Nov. 16, 1999 (Atlanta) -- Many patients with type 1 diabetes misjudge when their blood sugar is too low to safely drive a car, according to a study published in a recent issue of The Journal of the American Medical Association. Equally worrisome is a finding that some people with type 1 diabetes choose to drive even when they know that their blood sugar, or glucose, levels may be dangerously low.
"What this study showed is that a large percentage of the time, people with type 1 diabetes were making decisions to drive the car when their blood sugars were in a range that could impair their ability to drive safely," researcher William Clarke, MD, tells WebMD. Clarke is a professor of pediatrics at the University of Virginia Health Sciences Center in Charlottesville.
Other studies have shown that driving performance deteriorates significantly when a person's blood sugar is reduced to between 47 and 65 mg/dL. "At these levels of mild to moderate hypoglycemia, steering was disrupted, resulting in swerving, spinning, and increased time across the midline and off the road," the researchers write. Their study adds additional information to this disturbing picture.
For the study, two separate groups of patients with type 1 diabetes were enrolled from four medical centers. To participate, they had to be familiar with self-monitoring of blood sugar levels, and they had to measure their levels at least twice daily. Each person studied was trained to use a hand-held computer to do the following as they went through their normal daily activities: 1) record information about their symptoms; 2) perform two tests of thinking ability; 3) rate what they thought was their degree of impairment in performing each test; 4) enter whether their most recent insulin, food, and exercise were more, less, or usual in amount; 5) estimate their current blood sugar level; and 6) answer the question, "Based on your current blood glucose, would you drive now?"; and 7) obtain a blood sample and measure their actual blood sugar levels.
"We have never before had that kind of information from patients in their natural environment," says Clarke. "We can ask people questions about what they 'would' do, what they 'might' do, [when they're] in a lab situation or in a hospital bed, but these individuals were in their natural environment performing their everyday tasks."
Results showed that the people studied stated they would drive 43-44% of the time when they estimated their blood sugar at 60-70 mg/dL, and 38-47% of the time when their actual blood sugar was <40 mg/dL. Approximately 50% of drivers in each group decided to drive at least 50% of the time when their blood sugar level was <70 mg/dL. "There are a variety of reasons that people might drive a car even though they recognize that their glucose is low," says Clarke. "Those can be environmental or social factors, such as a need to pick up children, not having rapid-acting sugar available in the car, or previous experience with low blood sugar -- in much the way that people recognize that maybe they can drive the car when they have had two drinks and still do OK. I think that is [fine] when you don't stress the system, but what happens when a kid runs out in front of you? Your processing of an emergency situation is probably at a level [that prevents you from] respond[ing] appropriately."
Margaret Himelfarb, a board member of the Juvenile Diabetes Foundation International and the mother of a son with diabetes, adds two more reasons: the fact that people with low blood sugar have some impairment in their thinking ability or that they may have misunderstood the question asked in the study.
"The one thing that I don't see [in the study] is whether or not they might have expected that they would have a snack and then get behind the wheel," she says. "It is so obvious to them, such a part of their lives. ... It is conceivable they may have misunderstood the directive."
Clarke concedes that misunderstanding may be a possibility, but he considers it unlikely that so many of the people studied would have made the same mistake. "I don't think the numbers would bear that out," he says. "All of us physicians wish that our patients would make good decisions, ... and it is our responsibility to review with them the many things that could be dangerous in their lives and help them to be safer."
"Although the statistics from a variety of sources ... can't ... show that there are more accidents among people with type 1 diabetes, there is most likely a tremendous amount of underreporting of automobile accidents by people with type 1 diabetes," says Clarke. "I mean, you just wouldn't announce 'Oh, I have diabetes,' if you wrecked a car."
"It is important to note, as [the researchers] have, that there has never been any evidence that people with type 1 diabetes have higher accident rates, because I think that is the first conclusion that people are likely to draw," says Himelfarb.
The researchers stress the importance of counseling type 1 diabetics on the risks associated with low blood sugar and driving, and they suggest that diabetics test their blood sugar level before driving.
"One other thing the article mentions is the importance of carrying some form of glucose with you; that way, you always have something in the car so that ... you can immediately treat yourself," says Himelfarb. "It doesn't hurt to throw in another [test] before you get in the car -- obviously that is a great inconvenience, but it is important to do and make sure your blood sugar is where you want it to be. People who manage their diabetes well can do everything anyone else can with no greater risk. The key thing is testing blood glucose and keeping it within normal limits. It's important not only for driving but to prevent all the complications that go with diabetes."