmol/mol) criterion.”

So…What’s the Best Test to Diagnose Diabetes?

For our money it would be the oral glucose tolerance test (OGTT). Yes, it’s inconvenient. Yes, it’s time consuming. But from our reading of the research, it appears to be more accurate if you suspect you may be vulnerable to type 2 diabetes.

That doesn’t mean the A1C test should be thrown out with the bath water. It can be helpful in monitoring treatment progress. And if someone is borderline on the A1C test, it seems to us that a follow-up OGTT might be prudent.

And let’s not forget self-monitoring with a home blood glucose device. A finger-prick test will allow people with prediabetes or diabetes to monitor blood sugar periodically throughout the day. You will be able to determine the effects of exercise, diet and stress on your blood glucose and modify your life accordingly.

Share Your Own Diabetes Story!

How were you diagnosed with diabetes? Did you get the OGTT or the A1C test? How well are you controlling your blood sugar? Share your story in the comment section below.

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  1. Barry

    This exact same thing happened to me. For years I was told I was not diabetic because my A1c was always below 6.0, and my fasting blood sugar was often below 100. However, I was constantly hungry. Even after I had just eaten, and my stomach was full, my brain was still saying to eat. Finally I went to an endocrinologist, and he did the glucose tolerance test. After two hours my blood sugar was still over 200. I am thankful to that endocrinologist. People need to know that A1c is not a reliable test for diabetes.

  2. Anne
    Seattle, WA

    I have brought my A1C down more than once with just diet. I don’t want to take Metformin. My fasting glucose is often what I call high (125-140) but my post-prandial readings (2 hours after a meal) are always excellent. I don’t want to subject myself to a sugar overload and feel awful from it. I will continue to get the A1C and monitor my glucose after meals. The last test for A1C was 5.8. I’d like to get it down to 5.0 and think I can do that with diet and exercise. My maternal grandmother and maternal aunt both died from diabetic complications; my grandmother had a leg amputation. I am 75 years old.

  3. David
    Goffstown, NH

    Does anyone know how to interpret a huge DROP in glucose levels with the OGTT? I had one a couple of years ago, and in the third hour my levels dropped to about 35, dangerously low enough that they contacted my physician. I left the lab dizzy and vision-impaired and truly dangerous for driving. I went to Dunkin’s next door for something with sugar and a place to sit down and recover enough to drive home.

    No one has been able to answer what this might mean or whether it is anything to be concerned about. Most of the literature is not about being hypoglycemic!

  4. Abigail

    I hear there are things we may ingest,coffee and the diabetes drug metformin for example, that can influence A1C test results. I’m concerned about this. Please tell us what we should be aware of.

  5. Mollie

    Yes I would also like to say thank you to Joe and People’s Pharmacy! My daughter had an ear ache probably an infection following a head cold, and was given a script for a supposed Eustation Tube infection. The antibiotic was ciprofloxin. She took one dose and immediately experienced numbness and pins and needles in her face.

    I recalled reading the side effects of this drug on here which had been reported to the FDA for many years. I told my daughter about the drug, and she stopped it and asked another doctor she works with if she had to take them. He told her to stop immediately (and expressed surprise that she was prescribed Cipro for this condition) and to get a steroid puffer, etc, which worked so she did not have an ear infection after all.

    The info on this website is so useful. Thank you Joe and everybody!!

  6. Pam

    There is another test that is far more accurate as an indication of how your body deals with regular food. My friend has a doctorate in biochemistry and was commissioned into the Navy to do diabetes research at Bethesda. When I got pregnant 40 years ago, she told me to skip the usual tests for gestational diabetes and insist on the post-prandial test as diabetes researchers found it much more accurate as an indicator of how you handle normal food:

    You fast at midnight and then eat a set breakfast they tell you to (eggs, toast, orange juice– not sure exactly what, but they also specify quantities). Then you come in and have one blood draw a few hours later. Mine were always normal. By contrast, at 14 I had the OGTT because my brother suddenly had Type 1 diabetes at 25. I was sick as a dog drinking all that sugar glop, and my values spiked a bit in one hour.

    Fifty-two years later, and I am still fine, no diabetes.
    Ten years ago I had a hip replaced and went in last for surgery, so got up to the post-op floor too late for dinner. All I got for 38 hours, from fasting pre-op till breakfast the next day, was the sugary glop they offer in the recovery room and on the post-op floor: sugar jello, sugary soft drinks, popsicles, pudding, etc. A sugar fest.

    One of my blood values was a little high before I finally got real food, and the NP for my surgeon came in that next morning and glared at me and asked are you diabetic? No ma’am. no one ever told me I was, but you try fasting for 18 hours and then getting fed nothing but sugar for the next 18 hours and see how YOUR blood sugar looks!

    Why the heck do we expect people to eat nothing but sugar and then MASSIVE amounts for the OGTT on empty stomachs. and somehow the way your body reacts to this very abnormal eating pattern is supposed to be meaningful?

    I mentioned to my docs that maybe it would be a good idea for hospitals to stock something in unit fridges besides high sugar items, like protein shakes. I see at University of Pennsylvania Hospital in the unit kitchens now that patients can access they offer mainly higher protein/fiber items and very little in the way of high sugar items.

  7. Stephen
    Everett WA

    I’m 61 and have been prediabetic for a while. I have a fasting glucose of 114 and an A1C of 6.2. However with the OGTT for my test it was 75g of glucose. Within two hours my glucose was back down to 114. However my problem is that rarely can I get the sugar to go below 100 unless I don’t eat anything!

  8. Roger
    Quito, Ecuador

    I had prediabetes for more than 10 years when I was diagnosed with diabetes in September. After discussing it with my doctor, I went on a very low carbohydrate diet and soon started to eat some “nopal” (the meat of the leaves of the prickly pear cactus) along with the fruit I have on my breakfast oatmeal which has always included a sprinkling of cinnamon.
    My test must have been some kind of composite. I fasted from 7 pm and had my blood drawn the next morning at the doctor’s clinic. Then I ate my normal breakfast which I had been instructed to bring with me. Then two hours after completing my breakfast I had my blood drawn again.
    My test results were HbA1c: 6.68%, Glucosa: 84 fasting, Insulina: 5.56 fasting, Glucosa: 86 2 hpp. (with breakfast), Insulina: 16.96 2 hpp. (with breakfast.
    Recently I didn’t have my morning “nopal” for 5 days and my symptoms returned. I am once again eating my “nopal” and have decided to take it with both of my meals.

  9. NANCY
    Lakeland, Florida

    I just want to take this opportunity to thank you both, Joe and Terry Graedon for all the helpful information I have garnered from your newspaper columns and your Saturday morning Peoples Pharmacy NPR program. I have read your weekly articles and then listened to your weekly broadcast for many, many years. You have been such a wonderful resource for natural remedies and pharmaceutical information. I am 73 years old and we may never meet but because I have followed you so many years, I feel a connection to you. Thank you for your many years of dedication to the service of teaching anyone who wants to read, listen and learn for the benefit of health. I’ve never had any health professional explain in easy to understand language better than you. You have been a blessing to many, many people.

    • Joe Graedon


      Thank you so much for these very kind words. You are why we keep doing what we do after 40+ years. I too am 73 and will keep going as long as you keep listening and reading!


  10. John

    Thank you for publishing this. I was reassured by my latest glucose and A1C test results that used to be in the prediabetic range and then went down to the top of the normal range. But I will ask my doctor about the OGTT test that I never heard of before. It sounds tough though with so many blood draws.

  11. Nancilee
    suburban Chicago

    I can understand why most individuals would not want to undergo the OGTT for diabetes. So many medical tests are time consuming, expensive and uncomfortable.
    Of course most patients and insurance companies (not just those related to Obama Care), would prefer the cheaper, easier AIC.
    Is there possibly some way the AIC with it’s simplicity and cost could be made more accurate?

    • Terry Graedon

      We don’t know. But we are sure many people would welcome that.

  12. George Entenman
    Chapel Hill, NC

    If I understand this correctly, a HbA1c level below 7% does not guarantee that one does NOT have diabetes and that an OGTT is better at detecting it.

    But if the HbA1c level is ABOVE 7%, there’s no need for the OGTT, is there?

  13. Allan Landin
    College Station, TX

    I really don’t like comparing apples and oranges. The OGTT tells me almost nothing about three days ago and A1C isn’t designed to tell about the past hour. When used as designed, both tests provide a slice of information about a person.

  14. Sabrina
    San Diego

    No fluids for 8 hours and then several blood draws is pure torture. Ones veins are not easy to find so I understand why this test is not used. I’ll stick with my A1c every 3 months. I finally found a good combination of drugs to control my diabetes and came down from an 8.4 to 7.6 (time released metformin and glimperide twice a day).

  15. Luke

    People are less likely willing to get an oral glucose tolerance test (OGTT) because it is time consuming. You forget a lot of people still have to work for a living. The test may even cost more. If there are expensive deductibles which is very common with Obamacare, they are less likely to consider this. Check with your insurance first to see if it is covered under Obamacare. An OGTT may not be part of what is considered “routine exam” but rather “diagnostic” which mean the consumer pay for the entire test with $7,000 deductibles.

  16. Brigi

    This information concerns me. Regularly my fasting blood sugar is between 100 and 110. My A1C has been about 5.4 to 5.6 regularly. So it seems that I don’t manage my blood sugars as well as someone who has lower numbers. But I certainly don’t want to be diabetic and I consistently try to reduce my carbs and sugars but it doesn’t seem to have an effect on those numbers. I am 71 years old and this has been my story for the last 20 years or so. I try not to worry about it though.

  17. Lynda

    I had blood sugar issues and while my levels were close to 300 by A1C was below 6. Doctors never could figure out why. When I had a that nasty lengthy test it was confirmed that my levels were out of whack. I needed meds and was able to get my levels down but A1C NEVER indicated any issue.

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