Understanding Continuous Glucose Monitors (CGMs) for Type 1 Diabetes

How CGMs Work vs. Fingersticks

• 🩸 CGMs measure glucose in interstitial fluid, which surrounds cells, while blood glucose meters (BGMs) measure glucose directly in the blood.
• ⏳ This difference in measurement location leads to a lag time of 5-15 minutes between BGM readings and CGM readings, as glucose needs to transport from blood to interstitial fluid.
• 💧 Hydration is crucial because dehydration concentrates interstitial fluid, potentially affecting CGM accuracy and insulin effectiveness.

Factors Affecting CGM Accuracy

• 💧 Dehydration can make interstitial fluid more concentrated, impacting glucose readings.
• 🌊 Compression lows occur when pressure on the sensor displaces interstitial fluid, making readings appear lower than actual blood glucose.
• 🌡️ Extreme temperatures and changes in blood flow (e.g., from exercise or sedentary periods) can also affect glucose diffusion and CGM readings.
• ⏳ New sensors require a "marination" or "settling" period (often 1-3 days) to provide the most accurate readings, with some systems like Dexcom G7 offering a 12-hour grace period.

Best Practices for CGM Use

• ⏰ Be aware of lag time when treating lows; allow at least 10 minutes for the CGM to reflect changes after consuming fast-acting glucose.
• 🧐 Regular fingerstick checks are still recommended, especially in the morning, to compare with CGM readings and ensure accuracy, particularly before making treatment decisions.
• 📊 Dexcom Clarity reports provide a Glucose Management Indicator (GMI), which can be less accurate if fingerstick comparisons are not regularly performed.
• ⚙️ Calibration should be done when necessary and according to manufacturer guidelines, ideally when blood glucose is stable, to avoid confusing the sensor algorithm.

Sensor Placement and Skin Care

• 📍 Sensor placement can impact accuracy; while some sites are not officially approved, testing different locations can help find what works best for an individual.
• 🧼 For skin irritation, consider alternatives to alcohol wipes, such as mild soap and water, and explore hypoallergenic barriers like Skin Tac or Tegraderm.
• 🩹 Adhesives can cause issues; using skin prep products or specialized patches can help sensors stay on or come off more easily.

Other Factors and Considerations

• 💊 Acetaminophen and high doses of Vitamin C are known to potentially affect CGM accuracy, depending on the sensor model.
• ⚠️ Thyroid conditions (hypo- or hyperthyroidism) may influence metabolic rate and glucose uptake, potentially leading to variations in CGM readings.
• 📈 Stable blood glucose levels generally lead to more accurate and less variable CGM readings compared to a "roller coaster" of highs and lows.
• 🛠️ Follow manufacturer guidelines for calibration and sensor use; while alternative strategies exist, starting with official recommendations is advised.