Fasting Glucose is low but A1C is kind of high

Am I missing something about A1C and other BG measurements? I’m pre-diabetic so I had my daughter, age 18, tested. She got the 2 hour oral glucose tolerance test. Her fasting BG was 90 and she did not spike a bit over 2 hours. Yet her A1C was 5.6 which seems pretty high in the context of her other numbers. Am I missing something about fasting numbers and/or A1C? I’m not super worried about her but I’d like to learn more about BG in general.

I’m not a doctor, but I can see why you worry. From what I’ve read, an A1c of 5.6 is just below what they might clinically call “pre-diabetes” – I’m not a big fan of that label, but that’s not something I’d like to debate :wink:. Her OGTT apparently was good and her fasting BG is as well. I’m wondering what kind of fiet she’s eating. If she eats anything like my 19-year-old, who practically subsists on pizza and hot dogs, I would expect she had a lot of high-carb snacks and fast-food meals. A1c is used because it tends to reflect a 3-month average - lots of high-carb meals can push that average to the higher end.

Since you had her tested, I’m assuming you feel she is at a fair bit of risk of developing diabetes. I suggest that there’s no better time to work on better eating and exercise habits than now – habits, by the way, that are good for everyone, anyway!


I’m, with thas, i do think, it, time, to work, on better eating and exercise, habits, never to late to work on that,.

1 Like

Thanks for the advice! She does eat pretty well and gets exercise but I think I’ll mention to her that despite the good fasting glucose, the A1C isn’t normal.

If it were my child, I would be slightly wary about giving too much credence to a marginal HbA1c reading in light of the excellent fasting BG and OGTT.

HbA1c is a backwards view linked to weighted average BG over the previous 2-3 months. Doctors love HbA1c readings because they typically have to deal with large numbers of people with diabetes who often don’t test (or test too infrequently) and lie about their readings , diet etc. (sorry but that is sadly the case with many people with diabetes. ). OTOH, the HbA1c number never lies. For the minority that do test frequently and are careful about our diets, fingerstick (or CGM) test patterns are actually more informative than a 3-month average.

Furthermore, a small but still significant minority fall outside the normal relationship between HbA1c and average BG. For these individuals, although HbA1c is still dependent on average BG, the predicted BG based on HbA1c may be significantly higher or lower than the true average. If there are no other risk factors, it may be that the OP’s daughter falls into this group.


A1c depends on many things. One is the average lifetime of RBCs. They assume 120 days, but YMMV applies here too. Maybe because she’s young and healthy, her RBCs live longer, resulting in a higher A1c. It’s difficult to get an 18-year-old to focus on healthy eating, especially if her peers are living on pizza. I wouldn’t worry but you might want to have A1c tested once a year, and if you see it’s going up, then more stringent measures would be a good idea.

1 Like

Lie about diet? Really? No way! By the way, I did NOT eat that missing doughnut. I’m sure it was a mistake. Besides, it was a sweet roll…

1 Like

Even when we’re not lying we may very well underestimate what we’ve been eating. I have this “low-carb” granola I just love… but I have found I can be way off when I estimate how much of it I just ate… my meter often thinks I ate a lot more than I thought I did!

Generally true except in one very special case: labs DO make mistakes, and tests sometimes ARE invalid. When that happens you need to repeat the test. It’s happened to me a number of times that an immediate retest proved the first one was off the rails.

1 Like

It warrants keeping an eye on, but if her OGTT is totally normal and her fasting levels are good, I wouldn’t be losing sleep over it. Might be worth testing her blood sugar a handful of times 1-2 hours after hearty meals.

In the meanwhile, eating right, staying active, and maintaining a healthy body weight (not just weight but amount of body fat) are all things that can improve or help maintain the body’s healthy metabolism.

Good luck, I hope it ends up being a false alarm.

1 Like

This is a vert complicated disease no quick easy answer r solution. Testing over time should show causes/reasons r maybe not. Stay alert n good luck!

1 Like