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Is high blood glucose dangerous when driving?

Daniel Flanagan
Type 1 diabetes – September 2020 digest

Hypoglycaemia is known to impair driving ability, and national driving agencies restrict driving licences for those using insulin and other hypoglycaemic medications. However, there is no restriction on driving if blood glucose is elevated, as this is not thought to adversely affect the ability to drive. A new study by Haim and colleagues challenges this view.

A diagnosis of diabetes can affect an individual’s life in so many ways. Particularly for those using insulin, the ability to drive may be influenced by blood glucose control. It is clear that, for people with diabetes, low blood glucose can impair driving ability, and there are a wealth of research data supporting this. There are clearly defined mechanisms demonstrating why this may occur, laboratory data showing that brain function is impaired as blood glucose falls, and driving studies showing objective deterioration in driving ability. For this reason, national driving agencies restrict driving licences for those using insulin (and other medications that can cause hypoglycaemia). For some individuals for whom the ability to drive is particularly important, blood glucose is maintained at a higher level to avoid hypoglycaemia. There is no restriction on driving if blood glucose is elevated, as this is not thought to adversely affect the ability to drive.

Haim and colleagues have produced an interesting paper that challenges this view. They performed a laboratory-based study looking at the effect of a single high-sugar drink on driving ability using a driving simulator. The conclusion is that some of the measures of driving ability are adversely affected. There are already data supporting the view that high blood glucose may subtly influence brain function; however, such studies are limited and are dwarfed by the wealth of evidence for the impact of low blood glucose. Perhaps the most important question is whether these results provide evidence of a serious influence of hyperglycaemia on driving and whether we can/should extrapolate these results to current clinical practice.

Looking at the detail, the design is appropriate and the study was well performed. The sample was relatively small and complete data were not available for all individuals. Most of the study participants were young male drivers with limited driving experience. There were a large number of outcome measures and it is not clear if some of these were regarded as more or less important before the study was performed. It is also not clear that all of the outcomes – for example, braking more frequently – would necessarily result in a higher risk of accident.

The conclusion, as always, is that further research is required. Should this influence driving standards? Not yet – this is preliminary evidence from a small study. Can we use this paper to help people with diabetes? We already advise that running a high blood glucose to avoid hypoglycaemia may risk long-term complications. We could perhaps add that people should be cautious about driving if their blood sugar is significantly elevated.

Click here for the study summary.

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