Changes in HbA1c Level over a 12-week Follow-up in Patients with Type 2 Diabetes Following a Medication Change

Background: Current guidance about the interval needed before retesting HbA1c when monitoring for glycaemic control is based on expert opinion rather than well-powered studies. The aim of our work was to explore how fast HbA1c changes after a change in glucose-lowering medication. This has implications for whether routine HbA1c testing intervals before 12 weeks could inform diabetes medication adjustments.

Methods: This 12-week cohort study recruited patients from 18 general practices in the United Kingdom with non-insulin treated diabetes who were initiating or changing dose of oral glucose-lowering medication. HbA1c was measured at baseline and 2, 4, 8 and 12 weeks after recruitment. HbA1c levels at earlier time intervals were correlated with 12-week HbA1c. A ROC curve analysis was used to identify the 8-week threshold above which medication adjustment may be clinically appropriate.

Results: Ninety-three patients were recruited to the study. Seventy-nine patients with no change in medication and full 12-week follow-up had the following baseline characteristics: mean ± standard deviation age of 61.3 ± 10.8 years, 34% were female and diabetes duration of 6.0 ± 4.3 years. Mean HbA1c at baseline, 2, 4, 8 and 12 weeks was 8.7 ± 1.5%, (72.0 ± 16.8 mmol/mol) 8.6 ± 1.6% (70.7 ± 17.0 mmol/mol), 8.4 ± 1.5% (68.7 ± 15.9 mmol/mol), 8.2 ± 1.4% (66.3 ± 15.8 mmol/mol) and 8.1 ± 1.4% (64.8 ± 15.7 mmol/mol) respectively. At the end of the study 61% of patients had sub-optimal glycaemic control (HbA1c>7.5% or 59 mmol/mol). The 8-week change correlated significantly with the 12-week change in HbA1c and an HbA1c above 8.2% (66 mmol/mol) at 8 weeks correctly classified all 28 patients who had not achieved glycaemic control by 12 weeks.

Conclusions/interpretation: This is the first study designed with sufficient power to examine short-term changes in HbA1c. The 12-week change in HbA1c can be predicted 8 weeks after a medication change. Many participants who had not achieved glycaemic control after 12 weeks may have benefitted from an earlier review of their HbA1c and medication.