A continuous glucose monitor (CGM) is a small wearable sensor that tracks your glucose levels around the clock, sending readings to your phone or receiver every few minutes. Using one involves applying the sensor to your skin, waiting through a short warm-up period, then learning to read the data it produces. The process is straightforward once you understand the basics, but there are practical details that make the difference between a frustrating experience and a useful one.
Choosing and Preparing the Sensor Site
Most CGM sensors are approved for the back of the upper arm or the abdomen, depending on the brand. Your device’s instructions will specify which locations are cleared for your particular model. Whichever spot you choose, the skin should be clean, dry, and free of lotions, sunscreen, or moisturizer. Wash the area with soap and water, let it dry completely, and avoid using alcohol wipes right before application unless your device specifically calls for them, since residue from some products can weaken the adhesive bond.
Rotate your insertion site each time you apply a new sensor. Using the same spot repeatedly increases your risk of skin irritation and can affect sensor accuracy. Pick a location that won’t be compressed by tight clothing, a waistband, or a seatbelt for long periods.
Applying the Sensor
Today’s consumer CGMs come with a one-piece applicator that handles everything in a single push or click. You peel off the backing, press the applicator firmly against your skin, and activate it. A small, flexible filament inserts just beneath the skin’s surface to sit in the interstitial fluid (the fluid between your cells), where it measures glucose. The insertion needle retracts automatically into the applicator, so you never handle it directly.
Most people describe the sensation as a brief pinch or less. If you feel ongoing sharp pain after application, the sensor may be placed poorly, and it’s worth removing it and trying a new one at a different site.
The Warm-Up Period
Every new sensor needs time to calibrate itself before it begins reporting glucose values. The Dexcom G7 has a 30-minute warm-up that starts automatically the moment the sensor is applied. The FreeStyle Libre 3 requires 60 minutes and begins its warm-up only after you scan the sensor with your phone. During this window, you won’t receive any readings. If you need to know your glucose level during warm-up, a fingerstick meter is your backup.
A practical tip: if you want uninterrupted data coverage, apply your new sensor shortly before your old one expires so the warm-up finishes while the previous sensor is still active.
Reading Your Glucose Number
Your CGM displays a glucose value updated every one to five minutes, depending on the model. That number comes from interstitial fluid, not blood, which introduces a small time lag. Research measuring this delay found it averages about 5 to 6 minutes under resting conditions. The lag can stretch longer when glucose is changing quickly, like after a meal or during exercise. This means the number on your screen reflects where your glucose was a few minutes ago, not exactly where it is right now.
In practical terms, this matters most during rapid changes. If you just ate a high-carb meal and your CGM reads 130 mg/dL with a rising arrow, your actual blood glucose is likely already a bit higher. When glucose is stable, the lag is negligible.
What the Trend Arrows Mean
The trend arrows next to your glucose reading are arguably more useful than the number itself, because they tell you where your glucose is headed. While the exact thresholds vary slightly between brands, the general framework is consistent.
- Horizontal arrow: Glucose is stable, changing less than 1 mg/dL per minute.
- Single up or down arrow: Glucose is rising or falling at roughly 1 to 3 mg/dL per minute. That translates to a shift of 15 to 45 mg/dL over 15 minutes.
- Double up or down arrows: Glucose is moving fast, more than 3 mg/dL per minute on Dexcom systems. That’s a potential swing of over 45 mg/dL in just 15 minutes.
A reading of 110 mg/dL with a horizontal arrow is very different from 110 mg/dL with double down arrows. The first is stable and comfortable. The second means you could be below 70 mg/dL within 15 minutes and should act accordingly. Learning to pair the number with the arrow is the single most important skill in using a CGM effectively.
How Accurate Are CGM Readings?
Modern CGMs are accurate enough to make treatment decisions, but they aren’t perfect. Accuracy is measured by something called Mean Absolute Relative Difference (MARD), which represents the average percentage a CGM reading differs from a lab-grade blood glucose measurement. Lower is better. In a 2024 head-to-head study of adults with type 1 diabetes, the FreeStyle Libre 3, Dexcom G7, and Medtronic Simplera all posted MARDs between roughly 10% and 12% compared to venous lab draws. That means if your true glucose is 100 mg/dL, the CGM might read anywhere from about 88 to 112 mg/dL on average.
Manufacturer studies submitted for regulatory approval reported somewhat better numbers (7.5% to 10.2% MARD), which is common since those studies are conducted under more controlled conditions. Real-world accuracy is good, but not laboratory-grade. When you suspect a reading is off, or when glucose is changing faster than 2 mg/dL per minute, a fingerstick meter is the appropriate cross-check.
When to Use a Fingerstick Meter
Factory-calibrated CGMs have largely eliminated the need for routine fingerstick calibrations, but there are still specific situations where a blood glucose meter is necessary. The 2025 Standards of Care in Diabetes list these scenarios: when you suspect the CGM reading is inaccurate, during the sensor warm-up period, if there’s a gap in data transmission, when a warning message appears on the device, if your CGM supplies are delayed and you’re between sensors, and any time glucose is changing very rapidly.
Keep a meter and test strips accessible even if you go weeks without needing them. A CGM is a monitoring tool, not a replacement for every possible glucose check.
Compression Lows and False Alarms
One of the most common CGM quirks is the “compression low,” a falsely low reading caused by sleeping on the sensor. When you press your body weight against the sensor site, you reduce the flow of interstitial fluid around the filament. Less fluid means fewer glucose molecules for the sensor to detect, so it reports a low that isn’t real.
You can usually identify a compression low by its pattern: a sudden, steep drop during sleep that doesn’t match what was happening before. If your glucose was stable at 100 mg/dL and abruptly plunges to 55 mg/dL without any exercise or insulin to explain it, pressure on the sensor is the likely culprit. Genuine reactive hypoglycemia, by contrast, typically follows a recognizable arc of a rapid spike, sharp fall, and slow recovery over about an hour. If you get frequent compression lows, try placing the sensor on a site you’re less likely to roll onto at night.
Keeping the Sensor Stuck
Sensor adhesive is designed to last the full wear period (10 to 15 days depending on the brand), but sweat, humidity, swimming, and friction can loosen it well before that. Start with the least intervention possible: clean, dry skin and no lotions. If that’s not enough, liquid skin adhesive products applied around (not on) the sensor insertion point can significantly improve how well the patch holds. Apply the adhesive to the surrounding skin, let it dry completely until it feels tacky, and then place the sensor on top.
Transparent medical dressings placed over the sensor serve a dual purpose. They reinforce the adhesive and also act as a barrier between the sensor’s plastic or adhesive materials and your skin, which helps if you’ve experienced contact irritation. Some CGM manufacturers sell their own over-patches, but generic medical film dressings work the same way.
Swimming, Showering, and Water Exposure
Most current CGMs are water-resistant once the sensor is fully applied. Dexcom sensors are rated for submersion up to 2.4 meters (about 8 feet) for up to 24 hours. FreeStyle Libre sensors carry a similar water-resistance rating for shallow water. Normal showering, bathing, and recreational swimming are fine.
Two caveats: the receiver or phone displaying your data is not waterproof, so leave it at the pool’s edge. And prolonged water exposure, even within rated limits, can loosen adhesive over time. Patting the sensor dry promptly and using supplemental adhesive before water activities helps the sensor last its full lifespan.
Protecting Your Skin
Wearing an adhesive patch on your skin for 10 to 15 days at a time can cause irritation, especially with repeated use. The most effective prevention is site rotation, giving each area of skin a break before using it again. If you develop redness or itching under the adhesive, a thin transparent dressing placed on the skin before sensor insertion creates a physical barrier so the adhesive never directly contacts your skin. This technique also reduces the skin trauma that comes with removing the sensor at the end of its wear period.
If you notice persistent rashes, blistering, or worsening irritation that doesn’t resolve with rotation and barriers, you may be reacting to a specific component in the adhesive. Switching brands sometimes resolves the issue, since different manufacturers use different adhesive formulations.