Discrepancy Between High HbA1c and Optimal Self-Monitored Blood Glucose Readings
The most likely explanation for a patient having an elevated HbA1c of 12% despite optimal self-monitored blood glucose (SMBG) readings is a discordance between the testing methods due to technical errors in SMBG, inadequate testing frequency/timing, or physiological factors affecting HbA1c independent of glycemia.
Common Causes of HbA1c-SMBG Discrepancy
SMBG Technical and Procedural Issues
- Operator-related errors are a more significant source of error than instrument-related errors in SMBG 1
- Common technique errors include improper calibration of glucose meters, poor hand washing, dirty meters, and improper storage of test strips 1
- Patient failure to calibrate the glucose meter regularly is a common cause of error 1
Inadequate SMBG Testing Pattern
- Limited testing frequency may miss significant glycemic excursions, particularly postprandial hyperglycemia 1
- Testing at inappropriate times may not capture the full glycemic profile, especially if postprandial periods are missed 1
- The optimal timing of SMBG testing remains controversial, with some evidence suggesting that "extended post-lunch" (5 PM) values better predict HbA1c than fasting glucose 1
Physiological Factors Affecting HbA1c
- HbA1c reflects average glycemia over approximately 3 months, with recent glycemia having greater influence (50% from the past month) 2
- Conditions affecting red blood cell lifespan alter HbA1c independent of glycemia 1, 2:
- Shortened RBC lifespan (kidney disease, liver disease, hemolytic anemia, pregnancy) decreases HbA1c
- Prolonged RBC lifespan (iron deficiency anemia, aplastic anemia) increases HbA1c
Evaluation Steps
Assess SMBG Technique and Pattern
- Evaluate the patient's monitoring technique, as operator-related errors are a significant source of inaccuracy 1
- Review the frequency and timing of SMBG - inadequate testing may miss significant glycemic excursions 1
- Check if the patient is testing during postprandial periods, as these may be more predictive of overall glycemic control in some patients 1
Verify Meter Accuracy
- Compare the patient's meter readings with laboratory glucose measurements performed simultaneously 1
- Check if the meter is properly calibrated and maintained 1
- Ensure test strips are properly stored and not expired 1
Consider Confounding Factors for HbA1c
- Evaluate for conditions that might falsely elevate HbA1c such as iron deficiency anemia 1, 2
- Consider hemoglobin variants that can interfere with HbA1c measurements 1
- Assess kidney function, as renal disease is often associated with lower HbA1c than would be predicted based on glycemic levels 3
Management Recommendations
Optimize SMBG Practice
- Provide robust diabetes education on proper SMBG technique 1
- Increase testing frequency, especially for insulin-treated patients (6-10 times daily may be needed) 1
- Implement structured testing at various times of day, including fasting, pre-meal, post-meal, and bedtime readings 1
Consider Alternative Monitoring Methods
- Consider continuous glucose monitoring (CGM) to better understand glycemic patterns and identify missed excursions 1
- When HbA1c results seem discordant with clinical picture, consider potential confounding factors and alternative monitoring methods 2
Laboratory Evaluation
- If discrepancy persists, consider testing for conditions that affect red blood cell turnover 1
- Consider alternative measures of glycemic control such as fructosamine if HbA1c is suspected to be unreliable 1
Common Pitfalls to Avoid
- Assuming SMBG results are always accurate - operator-related errors are common 1
- Testing only during fasting or pre-meal times, which may miss significant postprandial excursions 1
- Ignoring the possibility of physiological factors affecting HbA1c independent of glycemia 2
- Failing to recognize that 20% of people with diabetes have HbA1c levels substantially higher or lower than would be predicted from mean blood glucose levels 3