An observational study suggests that in non-diabetic people, glycated haemoglobin (HbA1c) level is more strongly associated with risk of death than fasting glucose, whereas both are similarly associated with risk of frank diabetes.

Fasting blood glucose is the standard test for diabetes, however recent recommendations have moved to measurement of HbA1c. This analysis of data from a large pre-existing cohort aimed to determine the prognostic value of both measurements for onset of diabetes and also cardiovascular disease in those without diabetes at baseline. The data came from the ARIC (Atherosclerosis Risk in Communities) study, which was designed to investigate the etiology and natural history of atherosclerosis, related diseases, and variation in cardiovascular risk factors, medical care and disease by race, gender, location, and date. Recruitment took place between 1987-9 in four US communities, and participants were thoroughly examined at baseline and at three-year intervals until 1996-8. Annual assessment continues by telephone. HbA1c value was available for the 1990-92 visit, which was set as baseline, those with pre-existing diabetes or cardiovascular disease were excluded from analysis. Outcomes for this analysis were risk of cardiovascular disease and risk of diabetes according to HbA1c level (less than 5.0%, 5.0 to less than 5.5%, 5.5 to less than 6.0%, 6.0 to less than 6.5%, and 6.5% or greater) in comparison to standard fasting blood glucose categories (<100, 100 to <126, and ≥126 mg per deciliter [5.6, 5.6 to <7.0, and ≥7.0 mmol per litre, respectively]).

There were 15,792 people in the initial ARIC cohort; after exclusions, 11,092 were eligible for analysis. Median follow-up was about 14 years. The most common HbA1c category was 5.0 to 5.5% (n=4,950) so this was used as the reference category. In the analyses, HbA1c level at baseline was associated with newly diagnosed diabetes and cardiovascular disease: in the five categories, hazard ratios (95% CI) for diabetes were 0.52 (0.40 to 0.69), 1.00 (reference), 1.86 (1.67 to 2.08), 4.48 (3.92 to 5.13), and 16.47 (14.22 to 19.08), respectively.

There was a similar association for cardiovascular disease with HR 0.96 (0.74 to 1.24), 1.00 (reference), 1.23 (1.07 to 1.41), 1.78 (1.48 to 2.15), and 1.95 (1.53 to 2.48), respectively.

Fasting blood glucose categories were associated with risk of diabetes, however the association was less robust.

The authors conclude that in this population, HbA1c levels were similarly associated to risk of diabetes as fasting blood glucose, but were more strongly associated with risk of cardiovascular disease. They note limitations with their study, but suggest that it supports the use of HbA1c in screening for diabetes.