A1C vs. Glucose Downloads—Which Is the Better Gold Standard?

Abstract

Introduction: The ADAG Study Group defined the relationship between average blood glucose (AG) and A1C. They derived a linear equation valid across diabetes types and the complete spectrum of glucose control. This study aims to determine appropriate clinical use of the ADAG equation to substitute home glucose monitoring for A1C. Methods: A retrospective chart review was performed of downloaded glucose data from patients prescribed insulin at the UW Diabetes Center between 1/2011-10/2017. Eligible charts required: 1) 2 weeks of fingerstick glucose (SMBG) data with >14 checks or 1 month of CGM with >14 complete days and, 2) an A1C measured within 1 month of the encounter. The SMBG and CGM means were obtained from downloads. The associated A1C was used to extrapolate AG via the ADAG equation. The downloaded data and A1C-derived averages were compared. The analysis was repeated excluding comorbidities known to impact A1C. Results: The downloaded AG was within 15% of that predicted by ADAG in 68% of patients (N=537). This criterion was met by 65% of patients using SMBG (N=401) and 67% of those checking >1x/day (N=359). The percentage was not different in patients checking >3x/day (71%; N=189). For CGM data, 79% of AGs were within 15% of A1C projection (N=136). CGM had superior ADAG agreement compared to SMBG (p<0.01). After excluding comorbidities (N=209), 72% of AGs were within 15% of ADAG estimation. This was higher than when looking at comorbidities alone (57%, N=228, p<0.001). In CGM users (no comorbidities, N=72), 82% of AGs were within 15% of expected. CGM again had better ADAG agreement than SMBG (p<0.01). Diabetes type did not predict ADAG agreement in any population. Our data replicated the ADAG equation following exclusion of comorbidities. Conclusion: For most patients, SMBG and CGM data provide an important confirmatory assessment of A1C. However, in over 40% with (and about 30% without) a comorbidity, home glucose data are discordant with A1C. For many, downloaded glucose data gives a better assessment of overall diabetes control. Disclosure J.E. Perlman: None. J.L. Rosenbaum: None. B.K. McNulty: None. I.B. Hirsch: Research Support; Self; Medtronic MiniMed, Inc.. Consultant; Self; Abbott, Bigfoot Biomedical, Roche Diabetes Care Health and Digital Solutions, ADOCIA.