Is Standard Deviation more important than average BG or Time-In-Range?

This is a very personal question, but I just blurt it out. You don’t want to run low, this is just ugly and potentially deadly. I actually do everything to avoid the lows and will take some higher numbers in exchange.

For all practical purposes I set the lower range at 60 and the upper at 180. My A1C’s run anywhere from 5.5 - 6.1 with a TiR of 85%, above 180 10% and below 60 about 5% with an average deviation of 45.

That deviation is the number I believe is actually the most important. If your deviation is very small even though you run higher readings but you are constantly within a deviation of 20 between 140-160, you might be better off than with a deviation of 40 between 80-120 readings which should be the ideal numbers? I leave that to the real experts to chime in on that?

I don’t believe that you will mess anything up if you even run occasionally higher than that but maybe your Endos patience with you. I am a T1 for 30years now and maybe the last 10 years I control the numbers a bit tighter since I can with all new tools available. But I was absolutely not letting diabetes control my life and my work but tried to control it just as much that it would not bother me with any side-effects. In numbers that probably means A1C’s above 6.5 - 7 at badly controlled times and accepting the 240 plus readings. Gladly enough it has not shown any ugly long-term effects with me so far. But everyone is different, that is why it is such a personal question.

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To answer the first question, no one knows. There is some thoughts that Standard Deviation is important, but not enough research has been done to say that it is true. What is known, is that keeping your A1c below 7 will give you most of the protection possible from side effects.

TIR is such a personal thing, i.e. some people’s range is 60-120 and others are 80-180 means that TIR has to be bracketed with what your range actually is, and can only be compared to others with a similar range.

Finally, with a A1C of 5.5-6.1 you are doing great. Sure some people run lower, but you have minimized the chances of diabetic complications so congrats and keep up the good work. Hopefully additional data will come out in the near future that proves or disproves the role that standard deviation plays.

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I think the importance of deviation is overrated.

As an extreme case, compare these examples:

  • The first person spends 100% of his time with a BG of 300, but his deviation is zero.

  • The second person fluctuates between 80 and 120 all day long. His deviation is much higher than the first person.

Which one would you believe is handling it better?

Time in range is what you want. And a good non-diabetic range, not a range that they artificially inflate for diabetics.


This is true; however, IF the low A1C is being achieved with a large number of severe lows then that isn’t healthy either. Striking the right balance and maintaining this A1C WHILE ALSO achieving very minimum severe lows (especially prolonged severe lows), is the optimal approach, I believe.

Although, I think it’s been argued that severe lows aren’t AS big a deal to adults with fully developed brains than it is for those under that age.

I know our personal goal is to a) stay under 7%, but even more importantly is b) stay under the “severe low” (< 58) percentage of .04…we’ve yet to accomplish this, though…the best I’ve gotten Liam too is .06%. The Endo recommends us going up to (and being happy with) 7.5 or 8% AIC, but I want to exhaust every avenue to try and fit under the 7% while also staying near the .04 severe low percentage. It’s really a tough job…one bad day can shoot the severe low percentage to ■■■■.


Good point

My opinion is that TiR is more important.

This is the priority that I assign:

  • TiR (previous day) -Target >80%
  • TiR (5 day average) -Target >75%
  • BG Average (7 day average) -Target 132
  • Standard Deviation -Target 30

All of these are working towards the goal of maintaining lab test HbA1c of 6.5%

I think it’s important to have realistic and achievable targets and goals. For me an achievable target is something that you have achieved at least twice in the last 30 days. E.g. TiR 83%


There is no consensus on the importance of glycemic variability. In the D community, there are strong supporters on both sides—A1c vs standard deviation. In the research community, the research on deviation is fairly new because the measure used to be impossible to measure until CGMs.

I keep a good eye on literature because it is an issue that could change how we do things. Irt seems clear that oxidative stress is strongly affected by glycemic variability:

Oscillating Glucose Is More Deleterious to Endothelial Function and Oxidative Stress Than Mean Glucose in Normal and Type 2 Diabetic Patients [PDF]


Glycemic variability seems to have more deleterious effects than sustained hyperglycemia in the development of diabetic complications as both upward (postprandial glucose increments) and downward (interprandial glucose decrements) changes activate the oxidative stress. For instance, the urinary excretion rate of 8-iso-PGF2α, a reliable marker of oxidative stress, was found to be strongly, positively correlated ( r = 0.86, p < .001) with glycemic variability assessed from the mean amplitude of glycemic excursions (MAGE) as estimated by continuous glucose monitoring systems (CGMS).

But the fact is that there has not been a strong correlation established between glycemic variability and negative outcomes independently of A1c/ average glucose measures. Here is a 2015 debate that shows both points of view in The Diabetes Journal:

(Pro) Glycemic variability and diabetes complications: does it matter? Of course it does! [PDF]

(Con) Glycemic variability and diabetes complications: does it matter? Simply put, there are better glycemic markers [PDF]

So I think the jury is still out. As for us, we do try to moderate our SD, but our primary goal is still average glucose, whether you use GMI or A1c. For my purposes, the GMI is better.

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Do you mean 0.4%? ).04% is an astonishingly aggressive goal to target.

Based on the data, I think that the risk of severe lows on brain development in kids is way, way overstated by pediatric endos, while the risk of high BG on brain development is understated. On the other hand, I personally think severe lows are probably quite bad and that the issue is we just haven’t found the right way of getting the data and doing the right measurements and follow-up necessary to show exactly how they’re bad.

For instance, after Samson had his seizures, I found a study showing that youngsters who have had severe hypoglycemic events are 26 times more likely to be diagnosed with epilepsy as compared to non-diabetics. People diagnosed with T1D before age 6 are something like 6 times more likely to be diagnosed with epilepsy, while people diagnosed at an older age are 3 times likelier to be diagnosed with epilepsy. It’s hard for me to believe that those hypos have no effect on that risk. However, these are observational studies and other studies haven’t found a similar risk. And these studies are looking at severe events that require hospitalization and using finger-prick data, which is different from severe low percentages on CGM. For instance, what do you think the odds are that a person with 0.5% severe lows on finger prick data is only low that amount on time in reality? How often are they low and just not detecting it or correcting it?

There are other intriguing studies tying hypoglycemia to autonomic dysfunction. But the problem is that all the studies suffer from one or another defect. Either they’re too small, or they’re the opposite, large but observational and plagued by confounders.

For my part, I tend to think standard deviation is less relevant that percent time spent low, and percent time spent over 180. That’s partly because of my own personal preference – but also because stddev is a symmetric number, but most people’s BG is not a symmetric distribution – if you’re anything like my son you spend more time in the extremes of your high end relative to your median, for instance, than you do in your low end. So to me interpreting it just gets really confusing. Sure, a lower stddev and average BG is good, all things being equal, but I don’t know how to make that number actionable like i do time spent low or high.


Yes, it seems I can’t math or english well lately. 0.4%.

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Thanks for sharing that statistic. :stuck_out_tongue:

Zero percent chance. If I was ONLY basing my percentage on the fingerstick pricks each day that would give us nearly no data points for comparison to look at. But the Dexcom receiver data isn’t any better in our case because we have MANY low and severe lows that are going to be reflected in Clarity which actually were NOT lows or severe lows. I would say 50% of the time or more (estimate) when the receiver/G5 mobile app says he’s low or severe low, he’s actually not. But you can’t go back and “correct” those records in Clarity…our Endo looks at that data and bases their advice for us off of that data.

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Question re: how lab a1c is calculated…

But based on another thread

So it seems that a1c does not get effected by lows, and in fact, they are not even considered. I guess one might assume a PWD has lows if there avg BGs is rather high, but their a1c is lower than what would be expected. Is that the reasoning??

I don’t agree with this and I’m not sure that’s what Eric was saying (but I could be wrong. Maybe I’m reading/understanding incorrectly which happens A LOT).

Three scenarios:

Scenario 1:
The first is someone let’s say who stays at 50 all day long. That person, after converting their eAG (estimated average glucose) would end up having a 3.4% A1C (again, this isn’t a finger stick…I’m only talking about data on a receiver.) Very Low Great A1C…except that they had to average a BG of 50 to get that.

Scenario 2:
Then we have another person who stays at 300 all day long. Doing the same conversion, their A1C would be (or estimate to be) 12.1% A1C.

The “lower” the BG over a range of time, the lower the A1C…that’s the reason people fight to stay within low BG parameters.

The difference is that NEITHER of the two above examples are healthy…the first walks around hypoglycemic all day and faces all the potential consequences of that while the second example walks around with hyperglycemia all day and faces the consequences of that (perhaps DKA now, but more later as their body begins breaking down).

Scenario 3:
Now the third person finds that “happy place” where they walk around at 150 eAG for a given time…all day/week/month/year they are (or average out to be) 150. This persons A1C is going to be (or be estimated to be) 6.9. (This is where Liam is right now.)

I don’t want him to average 50 even though it would be a GREAT A1C…but it would also be an A1C that’s unhealthy and potentially life-threatening. Same with the average of 300.

I agree that the only way we can be certain of an actual A1C is to do the blood stick / test to get the “real” number…but the eA1C of a person is most certainly going to be impacted (whether estimated or actual with doctor tested data) by the BG level you walk around daily with. And this is going to be fairly close to your actual A1C…so both eA1c AND A1C are going to be effected by lows.

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Are you using a formula to calculate eAG based on the average glucose (from CGM)? Then yes, this makes total sense, and I agree with your scenarios and outcomes! These formulas DO take low BGs into consideration.

My question was related to the LAB a1c. I really don’t know how it is calculated, but I was assuming they have a lab test to measure the glycosylated hemoglobin, which does not correspond to the eAG formula. In this case, the lab a1c test would not consider low BGs. That was the question I had.

the extreme case of course is no contestant and very clear. My thought was more on those cases who have a hard time to bring their average down but seem to be comfortable in a range of 180 and can control their readings at this level.
With keeping in mind that traditional non-diabetic levels would be 80-120 which indicates a natural deviation of 40 those numbers are obviously the benchmark.

  1. Are you better off with readings 140-180 with a deviation of 40?
  2. Is it better to force yourself down to reach 120 and have readings from120-180 with a deviation of 60?
    Case 1. would have a better deviation number, case 2. the better A1C?
    Case 1. would be easy to obtain for some who have a hard time getting their readings down for whatever reason. Point in case: a friend of mine who is deeply afraid and reluctant of increasing units which has a whole other set of arguments pro and con.
    Case 1 would have very low risks of hypo’s, eliminating another host of complications.

Me, personally, have always dealt with T1 in a matter not to rule my life. Yes it affects my life, but stop right there. It does not rule it. If a doctor sets goals that are not reachable for me, I set my goals as where I feel comfortable. It is amazing what your body sometimes tells you. Hypo events are not comfortable, constantly being hyper is not comfortable, stressing over reaching those low numbers is not comfortable, constantly thinking about it is not comfortable. That applies more to the old days without the cgm’s. And on the other side: if you feel ok and well but your endo is bitching about your A1C and your numbers, you still feel ok and well. And that is probably for most of us the “ultimate” measurement of the state of our health. Rather unquantifiable but very human.


Yes, this is correct…although the eAG (and the resulting “eA1C”) and the A1C are calculated differently…eAG/eA1C are just Math conversions (and only serve as “estimates”) while the A1C measures the amount of glucose that is attached to the hemoglobin. The higher the glucose level in the bloodstream, naturally, the more glucose that is going to attach to the hemoglobin.

The A1C measures this for the past 3 months (90-days) because that’s how long red blood cells live.


I will say that there are few things in life that I would care less about than my endo complaining about my A1C or my numbers. It’s not his life, it’s mine.

“We all appear as dunces when feigning an interest in things we care nothing about.”
C. S. Lewis

You’ve gotta weigh it all out and pick what makes the most sense for you.


I feel like this is a great general rule for adults. I think with kids it gets so tricky. I want what’s best for my son and also want whatever we’re doing for management to feel comfortable and sustainable for him – but he can’t necessarily tell me whether it feels worse to spend 4 hours at 180 or 2 hours seesawing between 250 and 65. And we have to make sure he goes through childhood with a body that is as healthy as possible, so that he can make his own decisions when the time comes free of baggage. That’s the tricky part; what’s comfortable for us may not be ideally comfortable for him, and he might make different decisions if it were up to him.

But it does though. The lab A1c does measure glycated hemoglobin. Whether you are low or high, glucose still binds to hemoglobin, simply in different amounts. If you were low 100% of the time, for instance, it would bind in very small amounts, and your A1c would be unreasonably low—but it would still measure something related to your average BG. Essentially, the glycated hemoglobin represents a mathematical integral of your BG function, that is processed also mathematicallyl to provide an average of your BG–> A1c.

But—there are many issues in the physical process that make it so that the A1c does not represent a perfect average, far from it. It depends upon many factors, including the average life cycle of your red blood cells, and, even in ideal conditions, would only represent true average glucose if you have the same BG in a repeating cycle more or less every month.

In particular, the A1c tends to overweigh the last 30 days of BG across the 90-day sample.


According to what? How do you measure your time in range? This has been bugging me since my Auto Mode days when “TIR” was pushed but the only record of it was through the Guardian. If my CGMs can only reflect but so much, how do I know what my time in range is??


Although most of my doctors would set targets higher than what i do, because they perceive it means too many lows.

I think setting A1C, BG goals needs to be balanced with quality of life. My reference point is that i spent over 30 years with A1Cs very high (est 10-15), based on urine and early bg testing. It takes a long time at high bg for complications, and they are now detected sooner and treated more effectively… My first vision treatments were 23 years after diagnosis, but never limited my abilities.