What defines an adequate reading with the FreeStyle Libre continuous glucose monitoring system for adults with type 1 or type 2 diabetes?

What Defines an Adequate Reading with FreeStyle Libre

For FreeStyle Libre to provide clinically useful data, you need a minimum of 14 days of continuous wear with at least 70% active sensor time (approximately 10 days of actual data collection). 1, 2

Minimum Data Requirements

Duration and Active Time:

• Wear the sensor for at least 14 consecutive days to capture reliable glycemic patterns 1, 2
• Maintain ≥70% active CGM time during this period, which translates to approximately 10 days of usable data 1, 2
• This 14-day/70% threshold is the international consensus standard for all CGM systems, including FreeStyle Libre 1

Scanning Frequency:

• Scan the FreeStyle Libre sensor at minimum once every 8 hours to capture complete 24-hour glucose data 3
• More frequent scanning (mean 11.3 scans/day in clinical studies) provides better pattern recognition and improved outcomes 4
• The device stores 8 hours of glucose data; scanning less frequently results in data loss 3

Understanding Sensor Accuracy Across Its Lifespan

Accuracy varies significantly by sensor age:

• Days 0-1: Mean accuracy (MARD) of 14.5% - this is the least accurate period 5
• Days 5-7: Mean accuracy improves to 7.8% - this is the most accurate period 5
• Days 13-14: Mean accuracy returns to 14.7% - accuracy declines again at sensor end-of-life 5

Clinical accuracy remains acceptable throughout:

• Only 1.9% of readings fall into potentially dangerous zones (Clarke error grid zones C, D, or E) during days 0-1 5
• This drops to 0.2% during days 5-7 and 0.4% during days 13-14 5
• Overall MARD of 8.9% makes FreeStyle Libre 3 more accurate than competing systems like Dexcom G7 (13.6% MARD) 6

Essential Metrics for Adequate Assessment

Once you have sufficient data (14 days, ≥70% active time), evaluate these standardized metrics:

Time in Range (TIR):

• Target >70% of readings between 70-180 mg/dL (3.9-10.0 mmol/L) for most adults with diabetes 1, 2
• Each 5% increase in TIR provides clinically significant benefits 1, 2

Time Below Range (TBR):

• Target <4% total time below 70 mg/dL 1, 2
• Specifically: <1% below 54 mg/dL (level 2 hypoglycemia) 1, 2

Time Above Range (TAR):

• Target <25% total time above 180 mg/dL 1, 2
• Minimize time above 250 mg/dL (level 2 hyperglycemia) 1, 2

Glycemic Variability:

• Coefficient of variation (CV) should be ≤36% 1, 2
• Lower CV targets (<33%) provide additional hypoglycemia protection for insulin users 1, 2

Special Population Considerations

For older adults or high-risk individuals, adequate readings require modified targets:

• TIR >50% (≥12 hours/day) in range 70-180 mg/dL 1, 2
• TBR <1% (<15 minutes/day) below 70 mg/dL 1, 2
• TAR <10% (<2 hours, 24 minutes/day) above 250 mg/dL 1, 2

For pregnant individuals:

• Use tighter target range of 63-140 mg/dL (3.5-7.8 mmol/L) 1
• FreeStyle Libre shows good accuracy in gestational diabetes with overall MARD of 14.07% 7

Critical Limitations in Specific Populations

Use with extreme caution or avoid in advanced kidney disease:

• In hemodialysis patients, FreeStyle Libre accuracy drops significantly with MARD of 18.2% overall and 22.8% during dialysis 1
• Only 49-51% of readings fall into acceptable error grid zones A and B in hemodialysis patients 1
• The glucose oxidase-based technology in FreeStyle Libre is susceptible to interference from substances in kidney disease 1
• Do not use FreeStyle Libre in peritoneal dialysis patients - icodextrin interferes with glucose oxidase sensors 1

Common Pitfalls to Avoid

Insufficient data collection:

• Never make treatment decisions based on less than 14 days of data with <70% active time 1, 2
• Gaps in sensor wear reduce clinical value and limit ability to identify patterns 3

Ignoring sensor age effects:

• Be aware that readings during the first 24 hours and last 24 hours are less accurate 5
• Consider inserting a new sensor before removing the old one to avoid data gaps 3

Focusing only on mean glucose:

• Mean glucose alone misses critical information about hypoglycemia and variability 2
• Always evaluate the complete set of 10 standardized CGM metrics 1, 2

Medication and substance interference:

• Salicylic acid and ascorbic acid can affect FreeStyle Libre accuracy 1
• Acetaminophen does not interfere with FreeStyle Libre (unlike some other CGM systems) 1