Hemoglobin A1c Accurately Predicts Continuous Glucose Monitoring–Derived Average Glucose in Youth and Young Adults With Cystic Fibrosis

Abstract

OBJECTIVE In cystic fibrosis (CF), hemoglobin A1c (HbA1c) is thought to underestimate glycemia. However, few studies have directly assessed the relationship between HbA1c and average glucose in CF. We determined the relationships among glycemic markers—HbA1c, fructosamine (FA), glycated albumin (%GA), and 1,5-anhydroglucitol (1,5-AG)—and continuous glucose monitoring (CGM) in CF, hypothesizing that alternate markers would better predict average sensor glucose (ASG) than HbA1c. RESEARCH DESIGN AND METHODS CF participants and a group of healthy control subjects (HCs), ages 6–25 years, wore CGM for up to 7 days. Pearson correlations assessed the relationships between CGM variables and HbA1c, FA, %GA, and 1,5-AG. The regression line between HbA1c and ASG was compared in CF versus HC. Linear regressions determined whether alternate markers predicted ASG after adjustment for HbA1c. RESULTS CF (n = 93) and HC (n = 29) groups wore CGM for 5.2 ± 1 days. CF participants were 14 ± 3 years of age and 47% were male, with a BMI z score −0.1 ± 0.8 and no different from HCs in age, sex, or BMI. Mean HbA1c in CF was 5.7 ± 0.8% (39 ± 9 mmol/mol) vs. HC 5.1 ± 0.2% (32 ± 2 mmol/mol) (P < 0.0001). All glycemic markers correlated with ASG (P ≤ 0.01): HbA1c (r = 0.86), FA (r = 0.69), %GA (r = 0.83), and 1,5-AG (r = −0.26). The regression line between ASG and HbA1c did not differ in CF versus HC (P = 0.44). After adjustment for HbA1c, %GA continued to predict ASG (P = 0.0009) in CF. CONCLUSIONS HbA1c does not underestimate ASG in CF as previously assumed. No alternate glycemic marker correlated more strongly with ASG than HbA1c. %GA shows strong correlation with ASG and added to the prediction of ASG beyond HbA1c. However, we are not advocating use of HbA1c for diabetes screening in CF based on these results. Further study will determine whether glycemic measures other than ASG differ among different types of diabetes for a given HbA1c.