When is the best time to do Continuous Blood Glucose (CBG) monitoring?

Optimal Timing for Capillary Blood Glucose (CBG) Monitoring

For patients on intensive insulin regimens (multiple daily injections or insulin pump therapy), perform CBG testing 6-10 times daily: before each meal and snack, at bedtime, occasionally postprandially, prior to exercise, when hypoglycemia is suspected, after treating hypoglycemia until normoglycemia is confirmed, and before critical tasks such as driving. 1, 2, 3, 4

Specific Testing Times by Clinical Context

Intensive Insulin Therapy (Type 1 Diabetes or Insulin Pump Users)

• Pre-meal testing is mandatory before breakfast, lunch, and dinner to guide prandial insulin dosing 1, 2
• Bedtime testing is essential to prevent nocturnal hypoglycemia 1, 2
• Pre-snack testing should occur before any significant carbohydrate intake 1
• Postprandial testing (when performed) should occur approximately 75 minutes after starting the meal, as 80% of glucose peaks occur within 90 minutes 5, 6
• Exercise-related testing must occur before physical activity and during prolonged exercise 1
• Hypoglycemia confirmation requires testing when symptoms occur and repeated testing every 15-20 minutes after treatment until glucose normalizes 1
• Safety-critical testing is required before driving or operating machinery 1

Basal Insulin Only (With or Without Oral Agents)

• Fasting glucose measurement is the priority, as this directly informs basal insulin dose titration 3, 4
• One additional reading (ideally 2-hour postprandial) should be obtained daily while achieving glycemic targets 3
• Testing frequency can be reduced to twice daily once A1C and glucose remain at goal 3

Type 2 Diabetes on Oral Agents Alone

• Routine CBG monitoring is not recommended for patients managed with diet and oral agents alone, as more frequent testing paradoxically correlates with higher HbA1c in this population 2
• Intermittent monitoring schedules are appropriate once treatment goals are met 3

Hospitalized Patients

• Non-critical care setting: Test before each meal for eating patients, or every 4-6 hours for patients with restricted oral intake 2
• Critical care setting: Test every 30 minutes to 2 hours when using intravenous insulin infusion 2

Gestational Diabetes

• Fasting glucose should be measured with target <5.3 mmol/L (95 mg/dL) 2
• Postprandial testing at either 1-hour (target <7.8 mmol/L or 140 mg/dL) or 2-hour (target <6.7 mmol/L or 120 mg/dL) after meals 2
• Research suggests the physiological glucose peak occurs at approximately 60-82 minutes postprandially, making 1-hour testing more practical and clinically relevant than 2-hour testing 6

Clinical Outcomes and Testing Frequency

• Each additional CBG check per day is associated with a 0.2% reduction in HbA1c 4
• Patients achieving HbA1c <7.5% typically have approximately 60% of CBG readings in the target range (4-10 mmol/L or 72-180 mg/dL), with up to 30% of readings >10 mmol/L being acceptable 7
• Testing 4 or more times daily is associated with significantly better HbA1c control compared to less frequent monitoring 2

Critical Implementation Considerations

Common Pitfalls to Avoid

• Do not rely solely on scheduled testing times if hypoglycemia is suspected—test immediately regardless of the schedule 1
• Do not skip pre-meal testing even when feeling well, as asymptomatic hyperglycemia is common 1, 4
• Do not test only at convenient times—the timing must align with insulin action and meal patterns 1, 3

Continuous Glucose Monitoring (CGM) as Alternative

• Real-time CGM can replace most CBG testing when used properly in conjunction with intensive insulin therapy 1
• CGM detects significantly more hypoglycemic episodes than CBG alone (55 vs 12 episodes in one study) 8
• Some CGM systems still require periodic CBG for calibration 4
• Robust diabetes education, training, and ongoing support are required for optimal CGM implementation 1, 3

Quality Assurance

• Patients must receive ongoing instruction and regular evaluation of CBG technique 1
• Only FDA-approved meters with proven accuracy should be used, with unexpired strips from licensed sources 4
• Patients must be trained to use CBG data to adjust food intake, physical activity, or insulin doses—not just collect numbers 1, 3, 4
• Structured review of glucose patterns improves A1C by an additional 0.3 percentage points compared to unstructured monitoring 3