Intended for healthcare professionals

Letters Warfarin and atrial fibrillation

Problems with using numbers needed to harm

BMJ 2015; 350 doi: (Published 24 March 2015) Cite this as: BMJ 2015;350:h1581
  1. Benjamin D Williams, PhD student1
  1. 1University of Manchester, Manchester M13 9PL, UK
  1. benjamin.williams-4{at}

I have some questions about Jun and colleagues’ use of number needed to harm in their research article on the association between kidney function and major bleeding in older adults with atrial fibrillation starting warfarin treatment.1

In the methods they wrote: “Based on the adjusted incidence rates of major bleeding in chronic kidney disease (defined as <60 mL/min/1.73 m2) and …

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