What fasting glucose captures
A single morning blood draw after 8-12 hour fast. Reflects:
- Basal hepatic glucose production overnight
- Insulin's ability to suppress liver glucose output
- Background metabolic state
Optimal: under 90 mg/dL. Prediabetes: 100-125. Diabetes: 126+ (confirmed).
What CGM captures
Continuous measurement (every 1-5 minutes) for the wear period (typically 14 days). Captures:
- Postprandial glucose responses to specific foods
- Glucose variability across the day
- Time in range (typically 70-140 mg/dL)
- Nocturnal glucose patterns
- Exercise responses
- Stress responses
Complementary information
Same patient can have:
- Normal fasting glucose
- Normal HbA1c
- Substantial postprandial spikes on CGM
- High glucose variability
The CGM data reveals risk that fasting numbers miss.
When to use each
Fasting glucose is sufficient for:
- Annual screening
- Diabetes diagnosis
- Monitoring of established disease
- Initial metabolic evaluation (paired with fasting insulin for HOMA-IR)
CGM adds value for:
- Identifying glucose intolerance patterns missed by fasting
- Personalizing dietary patterns
- Troubleshooting unexplained metabolic patterns
- Optimizing in already-healthy patients
- Tracking response to interventions in granular detail
Interpretation
CGM key metrics:
- Time in range (70-140 mg/dL): aim >90% for healthy adults
- Glucose variability (CV): aim under 17%
- Postprandial peaks: aim under 140 mg/dL
- Nocturnal stability: minimal swings
The clinical pearl: Fasting glucose plus fasting insulin (HOMA-IR) is sufficient for most metabolic evaluation. CGM adds personalized resolution that's valuable for optimization but not essential for diagnosis.
Bottom line
Fasting glucose captures a single basal moment. CGM captures the full day. Each useful for different purposes. For routine assessment, fasting glucose with fasting insulin is sufficient. For optimization or troubleshooting, CGM adds substantial resolution.