The magic of math in assessing risk

Dear Dr. Roach: In a recent column, you reported using “the standard risk calculator” to determine that a person has a risk of heart attack or stroke in the next 10 years of “6.8%.” What exactly does this mean? How was the risk determined? — T.A.

A risk calculator takes important characteristics about a person and estimates the possibility that an event will happen. The American Heart Association and American College of Cardiology looked at data from tens of thousands of people and used a person’s age, gender, race, total and HDL cholesterol, blood pressure, presence or absence of diabetes and smoking history to estimate the likelihood of that person having a heart attack or stroke in the next 10 years. It is not appropriate for a person with a known history of heart disease or stroke. The person who wrote to me had a fairly low value, and this calculator is used to provide information that helps the person and their doctor decide what treatments are appropriate.

The calculator isn’t perfect. It does not take into account many other important risk factors Nonetheless, it’s a good place to start a conversation.

In order to calculate a number needed to treat, you need to know how the risk would be affected by that treatment.

The number needed to treat is just 100% divided by the absolute risk reduction. In this case, 50 people would need to be treated with the statin for 10 years to prevent one person from having a heart attack or stroke. The higher the absolute risk and the better the intervention, the larger the absolute benefit and the fewer people needed to be treated. Some calculators will give the estimated absolute risk reduction from treatment.

For a person who really wants to know whether a treatment is worth it, understanding the benefit meaningfully is critical.