The relationship between glycaemic control and mortality in patients with type 2 diabetes in general practice (ZODIAC-11)

Background The relationship between the degree of glycaemic control and mortality remains an important topic of discussion. Aim This study aimed to investigate this relationship. Design of study Prospective cohort study. Setting Primary care. Method A total of 1145 patients with type 2 diabetes were enrolled in the Zwolle Outpatient Diabetes project Integrating Available Care (ZODIAC) in 1998. Their survival status was recorded in September 2004. Mortality ratios were calculated using standardised mortality ratios (SMRs). Associations between haemoglobin A1c (HbA1c) levels and mortality were studied with a Cox proportional hazard model. HbA1c levels were studied as continuous and as categorical variables. Results A total of 335 patients died after a median follow-up period of 5.8 years. The SMR (95% confidence interval [CI]) for total mortality was 1.86 (95% CI = 1.66 to 2.06) and 2.24(95% CI = 1.91 to 2.61) for cardiovascular mortality. For each 1% increase in HbA1c there was a 21% increase in the hazard ratio for total mortality. When compared with the target HbA1c group (HbA1c 6.5-7%), the group with very poor glycaemic control (HbA1c >9%) had a hazard ratio of 2.21 (95% CI = 1.42 to 3.42) for total mortality. The group with normal glycaemic control (HbA1c <6.5%) had a hazard ratio of 1.00 (95% CI = 0.46 to 2.19) for total mortality. Conclusion HbA1c level was associated with mortality and this effect seemed largely attributable to patients who were in really poor glycaemic control. The absence of differences in mortality in the groups with lower HbA% levels supports the position that there is no basis for continually decreasing the therapeutic target HbA1c level in patients with type 2 diabetes mellitus.