Use of a model for A1c formation to estimate average glucose operative between short-interval measurements of %A1c.

BACKGROUND

Estimated average glucose (AG) is generally reported along with hemoglobin A1c measurements according to a standard calculation. Given a normal red blood cell lifetime of 120 days, serial A1c measurements at intervals <120 days are not completely independent. For short interval measurements, a change in AG (ΔAG) necessarily underestimates the change in average glucose operative during the interval (ΔG). We use a model for kinetics of HbA1c to evaluate the theoretical relationship between ΔAG and ΔG for HbA1c measurements made at intervals between 0 and 120 days.

METHODS

From any given starting point for A1c, step changes in G were simulated using model calculations to determine the extent to which A1c could change as a function of the interval of exposure. Values for ΔAG were compared to the operative ΔG as a function of the interval between A1c measurements.

RESULTS

Results of model simulations are a single graph for relationship of ΔAG to ΔG as a function of the interval between A1c measurements. ΔAG for (15, 30, 45, 60, 76, and 90) day intervals underestimated operative ΔG by (73, 51, 34, 21, 11, and 5)%, respectively.

CONCLUSIONS

Model calculations predict the relationship between changes in estimated average glucose to changes in operative glucose for serial A1c measurements made at intervals <120 days. Given that serial measurements of A1c made at short intervals are not uncommon in practice, physicians may find this information to be useful.