Using a continuous glucose monitor (CGM) starts with applying a small sensor to your skin, pairing it with your phone, and then learning to interpret the real-time glucose data it streams throughout the day. The process is straightforward, but getting the most out of your CGM means understanding warm-up times, trend arrows, target ranges, and a few practical tricks that make daily wear more comfortable.
Applying the Sensor
Every CGM comes with a one-touch applicator that inserts a tiny, flexible filament just beneath your skin. The filament sits in the interstitial fluid (the fluid between your cells) rather than in your blood, which is how the sensor estimates glucose levels continuously without repeated fingersticks.
Before applying, clean the site with an alcohol wipe and let it dry completely. Moisture under the adhesive is the most common reason sensors peel off early. Most people apply sensors to the back of the upper arm, though the abdomen is also approved for several devices. Pick a spot without much muscle movement, scarring, or body hair. Rotating your placement site each time you swap sensors helps prevent skin irritation.
Warm-Up and First Readings
Once the sensor is on, it needs time to acclimate to your body before readings appear. This warm-up period varies by device:
- Dexcom G7 and Stelo: 30 minutes
- FreeStyle Libre models (Libre 2, Libre 3, Libre 3 Plus): 1 hour
- Dexcom G6 and Medtronic Guardian Connect: 2 hours
- Eversense 365: 24 hours
During warm-up, no glucose data will appear on your phone or receiver. This is normal. If you’re switching sensors, it helps to apply the new one slightly before the old one expires so you avoid a gap in data.
Pairing With Your Phone or Watch
Most CGMs connect to a companion app on your smartphone via Bluetooth. Open the app, follow the on-screen pairing instructions, and keep your phone within Bluetooth range of the sensor (typically 20 feet or so). The Dexcom G7 also supports a direct Bluetooth connection to Apple Watch, meaning you can leave your phone behind during a run or errand and still see your glucose readings on your wrist. It’s currently the only CGM offering that independent watch connection.
You can usually share your data with a family member or partner through the app. This is especially useful for parents monitoring a child’s glucose or for anyone who wants a backup set of eyes on their numbers overnight.
Reading the Numbers and Trend Arrows
Your CGM displays two pieces of information: your current glucose reading and a trend arrow showing which direction it’s heading. The number tells you where you are; the arrow tells you where you’re going. Learning to use both together is the single most useful skill for CGM users.
Trend arrows represent your rate of glucose change per minute, projected forward about 30 minutes. A flat, horizontal arrow means glucose is relatively stable. A single arrow angling upward or downward suggests a moderate rise or fall. Double arrows mean glucose is changing rapidly, more than 3 mg/dL per minute, which translates to a projected shift of 90 mg/dL or more over the next half hour. Double arrows in either direction deserve attention.
There’s no universal standard for how different CGM brands display these arrows, so spend a few minutes with your device’s guide to understand exactly what each symbol means for your system.
Understanding Time in Range
The most important metric your CGM tracks over time isn’t any single reading. It’s “Time in Range,” or the percentage of the day your glucose stays between 70 and 180 mg/dL. An international consensus of diabetes specialists set the following targets for most adults with type 1 or type 2 diabetes:
- 70–180 mg/dL: Aim for more than 70% of the day (roughly 17 hours)
- Above 180 mg/dL: Less than 25% of the day (under 6 hours)
- Above 250 mg/dL: Less than 5% of the day (under about 1 hour)
- Below 70 mg/dL: Less than 4% of the day (under 1 hour)
- Below 54 mg/dL: Less than 1% of the day (under 15 minutes)
Your CGM app calculates these percentages automatically, usually displayed in a daily or weekly summary. Watching your Time in Range trend over weeks gives you a much clearer picture of your glucose management than any single fasting number or A1C test alone. Small improvements, even going from 60% to 70%, are clinically meaningful.
Calibration: Do You Need Fingersticks?
Older CGM systems required fingerstick calibrations every 12 hours, matching the sensor’s electrical signal to an actual blood glucose reading to maintain accuracy. Current-generation sensors from Dexcom and Abbott come factory-calibrated, meaning the calibration is built into the sensor during manufacturing. You can start using them without a single fingerstick.
That said, there are moments when a fingerstick is still wise. If your CGM reading doesn’t match how you feel, if you’re about to make a treatment decision based on a reading that seems off, or if you’re in the first few hours after insertion when readings can be less stable, confirming with a traditional meter adds a layer of safety. Some people also choose to manually calibrate factory-calibrated devices, which most systems allow as an optional step in the settings.
CGM accuracy has improved dramatically. Early systems had error margins around 25% compared to lab-grade analyzers. The latest generation has narrowed that to around 9–11%, making them reliable enough for insulin dosing decisions in most situations.
Wearing Your Sensor During Exercise and Water
CGM sensors are designed to stay on during your normal activities, including workouts and showers. Water resistance varies by device. Dexcom G6 and G7 sensors can be submerged in up to 8 feet of water for about 24 hours, making them suitable for swimming laps or a day at the pool. FreeStyle Libre sensors handle up to 3 feet of water for 30 minutes, which covers showers and casual splashing but may not hold up to extended swimming. Neither Libre model has been tested in salt water, so ocean swimming is a gray area.
During intense exercise, your glucose can change quickly, and sweat can loosen the adhesive. Applying an over-adhesive patch before a workout helps. Some people also find that readings lag slightly during rapid glucose shifts from exercise, since the sensor measures interstitial fluid rather than blood directly. There’s typically a 5 to 15 minute delay.
Keeping the Sensor Stuck
A sensor that peels off early is one of the most common frustrations. Several products can help. Skin prep wipes like Skin Tac or IV Prep clean the area and leave a tacky residue that improves adhesion. For extra hold, transparent film dressings like Tegaderm or Opsite Flexigrid can be placed over the sensor patch.
If you develop itching, redness, or a rash under the adhesive, you may be reacting to the sensor’s adhesive compound. A barrier film like Cavilon, applied to the skin before sensor insertion, creates a protective layer between your skin and the adhesive so the sensor components never touch you directly. For people with especially sensitive skin, zinc-oxide-based tapes like Hy-Tape cause less irritation than standard adhesive options.
Avoiding False Low Readings at Night
If your CGM alarm wakes you with a low glucose alert but you feel perfectly fine, you may be experiencing a “compression low.” This happens when you sleep on top of the sensor. Your body weight presses on the tissue around the filament, squeezing out interstitial fluid. With less fluid to measure, the sensor detects less glucose and reports a falsely low number.
The fix is simple: avoid sleeping directly on the arm or area where your sensor sits. If you tend to roll onto your sensor side, placing it on the back of your non-dominant arm (the side you’re less likely to sleep on) helps. Compression lows resolve on their own within minutes of shifting position, and they don’t require treatment.
Making Sense of Your Data Over Time
The real power of a CGM isn’t any single glucose reading. It’s the patterns you spot over days and weeks. Most apps generate reports showing your average glucose, Time in Range, and an “ambulatory glucose profile” that overlays multiple days on one graph so you can see recurring spikes or dips.
Look for patterns tied to meals, sleep, stress, and exercise. You might notice that your glucose spikes sharply after breakfast but stays flat after dinner, which tells you something useful about those specific meals. Or you might see a consistent rise in the early morning hours, a phenomenon driven by hormones that becomes obvious only with continuous data. These patterns are what make CGMs so much more informative than occasional fingerstick checks. Instead of snapshots, you’re watching the full movie of your glucose throughout the day, and that context changes how you eat, move, and manage your health.