Metformin adherence in patients with type 2 diabetes and its association with glycated haemoglobin levels

Abstract

INTRODUCTION: Metformin is the initial medication of choice for most patients with type 2 diabetes. Non-adherence results in poorer glycaemic control and increased risk of complications. AIM: The aim of this study was to characterise metformin adherence and association with glycated haemoglobin (HbA1c) levels in a cohort of patients with type 2 diabetes. METHODS: Prescription and dispensing data were used for this study. Primary care clinical and demographic data were collected from 10 general practices (October 2016-March 2018) and linked to pharmaceutical dispensing information. Metformin adherence was initially measured by calculating the proportion of patients who had optimal medication cover for at least 80% of days (defined as a medication possession ratio (MPR) of ≥0.8), calculated using dispensing data. Prescription adherence was assessed by comparing prescription and dispensing data. The association between non-adherence (MPR <0.8) and HbA1c levels was also assessed. RESULTS: Of the 1595 patients with ≥2 metformin prescriptions, the mean MPR was 0.87. Fewer Māori had an MPR ≥0.8 than New Zealand European (63.8% vs. 81.2%). Similarly, Māori received fewer metformin prescriptions (P = 0.02), although prescription adherence did not differ by ethnicity. Prescription adherence was lower in younger patients (P = 0.002). Mean HbA1c levels were reduced by 4.8 and 5.0 mmol/mol, respectively, in all and Māori patients with an MPR ≥0.8. Total prescription adherence reduced HbA1c by 3.2 mmol/mol (all P < 0.01). DISCUSSION: Ethnic disparity exists for metformin prescribing, leading to an overall reduction in metformin coverage for Māori patients. This needs to be explored further, including understanding whether this is a patient preference or health system issue.