CGM systems have transformed glucose management, offering a stream of data that far surpasses the information provided by a single finger-stick test. A CGM device uses a small sensor placed under the skin to measure glucose levels in the interstitial fluid every few minutes, providing hundreds of data points daily. Interpreting this continuous flow of information is necessary for understanding the body’s response to food, activity, and medication. Effective analysis moves beyond tracking the current number to understanding trends and patterns over time, leading to more informed health decisions.
Understanding Real-Time Readings and Trend Arrows
A CGM display provides two immediate pieces of information: the current glucose number and the trend arrow. Unlike a blood glucose meter, which offers a snapshot, the CGM reading shows the value in context. This number reflects the glucose concentration in the interstitial fluid, which lags slightly behind blood glucose levels.
The trend arrow is a dynamic indicator showing the direction and velocity of glucose change (Rate of Change or ROC). This helps users anticipate where their glucose level is headed. An arrow pointing straight sideways indicates stable glucose, changing less than 1 milligram per deciliter (mg/dL) per minute.
A single arrow (diagonally up or down) signifies a moderate change (1 to 2 mg/dL per minute). A double arrow signals a rapid change, often exceeding 2 mg/dL per minute. This rapid movement indicates the glucose level is increasing or decreasing quickly, potentially shifting by 90 mg/dL or more within 30 minutes.
Interpreting these arrows allows for proactive management and immediate decisions, such as adjusting insulin or consuming a fast-acting carbohydrate to prevent a low. This prevents glucose levels from moving significantly out of the target range.
Key Metrics for Long-Term Analysis
Aggregated metrics over 14 days or more are used to evaluate overall glucose control. The most referenced metric is Time In Range (TIR), which measures the percentage of time glucose levels remain within a predetermined target range (typically 70 to 180 mg/dL).
International experts recommend that individuals with diabetes aim for a TIR greater than 70% of the day (about 17 hours). TIR offers a clearer picture of glucose control than the traditional A1C test, as two individuals with the same A1C can have vastly different times spent in range. TIR is complemented by Time Above Range (TAR) and Time Below Range (TBR).
The goal is to increase TIR while minimizing TBR, especially severe lows. TBR is divided into Level 1 (54–69 mg/dL) and Level 2 (less than 54 mg/dL). TAR is categorized as Level 1 (181–250 mg/dL) and Level 2 (greater than 250 mg/dL). Achieving these targets suggests a balanced approach that avoids both hypo- and hyperglycemia.
The recommended targets for most adults are:
- Spending less than 4% of the day in Level 1 TBR.
- Spending less than 1% of the day in Level 2 TBR.
- Spending less than 25% of the day in Level 1 TAR.
- Spending less than 5% of the day in Level 2 TAR.
Beyond time in range, the data summary provides a measure of Glucose Variability (GV), which describes how much the glucose readings fluctuate throughout the day. High GV (frequent, large swings) is detrimental even if the average glucose level appears acceptable. GV is commonly quantified using the Standard Deviation (SD) or the Coefficient of Variation (CV).
The Coefficient of Variation (CV) is the ratio of the SD to the mean glucose, providing a standardized measure of variability. A CV of less than 36% is considered an indicator of stable glucose levels. Reducing GV is important because frequent fluctuations are linked to increased inflammation and oxidative stress, which can raise the risk of long-term complications.
Identifying Common Glucose Patterns
Deeper analysis involves reviewing the graphical output, often called an Ambulatory Glucose Profile (AGP). The AGP stacks daily glucose curves to identify recurring patterns and consistent trends that are less obvious in numerical metrics. These patterns are categorized by the time of day they occur, pointing toward specific causes.
A common pattern is the Post-meal Spike, appearing as a sharp, rapid ascent in the glucose curve shortly after eating. Analyzing the timing and magnitude of these spikes against food logs helps pinpoint which meals or carbohydrates lead to the fastest rises. A consistent post-meal rise above the target range suggests adjusting meal composition, medication timing, or the dose itself.
Nocturnal Hypoglycemia requires careful attention, appearing as a recurring dip into the Time Below Range overnight. Detecting these dips is crucial because they can occur without the person waking up, making the CGM the most reliable identification tool. Nocturnal lows suggest that evening medication doses or basal rates may be too aggressive.
The Dawn Phenomenon is a naturally occurring pattern characterized by an unexplained rise in glucose levels, usually between 3:00 AM and 8:00 AM. This rise is caused by a surge of hormones like cortisol, which prompt the liver to release stored glucose as the body prepares to wake up.
CGM data is necessary to distinguish the Dawn Phenomenon from the Somogyi effect, which is a rebound high caused by the body overcompensating for an earlier nocturnal low. The CGM confirms the Dawn Phenomenon when the glucose curve rises consistently in the morning without a preceding overnight dip. Recognizing this pattern helps inform the appropriate strategy, as treatment differs from the Somogyi effect.
Turning Interpretation into Actionable Steps
The ultimate goal of interpreting CGM data is translating observed metrics and patterns into specific, practical adjustments. This process requires prioritizing safety, focusing first on Time Below Range, as minimizing lows is the highest safety priority.
If long-term data shows a high TIR and a low CV, maintain the current strategy. If the AGP reveals consistent post-meal spikes, adjust the timing of pre-meal medication or change the carbohydrate content of that meal. These behavioral changes can be implemented immediately.
For patterns related to basal metabolism (e.g., Dawn Phenomenon or overnight lows), action involves modifying the basal insulin rate or the timing of long-acting medication. Correcting the Dawn Phenomenon often requires a slight increase in basal insulin or a shift in the timing of a morning dose.
Any change to medication dosage or timing should always be made in consultation with a healthcare provider. This collaborative approach uses detailed CGM insights to fine-tune the management plan, aiming for long-term improvement in glucose stability. The cycle of collecting data, reviewing patterns, and making targeted changes is an ongoing process.