Glucose sensors are wearable devices that measure your glucose continuously through a thin filament inserted just under the skin. They sample glucose in the interstitial fluid (not directly in the blood) every one to five minutes and stream readings to a phone or reader. Modern sensors wear for 10 to 15 days, do not need fingerstick calibration, and are now available both by prescription and over the counter.
How Glucose Sensors Work
Almost every glucose sensor on the market today uses the same fundamental chemistry: an enzyme called glucose oxidase coats a tiny wire-like filament that sits just under the skin in the fluid that bathes your cells (the interstitial fluid). When glucose molecules contact the enzyme, they react and produce a small electrical current proportional to glucose concentration. The sensor measures that current and converts it into a glucose reading.
A small adhesive transmitter sits on top of the skin and wirelessly sends readings to a smartphone app or dedicated receiver. Modern sensors are about the size of two stacked nickels, painless to insert, and can be worn through showers and most exercise.
Interstitial vs Blood Glucose
One important nuance is that sensors measure interstitial glucose, not capillary or venous blood glucose. Glucose moves from blood to interstitial fluid by diffusion, which takes time. The lag is typically 5 to 15 minutes. When glucose is stable, sensor and blood readings match closely. When glucose is rising or falling rapidly (after a meal, during exercise, or during a hypoglycemic episode), the sensor reading may temporarily lag behind the blood value. This is why CGMs include trend arrows that indicate the direction of change, not just the current number.
Major Glucose Sensors Available in the US
| Sensor | Maker | Wear Time | Reading Interval | Access |
|---|---|---|---|---|
| Dexcom G7 | Dexcom | 10 days + 12 hr grace | Every 5 min | Prescription |
| FreeStyle Libre 3 | Abbott | 14 days | Every 1 min | Prescription |
| FreeStyle Libre 2 Plus | Abbott | 15 days | Every 1 min | Prescription |
| Eversense E3 | Senseonics | 180 days (implanted) | Every 5 min | Prescription |
| Stelo | Dexcom | 15 days | Every 15 min | Over the counter |
| Lingo | Abbott | 14 days | Every 1 min | Over the counter |
Calibration and Accuracy
Older CGMs required fingerstick calibration once or twice a day. Modern sensors (Dexcom G7, Libre 3, Stelo, Lingo) are factory-calibrated and need no fingerstick under normal conditions. Accuracy is measured by mean absolute relative difference (MARD), the average percentage difference between sensor readings and lab-measured blood glucose. Today’s sensors typically achieve MARD values around 8 to 10 percent, which is comparable to many home blood glucose meters.
According to the American Diabetes Association Standards of Care, CGMs are recommended for all adults using insulin and increasingly considered for adults with type 2 diabetes managed with non-insulin medications. The shift toward over-the-counter access reflects growing interest in glucose data among people with prediabetes and the general public.
Over-the-Counter Sensors
In 2024 the FDA cleared the first OTC continuous glucose biosensors. Stelo from Dexcom and Lingo from Abbott are aimed at adults who are not on insulin and want general metabolic insights. They use the same underlying sensor technology as the prescription versions but pair with simpler apps that focus on glucose trends, meal responses, and lifestyle insights rather than alarms for hypoglycemia. They are not approved for diabetes treatment decisions and people on insulin should use a prescription-strength CGM.
OTC sensors typically cost 80 to 100 dollars per sensor (15 days of wear), so monthly costs run about 160 to 200 dollars without insurance. They are widely available through online retailers and pharmacies.
Implantable Sensors
The Eversense E3 sensor takes a different approach. Rather than being inserted by the user every two weeks, it is implanted by a clinician in the upper arm and lasts 180 days. A smart transmitter worn on the skin powers the sensor and sends readings to a phone. The implantable approach reduces the hassle of frequent insertions but requires a brief in-office procedure twice a year.
What Glucose Sensors Can Tell You
A typical CGM dashboard shows:
- Current glucose value and trend arrow
- 24-hour glucose curve with target range shaded
- Time in range (the percentage of readings between, for example, 70 and 180 mg/dL)
- Average glucose and estimated A1C
- Meal, exercise, and sleep tags for context
For people with prediabetes, glucose sensors can reveal how individual foods affect glucose, whether overnight glucose is climbing, and how exercise blunts post-meal spikes. To learn more about glucose targets and what readings mean, see our A1C levels guide.
Limitations to Know
- Lag during rapid changes: Sensor readings may differ from a fingerstick when glucose is rising or falling quickly.
- Compression lows: Lying on a sensor can produce a false low reading. Move and recheck.
- Adhesive irritation: Some users develop skin irritation. Barrier sprays and overpatches help.
- Signal dropout: Bluetooth connection between sensor and phone occasionally drops; readings backfill once reconnected.
- Cost: Sensors remain expensive, and OTC versions are not yet covered by insurance.
Are Glucose Sensors Useful in Prediabetes?
The clinical evidence for using CGMs in people with prediabetes (no diagnosed diabetes, not on glucose-lowering medication) is still developing. Studies suggest that glucose sensor feedback can prompt healthier food choices, motivate physical activity after meals, and identify post-meal hyperglycemia that fingerstick testing misses. Critics caution that in healthy people, normal post-meal glucose excursions can look alarming on a CGM but do not necessarily indicate a problem. For background on prediabetes detection more broadly, see our prediabetes 101 hub.
The Bottom Line
Glucose sensors are a major step forward in diabetes management and an emerging tool for the wider public. They use a tiny under-skin filament with a glucose-oxidase enzyme to measure interstitial glucose every few minutes, giving you a continuous picture of how food, exercise, sleep, and stress affect your numbers. Choosing among the prescription CGMs (Dexcom G7, FreeStyle Libre 3, Eversense) or the new OTC options (Stelo, Lingo) depends on your medical needs and budget. Talk with your clinician about whether a glucose sensor fits into your overall plan.
