Why Standard Deviation is Important in Blood Sugar Control

img_2357Standard deviation (SD) is an incredibly important indicator of health, but what is it?

SD is the variability of blood sugar values from the mean. Simpler yet, it measures the fluctuation of blood sugars. The smaller the value, the healthier. If SD is high, it’s not great for health, and you probably feel it too – imagine roller coaster blood sugars. Whereas with a low SD, your blood sugar isn’t changing much throughout the day, which feels great!

Why is SD important? Well, according to this study, “…glycemic variability predicted outcome better than mean blood glucose levels did.” If you have an A1C of 5.5%, but a SD of 40mg/dl or more, that indicates that your blood sugar is in a constant state of change with many fluctuations, which is not healthy. Whereas, and the same A1C with a SD of 20 mg/dl or less is much healthier, indicating very little blood sugar fluctuation. Furthermore, SD is considered to be a better indicator of developing diabetic complications than A1C (studied here).

So what SD number should you aim for? The magic number that I aim for, personally, is less than 20mg/dl. As indicated in the same study above, although focused on ill patients in a hospital setting, “a standard deviation of blood glucose levels of above 20 mg/dL was associated with a 9.6-fold increase of mortality compared with those with less than 20 mg/dL.” If that’s not reason enough to aim for less than 20mg/dl, than I don’t know what motivation means. Note: there is no official number that is recommended; however, there should be.

Reasons to keep your SD low:

  1. When your blood sugar does not fluctuate with high variability, you will more than likely feel better (physically, mentally, and cognitively). Think stable blood sugars VS yo-yo blood sugars!
  2. A low SD means less dangerous highs and lows.
  3. As mentioned above, SD is considered to be a better indicator of developing diabetic complications than A1C (studied here).

Ways to achieve a low SD:

  1. A low carb lifestyle will help dramatically with SD. The less carbs you eat, the less insulin you need, which automatically means less of a chance of blood sugar variation.
  2. Making sure to cover the food you eat, and pre-bolus, will ensure proper insulin to food coverage. Mitigating blood sugar spikes and drops will help keep SD low.
  3. If you wear a CGM, setting your high and low values to non-diabetic standards, or not far off, may help. This approach has helped me tremendously. I keep my low threshold at 65 and my high threshold at 110. I work on staying between these numbers each day – and it’s actually easier than one might think (especially following #1 on this list).
  4. Be diligent. Make better choices. Wait until you’re in range before eating. Correct lows with glucose tabs to hinder rebound highs. Correct high blood sugars. Pre-bolus. Think of what you can do next to help get you back in range, and stay there.

So go check your CGM and see what value your SD is. Make an action plan to lower it, if needed. You’ll probably even lower your average blood sugar in the process. A win-win!

If you don’t have a CGM, testing your blood sugar often will help create a more precise picture of your SD. You can calculate it on this website, by entering your blood sugar values.

I hope this has helped you better understand what SD is, why it’s important, and ways to lower it. Additionally, check out part two of this post and see visual examples of optimal vs poor standard deviation!




  1. Thank, this was very helpful and easy to understand. My guy is 5, and even though we are not new to this disease it’s constant learning and tweaking. MJ in MB


  2. Thanks for the post! I just got a Medtronic 670 g pump and needed to get a feel on what I could aim for as far of average and standard deviation! I see that I am dousing ok I improved my standard deviation to 14 with BG of around 90! It looks like this is almost as good as I can get! With time my number might slightly improve! By the way, I could not get these numbers on a Standard American diet! I am T1D on keto for 4 years and it works great for me!


    1. Hey Bernard, that’s an incredible average and SD. Like you, I also eat very low carb. Without doing so it’s got to be nearly impossible to have normal blood sugars. Awesome for you!!


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s