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

Abstract

ABSTRACT INTRODUCTIONMetformin 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. AIMThe 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. METHODSPrescription 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. RESULTSOf 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 5.0mmol/mol, respectively, in all and Māori patients with an MPR ≥0.8. Total prescription adherence reduced HbA1c by 3.2mmol/mol (all P<0.01). DISCUSSIONEthnic 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.