Assessing CGM Accuracy Performance

Which CGM systems pass basic study design criteria?

Based on the DSN Forum UK CGM comparison chart, only the CGM systems tested using study designs that meet basic international criteria with sufficient number of people with type 1 diabetes (at least 40, with more than 10,000 data points) are discussed. The CGM systems with adequate robust data can be split into systems that report, on average, true blood glucose exposure (between capillary and venous glucose), and those that read below.

• Accu-Chek SmartGuide
• FreeStyle Libre 2 (and Plus version)
• FreeStyle Libre 3 (and Plus version)
• Dexcom G6
• Dexcom G7
• Dexcom ONE
• Dexcom ONE+
• Medtronic Simplera / Simplera Sync
• Medtronic Guardian 4
• Eversense

Performance metrics – why averages can mislead

There are lots of accuracy metrics, each with limitations. The most common is MARD (Mean Absolute/Relative Difference), which gives an overall average error. But an “average” tells you very little about risk at the extremes — where clinical decisions matter most.

Agreement rates: 20/20 and 40/40

Agreement rates show the percentage of readings that fall within an acceptable error range, typically:

• 20/20 – Within this, no clinical risk
• 40/40 –Outside this, high clincial risk

A reading is within the 20/20 zone if:

• For CGM values < 5.5 mmol/L (100 mg/dL), the comparator is within ±1.1 mmol/L (±20 mg/dL).
  • Example: CGM = 4.5 mmol/L (80 mg/dL). The “true” value must be between 3.4–5.6 mmol/L (60–100 mg/dL).
• For CGM values ≥ 5.5 mmol/L (100 mg/dL), the comparator is within ±20%.
  • Example: CGM = 10.0 mmol/L (180 mg/dL). The reference must be 8.0–12.0 mmol/L (144–216 mg/dL).

The higher the % of readings within 20/20, the more dependable the CGM is for low-risk insulin dosing.

The 40/40 metric is a wider safety net that helps flag readings with a high chance of leading to incorrect insulin decisions.

• For CGM values < 5.5 mmol/L, the comparator must be within ±2.2 mmol/L (±40 mg/dL).
  • Example: CGM = 4.5 mmol/L (80 mg/dL) → reference must be 2.3–6.7 mmol/L (20–120 mg/dL).
• For CGM values ≥ 5.5 mmol/L, the comparator must be within ±40%.
  • Example: CGM = 10.0 mmol/L (180 mg/dL) → reference must be 6.0–14.0 mmol/L (108–252 mg/dL).

If a CGM frequently fails even the 40/40 window, those readings are very likely to cause incorrect dosing decisions.

Important: because different CGMs are calibrated and tested differently, the same 20/20 or 40/40 percentage does not always mean the same real-world accuracy.

How do the CGM systems stack up?

The DSN Forum UK CGM comparison chart summarises study design scores and 20/20 and 40/40 agreement rates.

If a CGM:

• Has a study design score of ≥ 4, and
• Achieves around 90% of readings within 20/20, and
• Has ≥ 99% within 40/40,

…then it can support insulin dosing decisions with a low rate of high-risk outliers. All of the CGM systems mentioned on this page meet this criterion.

However, this still leaves around 0.5–1% of readings that may be problematic. This is why occasional cross-checking with an ISO-standard blood glucose test will always be necessary, and why understanding the CGM black swan problem is so important.

Those with iCGM approval are the most accurate and have the greatest depth of data! That’s the Abbott, Dexcom and Eversnse CGM systems!

Time for the CGM Functionality: Bells and Whistles!