NAPLEX Question of the Week: Relative Risk Reduction

Biostatistics are an important part of literature evaluation that informs clinical practice. Are you up for the challenge?

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In a recent study, rivaroxaban was compared to enoxaparin for VTE prophylaxis following hip arthroplasty. VTE occurred in 18 of the 1595 patients in the rivaroxaban group and 58 of the 1558 patients in the enoxaparin group. What is the relative risk reduction of using rivaroxaban over enoxaparin? Answer should be in a percent rounded to the nearest whole number.




To find the relative risk reduction, we first must find the relative risk.

Relative risk = (Event rate in rivaroxaban group)/(Event rate in enoxaparin group)

Relative risk = (18/1595) ÷ (58/1558) = 0.303

To find the relative risk reduction, subtract the relative risk from 1

Relative risk reduction = 1 – 0.303 = 0.697

0.697 X 100 = 69.7% so 70%

Relative risk reduction tells you how much the treatment reduces bad outcomes in comparison to the other treatment, in this case VTE. In this trial, rivaroxaban showed a 70% risk reduction for a VTE compared to enoxaparin. Relative risk reduction calculations can be somewhat misleading, especially in outcomes for which major event rates are very low. 

A different and sometimes more useful calculation would be an absolute risk reduction which would be the event rate subtracted in absolute numbers, or in this case the event rate of enoxaparin (3.7%) - event rate of rivaroxaban (1.1%)= 2.6%. A number needed to treat can be calculated from the absolute risk reduction which is 1/ARR (decimal number not %) which would be 1/0.026 or 38. This is a helpful calculation meaning that 38 patients would need to be given rivaroxaban to prevent one additional VTE in the hip arthroplasty population compared to enoxaparin. Number needed to treat when calculated should be reported in whole numbers. 

NAPLEX Competency Statement 4.8 is "Biostatistics, epidemiological, or pharmacoeconomic measures". 

Reference: Eriksson BI, Borris LC, Friedman RJ, Haas S, Huisman MV, Kakkar AK, Bandel TJ, Beckmann H, Muehlhofer E, Misselwitz F, Geerts W; RECORD1 Study Group. Rivaroxaban versus enoxaparin for thromboprophylaxis after hip arthroplasty. N Engl J Med. 2008 Jun 26;358(26):2765-75. doi: 10.1056/NEJMoa0800374. PMID: 18579811

Christopher M. Bland

Clinical Professor, University of Georgia College of Pharmacy

Dr. Christopher M. Bland is a Clinical Professor at the University of Georgia College of Pharmacy at the Southeast GA campus in Savannah, GA. Dr. Bland has over 20 years of academic and clinical experience in a number of clinical areas. He is a Fellow of both the Infectious Diseases Society of America as well as the American College of Clinical Pharmacy. He is co-founder of the Southeastern Research Group Endeavor, SERGE-45, with over 80 practitioners across 14 states involved. Dr. Bland serves as Associate Editor for the NAPLEX Review Guide 4th edition as well as Editor-In-Chief for the Question of the Week. He has provided live, interactive reviews for more than 10 Colleges/Schools of Pharmacy over the course of his career.