I apologize in advance if this question has been asked before.

I'm just wondering what is a normal or what is a good number I had not heard of this until I downloaded my cgm and saw my SD number so I went on to google it to see what I could find, but I figured I get better answers here, so I wonder if anyone keeps track of that and if there is a number we should try to aim for and keep it at?

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The thing I've noticed is that if I try to arrange things so that my BG is 'uneventful', the CGM works a lot better. This sort of turns the CGM into a coach or "incentive" where having it behave itself is sort of "precious" (seriously...like Gollum...) and helps me focus on taking steps to keep it where I want it? It's sort of weird...

Hi Cindy,

If you can assume that your pre-meals are your lows and your post-meals are your highs, then you are going to have a pretty decent data set as far as your average and standard deviation goes. The assumption would be that everything you test in between those two extremes probably aren't affecting your average much, either way.

Simply by virtue of the fact that each day gives you more tests to add to your data set, your "pot" of BG readings", that will cause your standard deviation to go lower towards a more precise estimate, without affecting the spread around your average, if that makes sense.

FWIW, I do the same thing with my data. I break them into categories that I use to take action with. Those categories, premeal or postmeal, before or after exercise, are way more useful for me to determine what i should be doing than looking at my big bag of daily BGs. I usually test a couple of times between post-meal and the next premeal, though, just because my routine during those times change so much, which really affects my BGs.

I have had an exceptionaly good 2 weeks with only 2-4 readings above 200 mg/dl. My BG fingerstick SD is 39 (sample size 183) and my CGM SD is 25 (sample size 3,739).

This is interesting. My hypothesis is that we often test to catch or confirm highs or lows. I suspect that most people that use a CGM do some standard calibration tests, but probably use their meter to confirm a high or a low before treating. This would explain why you SD is 39 from your meter, markedly higher than your CGM. Is it fair to say that you often use a meter test to check or confirm a high or a low?

I check and confirm almost every high and low. I will sometimes test after correction to make sure my BG has/is moving as intended. I also test before every meal where I will often be in range.

My CGM does not read to the extremes like my BG does. If I test at 60 and then treat my CGM may only get down to the low 70s before it starts heading up. This example is far worse for high BGs. My BG may shoot up to 200 quickly while my CGM is only reading 150 (and climbing). I am aware of the issue and test and treat appropriately with my BG.

bsc has a point,

Standard deviation is trying to estimate your variation so if you want to know if your variation between the two methods really is different, you can start by looking at the high and lows measured by your fingersticks compared to the highs and lows measured by the CGM.

If you are managing to catch your BGs at their highest and lowest points, and your CGM is reading accurately and also catching your highs and lows, you wouldn't expect the average BG between the two methods to be that much different from each other.

But, the difference could just be the difference in the number of samples taken by the CGM compared to your fingersticks. The standard deviation is basically just the sum of how far each reading falls from the average divided by the number of BG readings taken. So, all things being equal, standard deviation drops as the number of sample BGs goes up, which is what your CGM could be doing.

I wasn't sure the MM had a different SD for the CGM and meter but it's 23 on the CGM and 27 on the "meter" reports? I think that I may dilute the meter ones by putting in fake readings, where I check the CGM, see 95 (or whatever) and just use that number for the BG reading to enter a bolus to check the otherwise inaccessible IOB "score" in the pump. I don't think I test too much less w/ the CGM but mostly am keeping an eye on the CGM and my BG by testing a lot in conjunction w/ the CGM data.

I watch my distribution, and SD but my distribution is a significant indicator as to where my A1c is going to be

on my next test. My 3O day SD was 26.9 today and my distribution is 89% (70-140 for the last 14 days). My 30

day BG average is 116.  My A1c is in normal range. Last month my SD was 52 one day when I uploaded

to CareLink but my distribution was about the same close to 90%.

Average: 108
SD: 24
Min: 58 mg/dL
Max: 185 mg/dL
Time Period: 11/14/2011-2/13/2011
N: 416

A1c: 6.0% as of Dec. 2011

While I find this SD topic interesting reading. I question it real life value for me.

I recently had to re-adjust my basal (Lantus) on split dose.

I was getting (on average) lows in the mornings and my later in the days reading on the high side. So my endo suggested I do an unequal split. I take a couple units less at night and equal more in the morning. By taking 11-12 units in the am and 9 at night my BG levels are better overall. NO lows but I still get a few highs once in a while later in the day. This has helped reduce my later-n-day highs and morning lows. Probably will do nothing for my a1c or SD, but it is a better more even BG pattern.

I see where averaging ALL morning, noon, dinner and evening BG readings are more important in getting insulin doses adjusted for a more stable BG.

BTW A1C is an average, problem is the most recent 2 weeks have a much bigger influence than the previous 2 weeks etc etc etc. So if ya wanna CHEAT on the CHEAT test (aka a1c) cheat the month after your a1c and clean up your act a month or so before your next scheduled a1c.

Like I said, I think this is an interesting topic, just not practical for me.

BTW its cheating time for me, paczki time.
.........(TIP; never schedule a1c time soon after fat Tuesday.

Gomer aka Sir Falls-A-Lot


My average SD is 29.

Your question reinforces the notion that WE as patients must take charge of managing our condition. Between my endo, gastroenterologist, urologist, dietician, neurologist and cardiologist, I've noticed that their curiosity ends at the limits of their specialty. For example, managing diabetic gastroparesis is a balancing act between managing carb absorbtion, digestive rate, gastric emptying, dietary components, insulin rate of action and resulting post meal BG level. None of these 3 speicalists coordinate what they are doing to me with each other - it is up to me to get their advice, ask questions and through their guidance figure out what works. It is maddening, and even though my health care all have electroc access to my treatment record they simply do not have the time to devote to me individually at a level necessary to determine the best treatment course. The trick is to engage their professional curiosity as it pertains to their specialty and use it to my benefit.

Great topic and well worth a disciplined scientific study using a large patient population.

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