The 20/20 rule, often applied to the accuracy of continuous glucose monitoring (CGM) systems and other home glucose testing devices, establishes criteria for how closely their readings must match laboratory reference values. Essentially, it specifies acceptable limits of deviation for glucose measurements to be considered clinically accurate.
Understanding the Accuracy Standard
For a CGM system to be deemed accurate in clinical studies, its readings must adhere to specific performance metrics relative to a laboratory reference method. This rule helps ensure the reliability of glucose data, which is crucial for making informed diabetes management decisions.
The 20/20 rule typically defines two main thresholds based on glucose concentration:
- For higher glucose values: When the laboratory reference glucose value is greater than or equal to 5.5 mmol/L (100 mg/dL), the CGM reading must be no more than 20% higher or lower than the laboratory result.
- For lower glucose values: When the laboratory reference glucose value is less than 5.5 mmol/L (100 mg/dL), the CGM reading must be within an absolute difference of ±0.83 mmol/L (±15 mg/dL) of the laboratory result.
This two-part criterion ensures that device accuracy is assessed appropriately across the full range of glucose levels, from hypoglycemia to hyperglycemia.
Why the 20/20 Rule Matters for CGM
The 20/20 rule, along with other similar accuracy standards (like those from the International Organization for Standardization, ISO), serves several vital purposes for CGM technology:
- Clinical Reliability: It provides a benchmark for manufacturers to demonstrate that their devices deliver data reliable enough for clinical use and treatment decisions.
- Regulatory Approval: Regulatory bodies often require devices to meet these accuracy standards before they can be marketed and sold.
- User Confidence: Knowing that a CGM system adheres to established accuracy rules builds trust among users and healthcare providers in the data it provides.
- Safety: Accurate glucose readings are paramount for preventing dangerous situations like severe hypoglycemia (low blood sugar) or hyperglycemia (high blood sugar).
Practical Implications for CGM Users
While CGM systems provide continuous glucose trends and real-time data, understanding the 20/20 rule can offer practical insights:
- Expected Variance: It's normal for CGM readings to vary slightly from a traditional fingerstick blood glucose meter (BGM) reading. This rule quantifies the acceptable amount of this difference.
- When to Calibrate or Verify: If your CGM reading significantly deviates from what you expect or from a fingerstick reading—especially outside the 20/20 rule's parameters for a given glucose level—it might be a sign to:
- Perform a fingerstick for verification.
- Consider calibrating your CGM if the system allows it.
- Check for sensor issues (e.g., compression low, sensor expired, sensor coming loose).
- Understanding Device Limitations: No glucose monitoring device is 100% accurate 100% of the time. The 20/20 rule acknowledges this and sets a realistic, clinically acceptable threshold for performance.
Example of the 20/20 Rule in Action
Let's consider an example for the higher glucose range:
| Lab Reference Glucose | 20% Range for CGM Reading |
|---|---|
| 150 mg/dL | 120 mg/dL to 180 mg/dL |
| 8.3 mmol/L | 6.64 mmol/L to 9.96 mmol/L |
If the laboratory reference value is 150 mg/dL, a CGM reading between 120 mg/dL and 180 mg/dL would fall within the 20% accuracy criterion.
For the lower glucose range:
| Lab Reference Glucose | Absolute Range for CGM Reading |
|---|---|
| 70 mg/dL | 55 mg/dL to 85 mg/dL |
| 3.9 mmol/L | 3.07 mmol/L to 4.73 mmol/L |
If the laboratory reference value is 70 mg/dL, a CGM reading between 55 mg/dL and 85 mg/dL would fall within the ±15 mg/dL accuracy criterion.
Adherence to the 20/20 rule ensures that CGM devices provide sufficiently accurate data to support effective diabetes management, allowing individuals to make timely adjustments to their insulin, diet, and activity levels.
