Home / Conditions / Prediabetes / HbA1c and OGTT Performance in Prediabetic Obese Adolescents

HbA1c and OGTT Performance in Prediabetic Obese Adolescents

Apr 3, 2015

Outcomes in A1c versus 2-hour OGTT performance using CGM in obese youths…

Recent studies in adolescents comparing A1c to 2-hour glucose and fasting plasma glucose (FPG) have reported poor agreement among the three tests in classifying youth with dysglycemia. Other studies show that A1c has been reported to have a low sensitivity for identifying prediabetes and diabetes when defined by OGTT.

Researchers from the University of Colorado Denver tested whether A1c or the oral glucose tolerance test (OGTT) is a better predictor of free-living glycemia as measured by continuous glucose monitoring (CGM).

This was a cross-sectional study of youth between 10–18 years old, body mass index (BMI) 85th percentile or greater, with known diabetes risk factors. A total of 118 participants with BMI 85th percentile or greater, not on anti-diabetic medications, were recruited from primary care and pediatric endocrinology clinics around Denver, Colorado. Exclusion criteria included an A1c >7.5% because these individuals required immediate initiation of glucose-lowering therapy, medications known to affect blood glucose, known anemia, hemoglobinopathy, chronic illness likely to affect red cell lifespan, and pregnancy. The study was approved by the Colorado Multiple Institutional Review Board (Aurora, Colorado). HbA1c, fasting plasma glucose, and 2-hour glucose were collected and all participants wore a blinded iPro Continuous Glucose Monitor (Medtronic Mini-Med, Inc) which was inserted for 72 hours. Continuous Glucose Monitor outcomes were determined and descriptive statistics were used and calculated. Performance characteristics at current American Diabetes Association cutpoints were compared with CGM outcomes during the study.

Results of CGM data were successfully collected on 98 obese youth participants out of the original 118 after further exclusions were made. Subjects with prediabetes displayed a significantly higher average glucose, area under the curve (AUC), peak glucose, and time greater than 120 and greater than 140 mg/dL (P < 0.01) on CGM than youth with normal A1c or OGTT. The HbA1c had a greater magnitude of correlation to CGM average glucose, AUC, and minimum glucose; 2-hour glucose had a greater magnitude of correlation to CGM SD, peak glucose, and time greater than 140 and greater than 200 mg/dL. The end result showed that there were no overall differences in the strength comparisons between 2-hour glucose and A1c correlations to CGM outcomes.

The researchers concluded that A1c and 2-hour glucose performed equally well at predicting free-living glycemia on CGM, which may suggest that both are valid tests for glycemic screening in this population.

Practice Pearls:

  • There were no differences in age, Tanner staging scale, or lipids among those with normal A1c (< 5.7%) or prediabetes A1c (5.7– 6.4%).
  • FPG and 2-hour glucose, as well as weight, BMI z-score, waist circumference, diastolic blood pressure, and ALT were higher in those with abnormal HbA1c.
  • The subgroups of obese adolescents with normal A1c and with normal 2-hour glucose, spent 17% and 20% of the time greater than 120 mg/dL, respectively.

Christine L. Chan, Laura Pyle, Lindsey Newnes, et al. “Continuous Glucose Monitoring and its Relationship to Hemoglobin A1c and Oral Glucose Tolerance Testing in Obese and Prediabetic Youth”. J Clin Endocrinol Metab, March 2015, 100(3):902–910