Glucose Monitors (CGM) — A Guide for Everyone

What Is a Continuous Glucose Monitor?

A continuous glucose monitor (CGM) is a small wearable device that measures glucose levels in the interstitial fluid (the fluid surrounding cells beneath the skin) continuously — typically every 1–5 minutes — and transmits the readings to a smartphone app or receiver in real time.

Unlike traditional blood glucose meters that give you a single point-in-time reading, a CGM shows you a continuous curve: how your glucose rises and falls throughout the day and night. This provides context that a single fasting reading or even an A1C test cannot give you.

How CGMs Work Technically

A typical CGM consists of three components:

• A small sensor: A thin flexible filament (usually about 5mm long) inserted just beneath the skin, typically on the upper arm or abdomen, using a small applicator. The sensor contains an enzyme (glucose oxidase) that reacts with glucose, generating a small electrical current proportional to the glucose concentration.
• A transmitter: Attached to the sensor; sends glucose data wirelessly (Bluetooth) to a reader or smartphone
• An app or reader: Displays current glucose, trends (arrows indicating whether glucose is rising, stable, or falling), historical graphs, and alerts for high/low readings

Most sensors are worn for 10–14 days before replacement. The readings are in the interstitial fluid, which lags behind blood glucose by approximately 5–15 minutes — this is important to understand during rapidly changing glucose situations.

Types of CGMs Available

Who Can Benefit From a CGM?

CGMs were originally developed for people with type 1 diabetes on insulin therapy, where precise glucose monitoring is critical. Their use has expanded significantly:

• Type 1 diabetes: Standard of care in many countries; significantly improves time-in-range and reduces hypoglycemia
• Type 2 diabetes on insulin: Strong evidence for improved glycemic outcomes and safety
• Type 2 diabetes not on insulin: Emerging evidence that real-time feedback motivates lifestyle changes; insurance coverage varies by country
• Prediabetes: Can reveal personalized glucose responses to foods and activities; helps motivate intervention before diabetes develops
• People without diabetes interested in metabolic optimization: Growing interest; OTC devices now make this accessible

Key Metrics a CGM Provides

Time-in-Range (TIR)

TIR is the percentage of time your glucose stays within a target range — typically 70–180 mg/dL for most people with diabetes. Research increasingly supports TIR as a clinically meaningful outcome that complements A1C. The general target for adults with diabetes is TIR above 70%.

Time Below Range (TBR)

The percentage of time your glucose is below 70 mg/dL. Target: less than 4% for most people. Time below 54 mg/dL (serious hypoglycemia): less than 1%.

Time Above Range (TAR)

The percentage of time above 180 mg/dL. Target: less than 25% for most people.

Glucose Variability (CV)

The coefficient of variation (CV) measures how much your glucose fluctuates. High variability — even within a "normal" average — may be associated with higher oxidative stress and cardiovascular risk. Target: CV below 36%.

Practical Insights CGMs Reveal

People who use CGMs often discover patterns they couldn't see with finger-stick testing:

• Which specific foods spike their glucose most (highly individual)
• How post-meal walks visibly flatten the glucose curve in real time
• How a poor night of sleep raises the next morning's baseline
• How stress during a difficult workday visibly raises glucose without eating
• The timing and magnitude of overnight glucose shifts (dawn phenomenon)

This real-time personalized feedback is one of the most powerful aspects of CGM use — providing the kind of individual data that general population studies cannot predict.

Accuracy and Limitations

• Calibration period: Most CGMs take 1–2 hours to stabilize after insertion before providing reliable readings
• Compression artifacts: Lying on the sensor while sleeping can cause falsely low readings
• Not suitable for dosing without confirmation: For people on insulin, dramatic readings should generally be confirmed with a finger-stick before making major medication decisions
• Lag time: During rapidly falling glucose (e.g., during exercise), the interstitial fluid reading may lag 5–15 minutes behind actual blood glucose
• Cost: Prescription CGMs can be expensive without insurance coverage. OTC options (Stelo, Libre Sense) are more affordable but have fewer features