A study compared drug A versus standard therapy in preventing recurrent pulmonary embolism (PE). The absolute risk reduction for drug A versus standard therapy was 4%. The incidence of recurrent PE in the standard therapy group was 6%. There were 24 patients who developed recurrent PE in the drug A group. How many total subjects were there in the drug A group?

Question

A study compared drug A versus standard therapy in preventing recurrent pulmonary embolism (PE).  The absolute risk reduction for drug A versus standard therapy was 4%.  The incidence of recurrent PE in the standard therapy group was 6%.  There were 24 patients who developed recurrent PE in the drug A group.  How many total subjects were there in the drug A group?

Quizplus · Accepted Answer

This question is meant to challenge your knowledge of how absolute risk reductions (ARR) are calculated. The ARR equals the event rate in the control group (ER_control) minus the event rate in the treatment group (ER_treatment). In this example, the event rate represents the incidence of recurrent pulmonary embolism (PE); ER_treatment is the incidence of recurrent PE in the drug A group, and ER_control is the incidence in the standard therapy group. ER_treatment can be determined from the ARR (given as 4%) and ER_control (given as 6%): This value (ER_treatment) also represents the number of events in the treatment arm divided by the number of subjects in the treatment arm. Therefore, knowing the total number of events in the treatment arm (24 instances of recurrent PE in the drug A group), the number of subjects in the treatment arm can be easily calculated: Educational objective: Absolute risk reduction = event rate in the control group − event rate in the treatment group.

A Study Compared Drug a Versus Standard Therapy in Preventing