AbstractBackgroundWe previously showed, in patients with diabetes, that >50% of monitoring tests for glycated haemoglobin (HbA_1c) are outside recommended intervals and that this is linked to diabetes control. Here, we examined the effect of tests/year on achievement of commonly utilised HbA_1ctargets and on HbA_1cchanges over time.MethodsData on 20,690 adults with diabetes with a baseline HbA_1cof >53 mmol/mol (7%) were extracted from Clinical Biochemistry Laboratory records at three UK hospitals. We examined the effect of HbA_1ctests/year on (i) the probability of achieving targets of ≤53 mmol/mol (7%) and ≤48 mmol/mol (6.5%) in a year using multi-state modelling and (ii) the changes in mean HbA_1cusing a linear mixed-effects model.ResultsThe probabilities of achieving ≤53 mmol/mol (7%) and ≤48 mmol/mol (6.5%) targets within 1 year were 0.20 (95% confidence interval: 0.19–0.21) and 0.10 (0.09–0.10), respectively. Compared with four tests/year, having one test or more than four tests/year were associated with lower likelihoods of achieving either target; two to three tests/year gave similar likelihoods to four tests/year. Mean HbA_1clevels were higher in patients who had one test/year compared to those with four tests/year (mean difference: 2.64 mmol/mol [0.24%], p<0.001).ConclusionsWe showed that ≥80% of patients with suboptimal control are not achieving commonly recommended HbA_1ctargets within 1 year, highlighting the major challenge facing healthcare services. We also demonstrated that, although appropriate monitoring frequency is important, testing every 6 months is as effective as quarterly testing, supporting international recommendations. We suggest that the importance HbA_1cmonitoring frequency is being insufficiently recognised in diabetes management.