Managing diabetes includes keeping track of seemingly endless sets of numbers. You might measure your blood glucose levels several times per day and get blood drawn for an A1C reading a few times a year. Each of these checks gives a snapshot of your blood glucose management, but neither gives you the whole picture.
In order to get a more complete picture of your blood glucose levels, you need to look beyond a single-number reading to a measurement that gives a sense of how much your blood glucose levels vary in a day, week, month, or quarter. You need to consider the standard deviation of your blood glucose readings.
What is standard deviation?
The standard deviation is a number that reflects how large the variation is in the numbers used when calculating an average (or mean). (If you’re curious, you can see a basic example of calculating the standard deviation here. In this case, the numbers we’re talking about are your blood glucose results. Thankfully the mySugr app does this calculation for you and displays the result automatically.)
Large swings in glucose values can result in the same average as values that are more steady, which can be misleading at first glance. Paying attention to your standard deviation shows whether the average is made up from readings that are relatively stable and close together, or swinging more wildly up and down.
The standard deviation can be calculated based on readings from a day, week, month, or quarter.
Let's look at two examples. The glucose readings for each day both equal the same average of 151 mg/dl, but come from quite different experiences.
Day one’s glucose readings are 197 mg/dl at breakfast, 138 mg/dl at lunch, 100 mg/dl at dinner, and 167 mg/dl at bedtime. The standard deviation for day one calculates out to ±36 mg/dL, which reflects a set of individual readings that are very close to each other – indicating more stable blood sugar values throughout the day.
Day two’s glucose readings vary much more widely. They are 144 dl/mL at breakfast, 147 dl/mL at lunch, 68 dl/mL at dinner, and 244 dl/mL at dinner. The readings from day two generate the same 151 mg/dL average, but the standard deviation is ±72 mg/dL. The higher deviation reflects the wider variation in this day’s glucose readings. In this particular example, the person also experienced both a hypoglycemic and a hyperglycemic episode.
Why does it matter if my glucose readings are close together or far apart?
In the long term, wide swings in glucose levels are linked to causing damage to small blood vessels (microvascular damage) and a greater likelihood of developing health complications. Microvascular damage can lead to serious complications. They include neuropathy which can lead to nerve damage or renal failure, retinopathy leading to loss of sight, and poor blood circulation which can undermine wound healing.
In the more immediate term, wide swings in glucose levels can affect mood and cognition. When glucose levels get too high or too low you can end up feeling nervous or anxious, angry or irritable. Confusion, brain fog, and physical weakness can be triggered by extremely low (hypoglycemia) and high (hyperglycemia) glucose levels. At its very worst, hypoglycemia can cause fainting or seizure.
By keeping glucose levels within a narrow range you are more likely to avoid developing complications from diabetes or experience the mood and cognition disruptions that come with extremely high or low glucose levels.
What is the target zone for standard deviation in diabetes?
Keeping the standard deviation at or below one-third of the average blood glucose level is considered a healthy target that will minimize the chances of microvascular damage and developing complications.
The calculation for the target is very simple: divide the average glucose reading by three. If your standard deviation equals or is less than that number you’re in the target zone. (In the mySugr app the color of the standard deviation indicates whether it’s in the target zone. Green means that it is in the target zone and red means it is not.)
Let’s take another look at the examples above. The average glucose reading for both days is 151 mg/dL, so the target range for the standard deviation is ≤50 mg/dL.
The example for day one has the standard deviation at ±36 mg/dL, which is in the target zone.
The example for day two has the standard deviation at ±72 mg/dL, which is outside the target zone.
What's the benefit of tracking standard deviation in your diabetes care?
No single reading or number can completely sum up your overall diabetes management. Each time you check your blood glucose levels you get a snapshot of a specific moment. Each A1C result gives you a little long picture, but it’s still a static image.
Standard deviation conveys a more nuanced picture—one that reflects the fluctuations in blood glucose levels and gives some indication of the likelihood of developing microvascular damage.
- Understanding Average Glucose, Standard Deviation, CV, and Blood Sugar Variability
- Time-in-Range: What's an Achievable Goal with Diabetes?