Diabetes as a Dummy Variable

“Adults with diabetes are nearly twice as likely to have heart disease or stroke as adults without diabetes.”

NIH

These alarming numbers are used to justify more pre-emptive testing, e.g., Calcium score for the heart, more aggressive use of statin therapy, etc., among diabetics.

The analyses that underlie the inference about risk that diabetics face compare the outcomes of people who are diagnosed with diabetes and similar non-diabetic others; the analyses treat diabetes as a dummy variable.

Let’s, for a moment, separate the diagnosis of diabetes (which may be based on having an H1A1c above six once) from how well diabetics control their blood sugar (“glycemic control”; H1A1c < 7). For argument’s sake, let’s also assume that glycemic control is the true causal variable for elevated risk of heart disease and stroke. The risk of heart disease and stroke rises with the extent of glycemic control (intensity) and the length of the period (duration) for which your blood sugar is uncontrolled, with risk profiles for people with glycemic control looking no different from those of ‘similar non-diabetic’ others. The predicates imply that excess heart attacks and strokes happen in patients who cannot control their blood sugar levels. Now, say that the percentage of diabetics who are able to achieve glycemic control is approximately 25% (see here for a study of patients on insulin therapy). It means that applying a policy learned on diabetes or not leads to overtreating some diabetics and undertreating others. Improving the precision of the targeting variable may improve outcomes.

p.s. It may be that the optimal policy with respect to some therapies is ‘overtreatment,’ as the famous Metformin study, which showed that diabetics on Metformin live longer than non-diabetics, suggests.