HbA1c and Glucose Management Indicator Discordance: A Real-World Analysis

Background: There can be marked discordance between laboratory and estimated (using the glucose management indicator [GMI]) glycated hemoglobin (HbA1c) from continuous glucose monitoring (CGM). This may cause errors in diabetes management. This study evaluates discordance between laboratory and CGM-estimated HbA1c (eA1C). Methods: We performed a retrospective review of patients with diabetes who use CGM. The patients were seen at the University of Washington (UW) Diabetes Care Center from 2012 to 2019. We used UW's Institute of Translational Health Sciences to extract eligible encounters from the electronic medical record. We required that patients use CGM and that HbA1c and sensor data be obtained fewer than 4 weeks apart. There were no exclusion criteria. We calculated HbA1c-GMI discordance for each subject and assessed for any impact of comorbidities. We defined HbA1c-GMI discordance as absolute difference between laboratory and eA1C. Results: This study included 641 separate office encounters. Ninety-one percent of patients had type 1 diabetes. Most patients had diabetes for greater than 20 years. The mean duration of CGM wear was 24.5 ± 8 days. Only 11% of patients had HbA1c-GMI discordance <0.1%, but 50% and 22% had differences ≥0.5% and ≥1%. There was increased discordance with advanced chronic kidney disease (estimated glomerular filtration rate <60). Discussion: We found substantial discordance between laboratory and eA1C in a real-world setting. Clinicians need be aware that HbA1c may not as accurately reflect mean glucose as previously appreciated.