Aims/hypothesis  

HbA_1c, expressed as the percentage of adult haemoglobin that is glycated, is the most widely used measure of chronic glycaemia. Achieving near-normal HbA_1c levels has been shown to reduce long-term complications and the HbA_1c assay is recommended to determine whether treatment is adequate and to guide adjustments. However, daily adjustments of therapy are guided by capillary glucose levels (mmol/l). We determined the relationship between an accurate measure of mean glucose levels over time and the HbA_1c level, and whether HbA_1c can be expressed in the same units as self-monitoring results.

Methods  

Twenty-two participants with diabetes and three non-diabetic participants were included in this longitudinal observational study. Mean glucose levels were measured by continuous glucose monitoring (CGM), which measures interstitial glucose levels every 5 min, for 12 weeks. Capillary measurements were obtained four times per day to confirm the accuracy of CGM. HbA_1c was measured at baseline and every 4 weeks.

Results  

The HbA_1c results at weeks 8 and 12 correlated strongly (r = 0.90) with the CGM results during the preceding 8 and 12 weeks. A curvilinear (exponential) relationship and a linear regression captured the relationship with similarly high correlations, which allowed transformation of HbA_1c values to a calculated mean glucose level.

Conclusions and interpretation  

HbA_1c correlates closely with a complete measure of average glycaemia over the preceding 8–12 weeks. The translation of HbA_1c to an average glucose level for reporting and management purposes is feasible.