Monitoring
CGM Data Interpretation: What Your Graphs Really Mean
A finger-prick gives you one number. A continuous glucose monitor gives you around 288 readings a day. That is a much better picture, but it is also a lot of data, and most patients are handed the app with no explanation of what to look at.
Here is how to read your own graphs.
Start with time in range, not the average
Your app shows an average glucose figure. Ignore it first. An average of 130 mg/dL can come from a steady day, or from a day that swung between 60 and 250. Those are completely different situations.
Look at time in range instead. This is the percentage of the day your glucose stayed inside your target band. For most patients that band is 70 to 180 mg/dL, though your doctor may set a tighter one during reversal.
Time in range tells you how steady you were. Average only tells you where the middle sat.
The shape of a meal
Every meal produces a curve. What matters is the height of the peak and how fast it comes back down.
- A good curve rises gently, peaks under about 140 mg/dL, and is back near your starting line within two hours.
- A sharp spike rises fast, goes well above 180, and takes three hours or more to settle. Usually a refined carbohydrate eaten on its own.
- A double peak rises, dips, then rises again. Often a large mixed meal, or a meal followed by a snack you forgot about.
- A slow drift down after eating, ending below where you started, can mean too much medicine for the food you ate. Report this one.
What your overnight line is telling you
The overnight stretch is the most useful part of the graph, because nothing is going in. Whatever your glucose does between midnight and 6 AM is your own metabolism.
A flat overnight line is a good sign. A line that climbs in the early morning is common and is known as the dawn effect. Your body releases hormones before you wake, and those hormones raise glucose. In insulin resistance this rise is larger.
If your fasting reading is your highest number of the day, that is a resistance signal, not a food signal. Eating less dinner will not fix it on its own.
Signals that your reversal is working
Over several weeks, look for these changes rather than for a single perfect day.
- Peaks after the same meal getting lower month to month
- The return to baseline getting faster
- Fasting readings drifting down
- Time in range climbing, even if your average moves slowly
- The overnight line flattening out
What to ignore
Do not chase every number. A CGM will show you readings you would never have seen on a meter, and some of them will look alarming without meaning anything.
Compression lows are a good example. If you sleep on the arm with the sensor, pressure can produce a false low reading. It usually corrects itself as soon as you move.
Sensors also drift in their first 24 hours and in their last day. Treat those readings as rough. If a reading does not match how you feel, confirm it with a finger-prick before acting on it.
Also read: HbA1c, Fasting Insulin and HOMA-IR: How to Read Your Metabolic Panel
Key takeaways
- Read time in range before you read your average.
- Judge a meal by its peak height and how fast it returns to baseline.
- A high morning reading points at insulin resistance, not at dinner alone.
- Watch trends across weeks, not single days.
- Confirm any reading that does not match your symptoms with a finger-prick.
Watch: Reading a week of CGM data
Dr. Das reads through a real patient week and points out what each pattern means.
