A continuous glucose monitor (CGM) uses a tiny sensor under the skin to track glucose levels around the clock, with initial setup taking about 10 minutes once you prepare the site correctly.
Knowing how to use a CGM correctly from day one saves you from wasted sensors, false readings, and a device that won’t pair. Whether you have type 1 or type 2 diabetes, or your clinician recommended one for other reasons, the setup follows a consistent sequence across major brands, and common pitfalls are easy to avoid once you know where to look.
What a CGM Does and Who Uses It
A CGM measures glucose levels in interstitial fluid — the fluid just beneath your skin — and sends readings to a receiver or smartphone app every few minutes. Unlike a standard fingerstick, which gives a single point-in-time number, a CGM shows trends, direction arrows, and alerts for highs and lows. In the U.S., CGMs are typically prescribed for people with diabetes, though some clinicians now recommend them for other metabolic conditions. The most common systems are the Dexcom G7 and FreeStyle Libre, both pairing with iPhone and Android apps via Bluetooth. Sensor wear time and warm-up vary by brand — Dexcom’s G7 warms up in about 30 minutes and lasts up to 10 days, while the FreeStyle Libre takes 60 minutes to start and can be worn for up to 14 days.
Step-by-Step: How to Apply and Start Your CGM
The exact steps differ slightly by brand, but the core workflow is the same for all modern CGMs. Always follow your specific device manual.
1. Choose and prepare the insertion site. The back of the upper arm is the most common approved location for both the Dexcom G7 and FreeStyle Libre. Some systems also allow the abdomen — check your device manual. Clean the area with the supplied alcohol wipe and let it dry completely. Moisture under the sensor pad weakens adhesion and can cause early failure. Give the alcohol a full 30 to 60 seconds to evaporate before you proceed.
2. Apply the sensor with the applicator. Remove the sensor pack from its sterile wrapper. Align the marks on the applicator with the sensor pod and press until it clicks. Place the applicator firmly on your skin and press the button to insert. Dexcom’s official training video instructs you to rub the adhesive patch firmly three times and press gently on the sensor for 10 seconds to ensure good adhesion.
3. Start the sensor and pair it with your device. Download the manufacturer’s app before you begin. Enable Bluetooth, then follow on-screen instructions — you may need to scan a QR code or enter a serial number. With the Dexcom G7, use the app or receiver to start a session. With the FreeStyle Libre, scan the sensor with the reader or app, and readings begin after the 60-minute warm-up. A blank screen or dashes during warm-up is normal, not a defect.
4. Verify and protect the sensor. Check that the adhesive lies flat with no lifted edges. If your manufacturer allows, apply an overpatch or medical tape for extra security — but never cover the transmitter itself, as blocking it can interfere with signal transmission. Inspect the site daily for redness, swelling, or irritation, and rotate locations with each new sensor.
If you’re weighing which system is right for your routine, our comparison of the best continuous glucose monitors for type 2 diabetes covers accuracy, cost, and real-world pros and cons of each option.
Common Mistakes That Lead to Failed Readings
Most CGM problems trace back to a handful of avoidable errors. Not letting the skin dry after cleaning is the most frequent cause of poor adhesion. Using a site the manufacturer hasn’t approved for your specific model can also cause problems — insertion sites are not interchangeable across brands. Skipping or rushing the warm-up period leads many users to think the device is broken when it is simply initializing. CGM readings come from interstitial fluid, not blood, so there is a natural lag during rapid glucose changes. If the reading seems off or does not match how you feel, confirm with a fingerstick before acting — the Mayo Clinic recommends this as a standard safety step. Also, some CGMs require occasional calibration with a fingerstick while others, like the Dexcom G7, are factory-calibrated and do not. Check your device manual.
FAQs
Can I shower or swim with a CGM sensor on?
Yes, most modern CGMs are water-resistant and designed to stay on during showering, swimming, and exercise. The Dexcom G7 and FreeStyle Libre are both rated for normal daily activities including brief immersion, but check your device manual for exact depth and duration limits.
Do I still need to check my blood sugar with fingersticks?
It depends on your CGM model. The Dexcom G7 is factory-calibrated and does not require routine fingersticks. Other systems may need occasional calibration. Regardless of your device, always confirm with a fingerstick if the reading seems wrong or if you feel symptoms that don’t match the display.
How should I store unused CGM sensors?
Store sensors in their sealed sterile packaging at room temperature, away from direct sunlight and extreme heat or cold. Most manufacturers recommend a storage temperature range between 36°F and 86°F. Do not use a sensor if the sterile package is damaged or if the expiration date has passed.
References & Sources
- Mayo Clinic. “Blood Sugar Testing: Why, When and How.” Covers CGM vs. fingerstick accuracy and when to confirm readings.
- Dexcom. “How to Insert Dexcom G7 Sensor.” Official training video for G7 sensor application and startup.
- diatribe. “So You Got a CGM — Now What?” Practical guidance for new CGM users on setup and common pitfalls.
Mo Maruf
I created WellFizz to bridge the gap between vague wellness advice and actionable solutions. My mission is simple: to decode the research and give you practical tools you can actually use.
Beyond the data, I am a passionate traveler. I believe that stepping away from the screen to explore new environments is essential for mental clarity and physical vitality.