Conflicting results of estimated average glucose (eAG) calculated from HbA1c and average glucose values (AG) derived from continuous glucose monitoring (CGM) are a very frequent source of confusion and frustrations in the dialogue with patients. Recently the term glucose management indicator (GMI) has been coined to describe HbA1c estimated from AG. The aim of this presentation is to elucidate the relation between calculated and estimated HbA1c and average glucose values on the basis of existing regression lines. The accepted equation translating eAG to HbA1c is: HbA1c (%)=0.63 X eAG (mmol/l) +1.63: (Nathan DM et al, Diabetes Care,2008; 31: 147380). In addition: GMI (mmol/mol) =4.70587 X AG (mmol/l) +12.71 (Bergenstal et al, Diabetes Care 2018; 41:227580). The master equation connecting HbA1c (mmol/mol) and HbA1c (%) is: HbA1c (mmol/l) =10.93 X HbA1c (%) 23.5. It follows that: GMI (%)=0.43 X AG +3.3. The two lines A: HbA1c (%)=0.63 X eAG (mmol/l) +1.63 and B: GMI (%)=0.43 X AG +3.3 plotted in the same diagram (figure) cross at a point corresponding to a glucose value of 8.4 mmol/l and HbA1c/GMI of 6.9%. A HbA1c value of 8.5% corresponds to eAG 10.9 mmol/l. A GMI of 8.5% corresponds to AG of 12.1 mmol/l. For HbA1c values higher than 6.9% the eAG value is progressively lower than AG for a similar GMI value. For patient with good glycemic control HbA1c/GMI and eAG/AG is nearly identical.
Conclusion: The present equation translating HbA1c to eAG underestimates AG based on CGM for patients with poor glycemic control. The diagram shown may be valuable when explaining a patient seemingly conflicting HbA1c and GMI results.
18 - 21 May 2019
European Society of Endocrinology