A study evaluated the role of angiotensin-converting enzyme (ACE) inhibitors in the prevention of coronary events in patients with diabetes. During 5 years of follow-up, 120 out of 400 diabetic patients who had taken an ACE inhibitor developed an acute coronary event. Over the same time, 100 out of 300 diabetic patients who had not taken an ACE inhibitor experienced coronary events. What was the relative risk of developing a coronary event in diabetic patients who were taking ACE inhibitors compared to diabetic patients who were not taking ACE inhibitors?

Question

A study evaluated the role of angiotensin-converting enzyme (ACE) inhibitors in the prevention of coronary events in patients with diabetes.  During 5 years of follow-up, 120 out of 400 diabetic patients who had taken an ACE inhibitor developed an acute coronary event.  Over the same time, 100 out of 300 diabetic patients who had not taken an ACE inhibitor experienced coronary events.  What was the relative risk of developing a coronary event in diabetic patients who were taking ACE inhibitors compared to diabetic patients who were not taking ACE inhibitors?

Quizplus · Accepted Answer

11ec160b_371d_218e_9249_b344cb1fa046_MD0001_00 It is important to know how to calculate some basic measures of association when given raw data. The first step is to organize the data into a 2×2 (contingency) table as shown below. The standard format includes the exposures in the rows (with "exposure present" listed above "exposure absent") and the outcomes in the columns (with "outcome present" listed before "outcome absent"). The relative risk (RR) represents the risk of an outcome (eg, coronary event) in the exposed (eg, angiotensin-converting enzyme [ACE] inhibitor) divided by the risk of that outcome in the unexposed (eg, no ACE inhibitor). For cohort studies, it is calculated using the following formula: RR = [a/(a+b)] / [c/(c+d)] = (120/400)/(100/300) = 0.90 Understanding the values calculated (rather than memorizing several formulas) would prevent errors that could occur if the standard format for the 2×2 table is not followed. By definition, RR = risk among the exposed/risk among the unexposed. In this example, 400 diabetic patients were taking ACE inhibitors (exposed) and 120 of those had a coronary event. Therefore, the risk among the exposed is 120/400. Similarly, 300 diabetic patients were not taking ACE inhibitors (unexposed) and 100 of those had a coronary event. Therefore, the risk among the unexposed is 100/300. The RR is the ratio of those 2 values: RR = (120/400)/(100/300). (Choice B) The odds ratio (OR) is calculated using the following formula: OR = ad/bc. The OR is often calculated in case-control studies where RR cannot be obtained because such studies do not follow patients over time to determine the incidence of a new outcome. Educational objective: The relative risk (RR) represents the risk of an outcome in the exposed divided by the risk of that outcome in the unexposed. Applying the correct formula for RR calculations depends on the proper formatting of a 2×2 (contingency) table.

A Study Evaluated the Role of Angiotensin-Converting Enzyme (ACE) Inhibitors