Your CGM numbers
What Is Glucose Variability? Understanding %CV on a CGM
Glucose variability describes how much your glucose moves around, not just where it averages. On a CGM report it is usually shown as %CV, the coefficient of variation, and the 2019 international consensus lists a target of 36% or less.
On this page
Why an average isn’t the whole picture
The consensus authors point out that A1C “fails to identify the magnitude and frequency of intra- and interday glucose variation.” Two people can share the same average glucose while one stays fairly steady and the other swings between highs and lows. Variability is the measure that tells them apart.
What %CV is
%CV is the standard deviation of your glucose readings divided by your average glucose, shown as a percentage. Because it is relative to your own average, it can be compared between people whose averages differ. The 2019 consensus recommends %CV as its variability metric.
A standard deviation on its own is measured in mg/dL. %CV removes the units, which is why it is the version CGM reports tend to show.
The target
The 2019 International Consensus on Time in Range lists a variability target of 36% or less. A footnote adds that some studies suggest a lower %CV, under 33%, gives additional protection against low glucose for people who take insulin or sulfonylureas.
These are consensus reference points, not a pass mark. Your own target is something to set with your care team.
What moves it
Meals, insulin, activity, stress and sleep can all move glucose, so variability reflects your whole day rather than any one thing. In a small study of adults with type 1 diabetes, more broken sleep was associated with higher glucose variability, though a study like that shows an association, not a cause.
Limits of the number
The consensus notes that, in type 1 diabetes, the link between shorter and longer sampling periods is weaker for low glucose and for variability than for other CGM metrics, and that people with more variable glucose may need a longer window of data. A single week’s %CV is a snapshot, not a verdict.
How Dia shows it
Dia’s “Your numbers” screen includes glucose variability alongside time in range and GMI, using the published definitions. It describes your data and doesn’t set targets for you.
Questions
What is a good %CV?
The 2019 international consensus lists a target of 36% or less, with some studies suggesting under 33% for people on insulin or sulfonylureas. Your own target is something to set with your care team.
Is %CV the same as standard deviation?
No. Standard deviation is in mg/dL. %CV divides it by your average glucose, so it is a percentage and can be compared between people with different averages.
How much data do I need for %CV?
The consensus recommends at least 14 days of CGM data with the sensor active at least 70% of the time, and notes that more variable glucose may need a longer window.
Sources
- Battelino T, et al. Clinical Targets for Continuous Glucose Monitoring Data Interpretation: Recommendations From the International Consensus on Time in Range. Diabetes Care. 2019;42(8):1593-1603.
- Cleveland Clinic Consult QD. Exploring the Relationship Between Sleep Amount and Type 1 Diabetes. April 29, 2022.