Continuous Glucose Monitoring and HbA1c in Cystic Fibrosis: Clinical Correlations and Implications for CFRD Diagnosis

Abstract

Context: The clinical utility and implications of continuous glucose monitoring (CGM) in cystic fibrosis (CF) are unclear.

Objective: We examined the correlation between CGM measures and clinical outcomes in adults with CF, investigated the relationship between hemoglobin A1c (HbA1c) and CGM-derived average glucose (AG), and explored CGM measures that distinguish cystic fibrosis-related diabetes (CFRD) from normal and abnormal glucose tolerance.

Methods: This prospective observational study included 77 adults with CF who had CGM and HbA1c measured at 2 to 3 time points 3 months apart.

Results: Thirty-one of the 77 participants met American Diabetes Association-recommended diagnostic criteria for CFRD by oral glucose tolerance testing and/or HbA1c. In all participants, CGM measures of hyperglycemia and glycemic variability correlated with nutritional status and pulmonary function. HbA1c was correlated with AG (R2 = 0.71, P < 0.001), with no significant difference between this regression line and that previously established in type 1 and type 2 diabetes and healthy volunteers. Cutoffs of 17.5% time > 140 mg/dL and 3.4% time > 180 mg/dL had sensitivities of 87% and 90%, respectively, and specificities of 95%, for identifying CFRD. Area under the curve and percent of participants correctly classified with CFRD were higher for AG, SD, % time > 140, > 180, and > 250 mg/dL than for HbA1c.

Conclusion: CGM measures of hyperglycemia and glycemic variability are superior to HbA1c in distinguishing those with and without CFRD. CGM-derived AG is strongly correlated with HbA1c in adults with CF, with a similar relationship to other diabetes populations. Future studies are needed to investigate CGM as a diagnostic and screening tool for CFRD.

Keywords: HbA1c; average glucose; continuous glucose monitoring; cystic fibrosis–related diabetes.

Figure 1.

Regression line and scatterplot of HbA1c vs CGM-derived AG divided into OGTT glycemic category (NGT, AGT, CFRD). Regression lines are included from the ADAG study (adults with type 1 and type 2 diabetes and healthy volunteers) (12) and from the study published by Chan et al (youth with CF) (16). Scully et al represents the regression line from the present analysis.

Figure 2.

ROC analyses and calculated cutoffs for the prediction of CFRD. All P values for AUC < 0.0001.