How to Use a Continuous Glucose Monitor? | Setup and Use

A Continuous Glucose Monitor (CGM) gives you real-time glucose readings without pricking your finger multiple times a day. The key to accurate data is proper sensor insertion and calibration — skip either step and your numbers won’t be reliable. This covers the correct procedure for the most common CGM models and the calibration rules each one requires.

How a CGM Actually Measures Your Glucose

A CGM measures glucose in the interstitial fluid just under the skin, not directly in the blood. That interstitial reading lags behind blood glucose by roughly 15 to 45 minutes — a detail that matters for calibration timing. The sensor takes a reading every few minutes and sends it wirelessly to a smartphone app or a dedicated receiver. Per the CDC’s CGM guidance, sensors are replaced every 7 to 14 days depending on the model.

How to Insert a CGM Sensor Correctly

The insertion procedure varies slightly by model, but the core steps are the same: clean the site firmly, apply the sensor, and secure it. Below are the official procedures for the two most widely used models.

Dexcom G7 Insertion (Official Dexcom Procedure)

1. Wash and dry your hands thoroughly.
2. Choose the back of the upper arm — enough fat to avoid muscle, and a spot you won’t bump regularly.
3. Clean the site with an alcohol wipe and let it dry for at least 10 seconds.
4. Unscrew the applicator cap. Do not touch the inside.
5. Press the applicator firmly against the skin and push the button.
6. Rub firmly around the patch three times, then press gently on the sensor for 10 seconds.
7. Apply the overpatch: peel one liner at a time without touching the white adhesive, rub around the overpatch, peel the colored liner, and rub again.

You’ll see the sensor’s first reading appear in the app within about 25 minutes. The G7 is factory-calibrated and requires no manual calibration for standard use, though an optional calibration is available via the receiver menu.

Dexcom G6 Insertion (Official US MED Procedure)

1. Wash and dry your hands. Check the sensor and transmitter “use by” date.
2. Choose the abdomen if you’re 18 or older. For ages 2–17, the lower back or upper buttocks is recommended.
3. Keep the site at least 3 inches from any injection or infusion set, 2 inches from the belly button, and away from scars, tattoos, hair, and bony areas.
4. Clean the site with rubbing alcohol and let it dry.
5. Remove the applicator’s paper tabs without touching the adhesive. Place the applicator horizontally and fold or break the safety guard.
6. Press the button to insert the sensor. Remove and dispose of the applicator.
7. Clean the transmitter with an alcohol wipe, let it dry, then insert the tab into the sensor slot — smaller end first, then the wider end. Listen for the click.
8. Rub the adhesive three times. Do not remove the transmitter until the sensor needs replacement.

The G6 is factory-calibrated, but you must enter the unique sensor code printed on the adhesive label during startup. No daily manual calibration is needed.

Using a Continuous Glucose Monitor Correctly: Calibration Rules for Every Model

Not every CGM requires daily finger-stick calibration, but the ones that do have strict timing and range rules. Calibrating with a stale blood glucose (BG) reading or during rapid glucose change produces bad data that lasts until the next calibration cycle.

CGM Model Comparison at a Glance

Choosing the right CGM affects everything from insertion frequency to out-of-pocket cost. The table below maps the key specs for the six most common models available in the US. Most private insurers and Medicare cover these devices with a diabetes diagnosis — Medicare Part D covers the Dexcom G7, while Part B covers the Eversense 365.

If you’re monitoring glucose for general wellness rather than diabetes management, our tested guide to the best continuous glucose monitors for non-diabetics compares the top models for fitness tracking and metabolic health.

Common CGM Mistakes That Skew Your Readings

Most accuracy issues trace back to a handful of preventable errors. Avoid these and your CGM data will stay reliable:

• Calibrating with an old BG number. Glucose changes constantly; enter the value within 3 minutes of the finger-stick test.
• Calibrating during rapid glucose change. Avoid calibrating when trend arrows point up or down — right after eating, insulin, or exercise.
• Using expired test strips or dirty hands. Both introduce error into the calibration BG value.
• Calibrating outside the 40–400 mg/dL range. Eversense 365 and MiniMed 780G require BG within this range to accept a calibration.
• Touching the adhesive or the sensor interior. Oil from your fingers reduces adhesion and can contaminate the sensor.
• Applying the sensor over lotion or moisturizing soap. Residue prevents the adhesive from bonding properly.
• Reusing the same sensor site repeatedly. Scar tissue and irritation build up; rotate sites each time.

Getting Reliable Data From Your CGM

A few habits separate accurate CGM users from the ones chasing bad readings. Rotate your sensor site every replacement. Never calibrate during rapid glucose swings. Keep the sensor dry and the overpatch sealed. And when your CGM reading seems off — especially with the 15 to 45 minute lag time in mind — confirm with a finger-stick test before making treatment decisions. The CDC’s CGM guidance notes that consistent site rotation and proper calibration timing are the two biggest factors in maintaining accuracy over the life of each sensor.