What if continuous blood glucose (CBG) monitoring shows hyperglycemia but capillary blood glucose (CBG) readings show hypoglycemia?

Discrepancies Between Continuous Glucose Monitoring and Capillary Blood Glucose Readings

When continuous glucose monitoring shows hyperglycemia but capillary blood glucose readings show hypoglycemia, trust the capillary blood glucose measurement and treat the hypoglycemia immediately, as this discrepancy likely represents a sensor error or physiological lag. 1

Understanding the Discrepancy

This discrepancy can occur due to several factors:

1. Physiological lag time: CGM measures interstitial fluid glucose, which naturally lags behind blood glucose by 5-15 minutes, especially during rapid glucose changes 1

2. Sensor accuracy issues:

   • CGM systems have varying degrees of accuracy, with Mean Absolute Relative Difference (MARD) ranging from 10-23% 1
   • Accuracy is particularly challenged during hypoglycemia 2

3. Interfering substances:

   • Medications like acetaminophen can cause falsely elevated CGM readings 1
   • Icodextrin in peritoneal dialysis solutions can interfere with certain glucose monitoring systems 1

4. Technical factors:

   • Sensor placement, calibration errors, or sensor wear time 1
   • Sensor compression or pressure on the insertion site 1

Clinical Decision Algorithm

When facing this discrepancy:

1. Verify hypoglycemia with a second capillary blood glucose test

   • Use a reliable glucose meter with fresh test strips 1
   • Ensure proper hand washing before testing to avoid contamination

2. If hypoglycemia is confirmed by capillary testing:

   • Treat the hypoglycemia immediately according to standard protocols
   • Consider the capillary reading as the more accurate value 1

3. After treating hypoglycemia:

   • Check for potential CGM interference factors
   • Consider recalibrating or replacing the CGM sensor if discrepancies persist

4. Document the discrepancy for discussion with healthcare provider

Factors Affecting Accuracy

CGM-Specific Factors

• Sensor age (accuracy decreases over wear time) 1
• Calibration timing and frequency 1
• Sensor placement (areas with less subcutaneous fat may show more variability) 1
• Type of CGM system (different systems have different accuracy profiles) 1

Patient-Specific Factors

• Rapid glucose fluctuations increase lag time between blood and interstitial fluid 1
• Dehydration or edema can affect interstitial fluid glucose concentrations 1
• Medications that interfere with sensor enzymes 1
• Conditions affecting peripheral circulation (shock, hypotension) 1

Special Considerations

For Patients with Kidney Disease

• CGM accuracy may be further compromised in patients with advanced chronic kidney disease or on dialysis 1
• Certain glucose dehydrogenase-based meters (GDH-PQQ) should not be used in patients on peritoneal dialysis with icodextrin solutions due to risk of falsely elevated readings 1

For Critical Care Settings

• Arterial or central venous blood sampling is preferred over capillary sampling for glucose monitoring in critically ill patients 1
• CGM systems in critical care require additional validation and may have specific accuracy requirements 1

Quality Assessment of Readings

CGM accuracy is typically assessed using:

• Mean Absolute Relative Difference (MARD) - lower values indicate better accuracy 3
• Clarke Error Grid Analysis - zones A and B represent clinically acceptable readings 2
• Time in Range metrics - percentage of readings within target glucose range 4

Key Takeaway

In the case of discrepancy between CGM showing hyperglycemia and capillary blood glucose showing hypoglycemia, the capillary reading should take precedence for immediate clinical decision-making. Hypoglycemia poses an immediate risk to patient safety and should be addressed promptly, while the CGM discrepancy should be investigated after the patient is stabilized.