What is continuous glucose monitoring?
Continuous glucose monitoring (CGM) is a method of tracking blood glucose (blood sugar) levels automatically and continuously throughout the day and night. A small sensor worn on the body measures glucose in the fluid between cells (interstitial fluid) every one to five minutes and transmits the data wirelessly to a receiver, smartphone, or smartwatch.
Unlike traditional blood glucose meters that require a finger-stick blood sample to get a single reading, CGM provides a continuous stream of data — showing not just where your glucose is right now, but where it has been over the past hours, and trend arrows that indicate where it is heading.
CGM does not measure glucose in the blood directly. It measures glucose in interstitial fluid — the fluid surrounding cells in tissue just below the skin. This is closely correlated with blood glucose but typically lags blood glucose by 5–15 minutes, which is important to understand when interpreting readings.
How does CGM work?
A CGM system has three main components that work together to deliver continuous glucose data to the user.
The sensor
A small, flexible filament — typically 5–7mm long and thinner than a human hair — is inserted just below the skin surface, usually on the abdomen or upper arm. The sensor contains an enzyme (glucose oxidase) that reacts with glucose in the interstitial fluid and generates a tiny electrical current proportional to the glucose concentration. This electrochemical signal is the raw glucose measurement.
The transmitter
Attached to the sensor, the transmitter reads the electrical signal from the sensor, applies calibration algorithms, converts it to a glucose value in mg/dL or mmol/L, and transmits the data wirelessly via Bluetooth to a display device. Transmitters are either integrated into disposable sensors or reusable components that attach to each new sensor.
The receiver / app
The glucose data is received by a dedicated receiver, smartphone app, or smartwatch. The display shows the current glucose value, a trend arrow indicating direction and rate of change, a graph of recent glucose history, and alerts when glucose goes out of the user's target range. Data can be shared with family members or healthcare providers in real time.
Most modern CGM sensors last 10–15 days before requiring replacement. Insertion is done with a small applicator device — the process takes seconds and causes minimal discomfort for most users. After a warm-up period of one to two hours, the sensor begins delivering readings.
CGM vs traditional blood glucose monitoring
Traditional self-monitoring of blood glucose (SMBG) using a finger-stick meter has been the standard for decades. CGM does not fully replace it in all situations, but provides a fundamentally different and richer picture of glucose patterns.
| Feature | CGM | Finger-stick meter |
|---|---|---|
| Readings per day | 288 (every 5 min) | 4–8 (manual) |
| Trend information | Yes — direction and rate | No — single snapshot |
| Overnight monitoring | Continuous, with alerts | Not practical |
| Finger-stick required | No (most devices) | Every reading |
| Real-time alerts | Yes — high and low alerts | No |
| Data sharing | Real-time to caregivers | Manual log only |
| Cost | Higher (sensor subscription) | Lower upfront |
| Accuracy in rapid change | May lag by 5–15 min | Immediate blood value |
Even with CGM, finger-stick verification is recommended when CGM readings seem inconsistent with symptoms, before making critical treatment decisions (such as insulin dosing in some circumstances), and when CGM indicates rapidly changing glucose. Always follow your healthcare provider's guidance on when to verify with a finger-stick.
Benefits of continuous glucose monitoring
The clinical evidence for CGM benefits in people with diabetes is substantial and growing. Beyond diabetes management, CGM is revealing new insights about metabolic health in broader populations.
The benefits described above are based on published clinical research in specific patient populations. Individual results vary. CGM is a monitoring tool, not a treatment. Any changes to your diabetes management — including insulin dosing, medication, or diet — should be made in consultation with your healthcare provider.
Who should use CGM?
CGM was originally developed for people with Type 1 diabetes who require intensive insulin therapy. Its indications have expanded substantially and it is now used across a much broader population.
How accurate is CGM?
CGM accuracy is measured using Mean Absolute Relative Difference (MARD) — the average percentage difference between CGM readings and simultaneous blood glucose measurements. Lower MARD indicates higher accuracy.
Current generation CGM devices from major manufacturers achieve MARD values of 8–10%, compared to approximately 5% for a high-quality finger-stick meter. This means CGM readings are clinically accurate for most diabetes management decisions, though individual readings may differ from blood glucose by up to 15–20% in some circumstances.
Factors that affect CGM accuracy
Rapid glucose changes: Because CGM measures interstitial glucose, it lags blood glucose by 5–15 minutes during periods of rapid change — such as during or after eating, during intense exercise, or during hypoglycemia. Trend arrows help compensate for this lag.
Sensor warm-up: The first 1–2 hours after sensor insertion may show reduced accuracy as the sensor stabilizes in the tissue. Most devices display a warm-up countdown and do not provide readings during this period.
Sensor placement: Accuracy can vary by body site. Follow manufacturer guidelines for approved insertion sites and rotate locations to avoid tissue buildup that may reduce sensor performance.
Certain medications: Some medications, including high-dose acetaminophen (paracetamol), can interfere with CGM accuracy. Check the device's prescribing information for a complete list of potential interferences.
How to get started with CGM
For people with diabetes
Talk to your endocrinologist, diabetes care specialist, or primary care provider. CGM devices are prescription medical devices in most countries. Your healthcare provider can assess whether CGM is appropriate for your situation, prescribe the right device, and help you interpret your data in the context of your treatment plan. Insurance coverage varies — your provider's office can help navigate coverage requirements.
For health optimization without diabetes
Several CGM devices are now available over the counter without a prescription in the United States, including Abbott's Lingo and Dexcom's Stelo — both approved for adults without diabetes. These devices are available directly from manufacturers and major pharmacies. They provide the same continuous glucose data but are positioned as wellness tools rather than medical devices.
If you have diabetes or any other medical condition, do not start using a CGM device or make changes to your treatment based on CGM data without first consulting your healthcare provider. CGM data requires clinical context to interpret safely and act upon.
What to expect
Most new CGM users go through a learning curve. The volume of data — up to 288 readings per day — can be overwhelming at first. Focus on patterns rather than individual readings. Look at overnight glucose stability, post-meal responses to specific foods, and how exercise affects your levels. Most CGM apps provide pattern summaries that make this easier over time.