New aspects of HbA1c as a risk factor for cardiovascular diseases in type 2 diabetes: an observational study from the Swedish National Diabetes Register (NDR)

Eeg-Olofsson K, Cederholm J, Nilsson PM, Zethelius B, Svensson A-M, Gudbjornsdottir S, Eliasson B (Institute of Medicine, Sahlgrenska University Hospital, University of Gothenburg, Gothenburg; Department of Public Health and Caring Sciences/Family Medicine and Clinical Epidemiology, Uppsala University, Uppsala; Department of Clinical Sciences, Lund University, University Hospital, Malmo; Department of Public Health and Caring Sciences/Geriatrics, Uppsala University, Uppsala; and Center of Registers in Region Vastra Gotaland, Gothenburg, Sweden) New aspects of HbA1c as a risk factor for cardiovascular diseases in type 2 diabetes: an observational study from the Swedish National Diabetes Register (NDR). J Intern Med 2010; 268: 471-482. Aims. To analyse the association between glycosylated haemoglobin A1c (HbA1c) and cardiovascular disease (CVD) in patients with type 2 diabetes in the Swedish National Diabetes Register (NDR). Methods. An observational study of 18 334 patients (age 30-79 years, previous CVD in 18%, baseline HbA1c 5.0-10.9%) who were followed for 6 years (mean 5.6 years) from 1997/1998 until 2003. Results. Hazard ratios per 1% unit increase in baseline or updated mean HbA1c for fatal/nonfatal coronary heart disease (CHD), CVD and total mortality were 1.11-1.13, 1.10-1.11 and 1.09-1.10, respectively (all P < 0.001), adjusted for several risk factors and clinical characteristics in Cox regression. Adjusted 6-year event rates increased with higher baseline or updated mean HbA1c with no J-shaped risk curves, in all patients and also when subgrouping by shorter (mean 3 years) or longer (mean 14 years) diabetes duration, by presence or absence of previous CVD, or by treatment with oral hypoglycaemic agents (OHAs) or insulin. Risk reductions of 20% for CHD and 16% for CVD (P < 0.001) were found in patients with a baseline mean HbA1c of 6.5%, compared to those with a mean level of 7.5%. Compared to OHA-treated patients, insulin-treated patients had an increased risk of total mortality, due almost exclusively to an increased risk of non-CVD mortality, and due less to a weakly significant increased risk of fatal CVD. HbA1c was not associated with non-CVD mortality. Conclusions. This observational study showed progressively increasing risks of CHD, CVD and total mortality with higher HbA1c, and no risk increase at low HbA1c levels even with longer diabetes duration, previous CVD or treatment with either insulin or OHAs. Patients achieving HbA1c < 7% showed benefits for risk reduction.