Nearly half of Alzheimer’s disease patients can pass a standard driving test, an indication that no single tool can effectively evaluate them, a new study shows.

Many people with Alzheimer's disease and mild cognitive impairment are at an increased risk of motor vehicle accidents but others retain the ability to drive safely. But the complex nature of these medical conditions – and of driving itself –makes deeming a patient an unsafe driver extremely difficult, researchers say.

Canadian researchers reviewed  32 studies that looked at various cognitive tests in conjunction with driving outcomes, on-road evaluations and driving simulations, and found inconsistent results. Of these studies, 29 assessed driving performance of patients with Alzheimer's and four in patients with mild cognitive impairment.

Several skills involved in driving were predictors of performance, they found. These included  the ability to pay attention; perform high level thinking skills (executive function); assess objects in spatial relation to each other (visuospatial function), as well as the ability to perceive, remember and to think (global cognition).  However, they found there was no consistent measure across all studies that could determine safe driving ability.

Individuals with very mild and mild Alzheimer's disease who took a road test had a failure rate of 13.6 and 33.3 per cent respectively, compared to a failure rate of 1.6 per cent in drivers without Alzheimer's disease. However, in general, patients with any degree of Alzheimer's disease still had a pass rate of almost more than 46 per cent, the researchers say.

The study, which appears in the Journal of Alzheimer’s Disease, underscores the need for more comprehensive tools to assess drivers’ skills, the researchers noted.