Introduction to CGM - Understanding Continuous Glucose Monitors 📡
Technology
The essentials
An Essentials guide gets straight to the point: the key information on the topic, no detours. To go further, check out our full guides.
Learn what a Continuous Glucose Monitor (CGM) is, how it works, the main systems available today, how to read trend arrows and Time in Range, and how to talk to your doctor about getting one. A beginner-friendly guide for anyone curious about CGM technology.
What this guide gives you
What a CGM is and how it measures glucose
The main CGM systems available today (Dexcom G7, FreeStyle Libre 3, Medtronic Simplera)
How to read your CGM: numbers, trend arrows, and graphs
What Time in Range (TIR) means and why it matters
Limitations you should know about
How to talk to your doctor about getting a CGM
Who is this for?
This guide is for **anyone** with diabetes (Type 1 or Type 2) who is curious about CGM technology — whether you have never used one before or you have just started and want to understand it better.
Newly diagnosed
Understanding Continuous Glucose Monitors
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Know how to start a conversation with your doctor about CGM
What is a CGM and how does it work?
A Continuous Glucose Monitor (CGM) is a small device that tracks your glucose levels around the clock. It consists of a tiny sensor — about the size of a coin — that you wear on your skin, usually on the back of your upper arm or your abdomen.
A thin, flexible filament sits just under your skin in the interstitial fluid (the fluid between your cells). This filament measures the glucose concentration in that fluid every 1 to 5 minutes and sends the data wirelessly to a receiver or your smartphone.
The result: instead of a handful of fingerstick snapshots per day, you get a continuous stream of glucose data — often 288 readings in a single day.
Info
A CGM does NOT measure blood glucose directly. It measures glucose in the interstitial fluid. There is a physiological delay of about 5 to 10 minutes between your blood glucose and your interstitial glucose. This means your CGM reading may lag slightly behind a fingerstick, especially when glucose is changing rapidly (after a meal, during exercise, etc.).
How the sensor works — step by step
Application: You apply the sensor using a one-press applicator. A spring-loaded mechanism inserts the filament painlessly under the skin.
Warm-up: Most sensors require a warm-up period (30 minutes to 1 hour) before they start reporting data.
Measurement: The filament uses an electrochemical reaction with glucose oxidase to generate a tiny electrical signal proportional to the glucose level.
Transmission: The sensor transmits data wirelessly (Bluetooth) to your phone app or a dedicated receiver.
Display: You see your current glucose number, a trend arrow, and a graph of your recent history.
Most modern CGMs do not require fingerstick calibrations — they come factory-calibrated.
Quick Check: CGM Basics
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What does a CGM actually measure?
Main CGM systems (2025)
There are three major CGM manufacturers today. Here is an objective comparison of the current systems:
Feature
Dexcom G7
FreeStyle Libre 3 (Abbott)
Medtronic Simplera
Wear time
10 days (15-day version also available)
14 days
7 days
Accuracy (MARD)
~8-9%
~8-9%
Comparable; strong in low glucose range
Alerts
Real-time customizable alerts
Optional alerts (can be enabled)
Tip
What is MARD? Mean Absolute Relative Difference — it measures how close the CGM reading is to the actual blood glucose value. A lower MARD means better accuracy. All three current systems have accuracy around 8-9%, which is considered very good.
Important notes on availability
Country availability varies. Not all CGM systems are available in every country. Check with your local diabetes team.
Insurance coverage varies. Most Type 1 patients in the EU have CGM coverage. In the US, coverage depends on your insurance plan and diagnosis. Many insurers now cover CGM for Type 2 diabetes on insulin as well.
All three systems are continuously improving. New versions and features are released regularly.
Info
This guide does not recommend one CGM brand over another. The best CGM for you depends on your specific needs, your pump system (if any), your insurance, and your healthcare team's recommendation.
Reading your CGM: the basics
Your CGM display shows three key pieces of information:
1. The current glucose number
This is the big number on your screen — your estimated glucose level right now, typically shown in mg/dL or mmol/L depending on your region.
2. Trend arrows
Trend arrows are one of the most powerful features of a CGM. They tell you not just WHERE your glucose is, but WHERE IT IS GOING:
Arrow
Meaning
Rate of change
What to consider
↑↑
Rising fast
More than +3 mg/dL per minute (+0.17 mmol/L/min)
Glucose may rise 45+ mg/dL (2.5+ mmol/L) in the next 15 minutes. Consider a correction if appropriate.
↑
Rising
+2 to +3 mg/dL per minute
Glucose is climbing steadily. Watch for it approaching your high threshold.
↗
Rising slowly
+1 to +2 mg/dL per minute
Mild upward trend. May stabilize on its own.
Warning
Trend arrows show direction and speed, but they should not trigger panicked corrections. A glucose of 150 mg/dL (8.3 mmol/L) with a stable arrow (→) is very different from 150 mg/dL with a fast-rising arrow (↑↑). Always consider both the number AND the arrow together before making insulin or carb decisions.
CGM Trend Arrows
What each arrow tells you about how fast your glucose is changing
↑↑> +3 mg/dL/min (+45/h)Rising very fast — check injection site, correct if needed
↑+2 to +3 mg/dL/minRising quickly — consider a correction bolus
↗+1 to +2 mg/dL/minRising gradually — monitor after meals
→< ±1 mg/dL/minGlucose is stable — good situation
↘−1 to −2 mg/dL/minDropping slowly — watch if exercising or insulin still active
↓−2 to −3 mg/dL/minDropping fast — take fast-acting carbs now
↓↓< −3 mg/dL/min (−45/h)Rapid drop — act immediately, fast sugar required
Based on Dexcom / FreeStyle Libre standards — thresholds may vary slightly by device
3. The graph
Your CGM also shows a graph of your glucose over the past several hours (usually 3, 6, 12, or 24 hours). This graph helps you see patterns: post-meal spikes, overnight stability, the impact of exercise, and more.
Tip
Many CGM users find that reviewing the 24-hour graph each morning is one of the most useful habits. It takes 30 seconds and helps you spot patterns you would miss with individual fingerstick readings.
Time in Range (TIR)
Time in Range (TIR) is a metric that tells you what percentage of the day your glucose stays within a target range. For most adults with Type 1 or Type 2 diabetes, the standard target range is:
Target range: 70-180 mg/dL (3.9-10.0 mmol/L)
The international consensus (Battelino et al., 2019) recommends these goals:
Metric
Target
What it means
Time in Range (70-180)
> 70%
More than ~17 hours per day in target
Time below range (< 70)
< 4%
Less than ~1 hour per day below 70 mg/dL
Time below range (< 54)
< 1%
Less than ~15 minutes per day in serious hypoglycemia
Time above range (> 180)
< 25%
Less than ~6 hours per day above target
Info
TIR is now considered just as important as HbA1c for assessing diabetes management. While HbA1c gives a 2-3 month average, TIR shows you the day-to-day picture — including highs AND lows that HbA1c can miss. A person can have a "good" HbA1c of 7% but still experience frequent dangerous lows and extreme highs that average out.
Don't be discouraged if you are not at these targets yet. Every percentage point increase in TIR is meaningful. Going from 50% to 60% TIR is a significant improvement with real health benefits. Work with your healthcare team to set personalized, achievable goals.
Limitations and precautions
CGMs are remarkable devices, but they are not perfect. Understanding their limitations helps you use them more effectively:
1. Interstitial lag
As mentioned earlier, CGM readings lag about 5-10 minutes behind actual blood glucose. This matters most when glucose is changing rapidly:
After eating: Your blood glucose may be rising, but the CGM number hasn't caught up yet.
During a hypo: Your blood glucose may already be recovering, but the CGM still shows a low number.
Practical rule: If your CGM reading doesn't match how you feel, do a fingerstick to confirm before making treatment decisions.
2. First-day accuracy
Many CGM sensors are less accurate during the first 12-24 hours after insertion. The sensor needs time to stabilize in the interstitial fluid. Consider doing a confirmatory fingerstick if readings seem unusually off during the first day.
3. Compression lows
If you sleep on the arm where your sensor is placed, the pressure can restrict interstitial fluid flow and produce falsely low readings — sometimes triggering low glucose alarms in the middle of the night even though your actual blood glucose is fine.
Solution: Try wearing the sensor on different locations, or learn to recognize compression low patterns (sudden drop that recovers quickly when you change position).
4. Adhesion issues
Sensors need to stay securely attached to your skin for their full wear period (7-14 days). Sweat, water, exercise, and humidity can weaken adhesion over time.
Solutions: Use skin-prep wipes before application, consider over-patches or adhesive tapes (many brands offer CGM-specific patches), and pat the sensor dry after swimming or showering.
5. Acetaminophen / Paracetamol
Older CGM models were significantly affected by acetaminophen (paracetamol/Tylenol), which caused falsely elevated readings. Newer sensors (Dexcom G7, Libre 3) have largely resolved this issue, but it is worth being aware of — particularly if you use an older sensor model.
Warning
A CGM is a powerful tool, but it does not replace clinical judgment. If your CGM reading does not match your symptoms — for example, you feel shaky and low but your CGM shows a normal number — always do a confirmatory fingerstick. Trust your body.
Talking to your doctor about CGM
If you are interested in getting a CGM, here is how to prepare for that conversation:
What to bring to the appointment
Your current testing frequency: How many fingersticks do you do per day? If you struggle to test regularly, CGM can help.
Your challenges: Do you have frequent lows? Highs you can't explain? Fear of nighttime hypoglycemia? These are all strong reasons for CGM.
Insurance information: Know your plan details. Ask your insurance provider in advance if CGM is covered and what documentation they require.
Your goals: What do you hope to gain? Better TIR? Fewer lows? Peace of mind? Being specific helps your doctor advocate for you.
What the guidelines say
The ADA Standards of Care 2024 recommends CGM for:
All adults with Type 1 diabetes — from diagnosis onward
Adults with Type 2 diabetes on intensive insulin therapy (multiple daily injections or pump)
People with diabetes who have impaired hypoglycemia awareness
Pregnant individuals with Type 1 diabetes
These recommendations are getting broader every year as evidence for CGM benefits continues to grow.
Tip
If your doctor is not familiar with CGM technology, consider asking for a referral to an endocrinologist or a diabetes educator who specializes in diabetes technology. Many clinics now have dedicated "diabetes tech" specialists.
Key takeaways
A CGM measures glucose in interstitial fluid every 1-5 minutes, replacing most fingersticks with continuous data
Current systems (Dexcom G7, FreeStyle Libre 3, Medtronic Simplera) are accurate (~8-9% MARD), factory-calibrated, and do not require fingerstick calibrations
Trend arrows show direction AND speed of glucose change — always consider both the number and the arrow before acting
Time in Range target is >70% between 70-180 mg/dL (3.9-10.0 mmol/L), with <4% below 70 mg/dL
