What laboratory tests are recommended to monitor insulin treatment?

Laboratory Tests for Monitoring Insulin Treatment

The two essential laboratory tests for monitoring insulin treatment are hemoglobin A1c (HbA1c) measured every 3 months until glycemic targets are achieved, then every 6 months when stable, and self-monitoring of blood glucose (SMBG) performed multiple times daily based on insulin regimen intensity. 1

HbA1c Testing Schedule

HbA1c is the primary laboratory test for assessing long-term glycemic control and should be performed quarterly (every 3 months) in patients whose therapy has changed or who are not meeting glycemic goals. 1 Once treatment goals are achieved and glycemic control is stable, testing frequency can be reduced to every 6 months. 1

• Point-of-care A1c testing provides the opportunity for more timely treatment adjustments during clinic visits 1
• HbA1c reflects average glucose control over the preceding 2-3 months and is the gold standard for assessing overall diabetes management 1

Important Caveats for HbA1c

HbA1c may be unreliable in specific clinical situations and should not be used alone in these cases: 1, 2

• Anemia, hemoglobinopathies, or conditions with increased red cell turnover 2
• Recent significant blood loss 2
• HIV infection (A1c may underestimate glycemia) 2
• In these situations, rely exclusively on blood glucose monitoring for treatment decisions 2

Self-Monitoring of Blood Glucose (SMBG)

For Intensive Insulin Regimens (Multiple Daily Injections or Pump Therapy)

Patients on intensive insulin regimens should perform SMBG at least 6-10 times daily, including: 1, 3

• Before each meal and snack 1, 3
• At bedtime 1, 3
• Occasionally postprandially 1
• Prior to exercise 1, 3
• When suspecting hypoglycemia 1, 3
• After treating hypoglycemia until normoglycemic 1, 3
• Before critical tasks such as driving 1, 3

More frequent SMBG is associated with better glycemic control, with each additional daily test associated with a 0.2% reduction in A1c. 1

For Basal Insulin Only

Patients using basal insulin without intensive prandial coverage should assess fasting glucose with SMBG to inform dose adjustments, which results in lower A1c. 1 Testing frequency can be less intensive than with multiple daily injections, but should include at minimum fasting glucose checks and periodic bedtime monitoring. 1

Special Monitoring Situations

Increase SMBG frequency when making changes to insulin regimens, as medication errors and hypoglycemia risk are heightened during transitions. 4 The FDA label for insulin products specifically warns that changes in insulin strength, manufacturer, type, injection site, or method of administration require close medical supervision with increased frequency of blood glucose monitoring. 4

For patients using insulin pumps, SMBG is critical because inadvertent interruption of insulin delivery can lead to rapid development of ketoacidosis. 1 These patients require frequent monitoring both to guide bolus dosing and to detect pump malfunction. 1, 4

Continuous Glucose Monitoring (CGM)

CGM can replace most SMBG testing in appropriate patients and detects significantly more hypoglycemic episodes than SMBG alone. 5 More frequent CGM use (>70% sensor use) is associated with lower A1c levels. 1 However, some CGM systems still require periodic SMBG for calibration. 5

Additional Laboratory Tests in Specific Situations

C-Peptide Measurement

C-peptide measurement is not routinely needed for insulin monitoring but has specific indications: 1

• Differentiating type 1 from type 2 diabetes in ambiguous cases (e.g., patients with type 2 phenotype presenting in ketoacidosis) 1
• Required by some payers for insulin pump therapy coverage when fasting plasma glucose is ≤220 mg/dL (12.5 mmol/L) 1
• Investigating nondiabetic hypoglycemia or suspected surreptitious insulin administration 1

Fasting and Postprandial Glucose Patterns

When there is a discrepancy between SMBG results and the average glucose expected from measured HbA1c, there may be periods of undetected hyper- or hypoglycemia requiring insulin therapy adjustments. 6 In well-controlled patients with preprandial SMBG and HbA1c in good range, routine postprandial testing adds little value beyond its effect on HbA1c. 1

Critical Monitoring Pitfalls to Avoid

• Never rely solely on HbA1c without SMBG data in insulin-treated patients, as HbA1c does not detect hypoglycemia or inform day-to-day insulin dosing decisions 1
• Ensure glucose meters are FDA-approved with unexpired strips from licensed distributors, as accuracy varies significantly between devices 1, 5
• Be aware that high-dose vitamin C, hypoxemia, maltose, galactose, and xylose can interfere with glucose meter accuracy 1, 5
• Patients must be taught how to use SMBG data to adjust therapy; performing SMBG alone without action does not improve outcomes 1
• Regularly evaluate patient monitoring technique and their ability to interpret and act on results 1