What is the difference between Fasting Blood Sugar (FBS) and Continuous Blood Glucose (CBG) monitoring in patients with diabetes or at risk of developing diabetes?

Difference Between FBS and CBG

FBS (Fasting Blood Sugar) is a single-point laboratory or capillary blood glucose measurement taken after an 8-hour fast, while CBG (Capillary Blood Glucose, often confused with CGM - Continuous Glucose Monitoring) refers to point-of-care fingerstick glucose testing that provides intermittent snapshots of current glucose levels. 1

Key Distinctions

FBS (Fasting Blood Sugar)

• Timing and Purpose: FBS is specifically measured after an overnight fast (typically 8 hours) and represents a single time-point assessment of baseline glucose metabolism 1
• Clinical Use: Primarily used for diabetes screening, diagnosis, and assessing basal insulin requirements in patients on basal insulin therapy 1, 2
• Limitations: Provides only one data point and cannot capture glucose fluctuations throughout the day, missing hyperglycemic or hypoglycemic excursions 3, 4

CBG/SMBG (Capillary Blood Glucose/Self-Monitoring of Blood Glucose)

• Measurement Method: Involves fingerstick testing using glucose meters that can be performed multiple times daily at various time points 1
• Frequency: For intensive insulin users, requires 6-10 tests daily including pre-meals, bedtime, post-exercise, and when hypoglycemia is suspected 1, 2
• Real-time Decision Making: Allows immediate therapy adjustments for insulin dosing, particularly prandial insulin calculations 1

Clinical Applications by Patient Population

For Insulin-Treated Patients

• SMBG is essential for patients on intensive insulin regimens (multiple daily injections or pump therapy) to prevent hypoglycemia and adjust insulin doses 1
• Testing frequency: Before meals and snacks, at bedtime, occasionally postprandially, before exercise, when suspecting hypoglycemia, and before critical tasks like driving 1, 2
• Evidence: Meta-analyses show SMBG reduces A1C by 0.25-0.3% at 6 months when data is used to adjust medications 1

For Non-Insulin Treated Type 2 Diabetes

• Routine SMBG has limited benefit in patients not using insulin, as randomized trials show minimal improvement in outcomes 1
• Selective use: May provide insight into effects of diet, physical activity, and medications, or assess hypoglycemia during intercurrent illness 1
• Intermittent monitoring is adequate once treatment goals are met for patients on lifestyle modifications alone 5

Important Accuracy Considerations

Glucose Meter Limitations

• Only FDA-approved meters with unexpired strips from licensed distributors should be used, as accuracy varies significantly among devices 1, 2
• Interference factors: High-dose vitamin C, hypoxemia, and certain medications can impair meter accuracy 1, 2
• FDA accuracy standard: 95% of readings must be within 15% of laboratory values, but only 6 of 18 top meters met this standard in recent testing 1

Continuous Glucose Monitoring (CGM) as Alternative

When CGM Replaces Traditional Monitoring

• All patients with type 1 diabetes should use CGM regardless of age or glycemic control, as it consistently reduces hypoglycemia and improves time in range 6
• Type 2 diabetes on intensive insulin (≥3 injections daily) should use CGM to lower A1C and reduce hypoglycemia 6
• CGM provides continuous data every 5 minutes, capturing fluctuations that SMBG misses, though it measures interstitial fluid glucose which lags blood glucose by 5-15 minutes 1, 6

CGM Limitations

• Not suitable for ICU settings due to skin edema, vasoconstrictor drugs, and factors affecting sensor accuracy 6
• Requires calibration (device-dependent) using capillary blood glucose for some systems 1, 6
• Lower accuracy in hypoglycemic ranges, a critical limitation for patients with problematic hypoglycemia 6

Common Pitfalls to Avoid

• Testing without action: SMBG does not itself lower glucose levels; data must be integrated into treatment adjustments to be effective 1, 5
• Inadequate testing frequency: Patients on intensive insulin who test less than 6 times daily miss critical information for safe insulin dosing 1, 2
• Ignoring technique evaluation: Providers must regularly assess patients' monitoring technique and ability to interpret results 1
• Overuse in non-insulin users: Routine SMBG in well-controlled type 2 diabetes patients not on insulin adds cost without proven benefit 1