A Dexcom blood sugar monitor is a continuous glucose monitor (CGM) that uses a small, under-skin sensor to measure interstitial glucose every one to five minutes and stream those readings to your phone or receiver — no routine fingersticks required.
For anyone managing diabetes, navigating prediabetes, or simply trying to understand how food, sleep, and stress shift their glucose, a Dexcom CGM offers a level of detail a glucometer cannot match. This guide walks through how the device works, how accurate it is, and what to expect from your first 10 days of wear.
How a Dexcom Sensor Actually Reads Glucose
The Dexcom sensor sits just under the skin — typically on the back of the upper arm for the G7 model — and uses a tiny filament coated with glucose oxidase enzyme. When glucose in your interstitial fluid hits that enzyme, it produces a small electrical current. The transmitter translates that current into a glucose value and sends it via Bluetooth to your phone every five minutes.
Because Dexcom reads interstitial fluid rather than blood, there is a 5- to 10-minute lag compared with a fingerstick taken at the same moment. That delay is rarely a problem during stable periods but matters during rapid rises (like after a sugary meal) or rapid drops (during exercise). The G7 algorithm smooths and projects readings to narrow that gap.
How Accurate Is It? The MARD Numbers
Accuracy in the CGM world is measured as Mean Absolute Relative Difference, or MARD — the average percentage difference between sensor readings and a lab reference. Lower is better.
| Device | Overall MARD | Sensor Wear | Fingerstick Calibration |
|---|---|---|---|
| Dexcom G7 | 8.2% | 10 days (+12 hr grace) | Not required |
| Dexcom G7 15-Day | 8.0% | 15 days | Not required |
| Dexcom Stelo (OTC) | ~10% | 15 days | Not required |
| FreeStyle Libre 3 | 7.9% | 14 days | Not required |
| Fingerstick glucometer | ~5-7% | Per-reading | N/A |
A MARD under 10% is considered clinically excellent. According to FDA 510(k) documentation, the G7 meets the agency’s integrated CGM (iCGM) special controls, meaning its data is accurate enough to dose insulin from directly.
Setting Up and Wearing Your First Sensor
The G7 applicator is a single piece — no separate transmitter. You peel the sticker, press the applicator firmly against clean upper-arm skin, and push the button. The sensor inserts in a fraction of a second, usually painlessly. The 30-minute warm-up begins immediately; after that your phone shows a live glucose curve.
A few first-time wearer tips:
- Clean the skin with alcohol and let it fully dry before applying.
- Rotate sites between arms every session to avoid skin irritation.
- Use an overpatch (Dexcom provides one per sensor) if you sweat heavily or sleep hot.
- Keep your phone within 20 feet to avoid data gaps.
What Dexcom Data Shows You
The Dexcom app presents glucose as a live trend line with directional arrows (rising fast, rising slow, steady, falling, falling fast). Over time, the Clarity app aggregates your data into three metrics that matter most:
- Time in Range (TIR): Percentage of time between 70-180 mg/dL (or 70-140 mg/dL if you are using the monitor to chase tighter prediabetes control).
- Glucose Management Indicator (GMI): An estimated A1C based on your average glucose. It’s useful for tracking progress between lab A1C tests.
- Coefficient of Variation (CV): How “bumpy” your glucose is. Under 36% is the target.
Who Should Consider One
Dexcom G7 is FDA-cleared for people aged 2 and older with diabetes. Dexcom Stelo, the over-the-counter option, is cleared for adults 18 and up who are not on insulin — which covers most people with prediabetes or diet-controlled type 2.
A CGM is particularly valuable if you have trouble identifying which foods spike you, want to see how sleep or stress affects your numbers, or are actively working to reverse prediabetes through lifestyle change. Seeing real-time feedback after a bowl of oatmeal versus eggs can change behavior more quickly than a quarterly lab report.
Insurance, Cost, and OTC Options
With insurance, G7 often runs $35-75 per month out of pocket for people with type 1 or insulin-dependent type 2 diabetes. Without insurance, cash pay runs $100-180 per 10-day sensor. Stelo, the OTC version, is currently priced at $89 for a 30-day supply (two 15-day sensors) direct from Dexcom — a more realistic on-ramp for prediabetes users.
The Bottom Line
A Dexcom blood sugar monitor gives you continuous, research-grade glucose data without the routine pain of fingersticks. With a MARD near 8% and no calibration required, the G7 and Stelo products are reliable tools for catching patterns you would otherwise miss. Talk with your clinician about whether a 2-4 week CGM trial fits into your prediabetes or diabetes plan — the data may reshape how you eat, move, and sleep.
This article is educational and not a substitute for personalized medical advice. Consult your healthcare provider before starting or changing any glucose-monitoring or treatment plan.
