A glucose number is only half the information
Two Dexcom readings can show exactly the same glucose value and still tell very different stories.
{Side note, I am a Dexcom user and have been for over 5 years, it is a long time since I have used any other CGM, and therefore always use Dexcom as my point of reference)
For example, 6.0 mmol/L with a steady arrow is not the same situation as 6.0 mmol/L with arrows falling quickly. The first suggests glucose has been relatively stable. The second says the number could soon be much lower.
Trend arrows add direction and speed to the current reading. They do not predict the future perfectly, but they help show where glucose appears to be heading based on recent sensor data.
What the arrows generally mean
The exact symbols and rates depend on the Dexcom model and app, so the official user guide for your own system should always be the final authority. In general, arrows communicate patterns such as:
-
Steady: Glucose is changing slowly
-
Slightly rising or falling: A gradual movement
-
Rising or falling: A more noticeable change
-
Rapidly rising or falling: A fast change that deserves closer attention
An arrow is not a promise that glucose will continue in the same direction. Food, insulin, exercise, stress and automated insulin delivery may already be changing what happens next.
Before eating
Trend arrows can add useful context to a pre-meal reading. If glucose is already climbing, I might think about what has caused it, whether insulin is still active and the bolus plan agreed with my healthcare team.
If it is falling, I need to consider whether glucose could become low before the meal is absorbed.
This does not mean making up an insulin adjustment from an online formula. Different systems, insulin sensitivities and clinical plans require different approaches. The arrow is information to use within my own agreed treatment guidance.
After eating
A rising arrow soon after food does not automatically mean the meal bolus has failed. Carbohydrate can begin affecting glucose before rapid-acting insulin reaches its strongest effect.
Correcting too quickly can lead to insulin stacking: several doses remain active and later combine to cause a low. Before reacting, I consider meal timing, active insulin and what my pump or automated system is already doing.
Sometimes the hardest—but safest—action is giving the insulin already taken time to work.
Before exercise
Exercise can make the same reading mean something entirely different. Starting activity at 7 mmol/L with a steady arrow is not the same as starting at 7 mmol/L while falling quickly.
I consider:
-
The arrow direction
-
Active insulin
-
The type and intensity of activity
-
How my body usually responds
-
Whether carbohydrate is available
-
The exercise guidance agreed with my team
Aerobic activity often lowers glucose, while intense or resistance-based exercise can sometimes raise it temporarily. Delayed lows can also occur hours later, so the end of a workout is not always the end of its glucose effect.
Before driving
A CGM is extremely helpful, but driving requires particular care. In the UK, people using insulin must follow current DVLA glucose-testing rules and their clinician’s advice.
A downward arrow near the lower end of a safe range may warn that glucose is heading towards a level at which driving would not be safe. Never rely on a blog post in place of the current legal guidance, and do not drive while hypoglycaemic or if you cannot confirm that it is safe.
Read more on driving in the UK with diabetes on GOV.UK
At bedtime
The bedtime number is only a snapshot. A downward trend, recent exercise, alcohol, active insulin or an earlier hypo may all alter overnight risk.
Equally, a rising arrow may reflect food that is still digesting or a treatment for a previous low. I use the arrow as part of the picture, not as an invitation to chase every movement before sleep.
When arrows and symptoms disagree
Dexcom advises using a blood glucose meter when sensor readings do not match symptoms or expectations. This is especially important during rapid changes, suspected compression lows, sensor errors or unexpected readings.
Trend arrows are best viewed as a weather forecast rather than a crystal ball. They show the current direction of travel, but conditions can change. Used alongside symptoms, active insulin, food, activity and an individual treatment plan, they make a glucose number far more useful.