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The association between kidney function and major bleeding in older adults with atrial fibrillation starting warfarin treatment: population based observational study

BMJ 2015; 350 doi: (Published 03 February 2015) Cite this as: BMJ 2015;350:h246

Numbers needed to harm

Dear Editor,

I would like to thank the authors for an interesting, generally very good and clinically relevant paper. However, I did have some questions about their use of number needed to harm. In their methods they write:
“Based on the adjusted incidence rates of major bleeding in chronic kidney disease (defined as <60 mL/min/1.73m2) and non-chronic kidney disease (≥60 mL/min/1.73m2), we determined the number needed to treat to harm”

Later in their results they write:
“Among participants with chronic kidney disease, the number needed to treat to harm was 22 (95% confidence interval 18 to 27) during the first 30 days of warfarin treatment”

At face value these figures seem quite alarming, given the number of patients with AF and an eGFR<60 one seems to encounter in hospital medicine. However, it seems they derive their number needed to harm from the excess in incidence rate amongst those with an eGFR <60 compared to those with an eGFR >60. If I have read this correctly then isn’t this the number needed to harm for inducing CKD in patients with atrial fibrillation on warfarin? Shouldn’t the number needed to harm of warfarin therapy be calculated based on comparison with a control group who were not started on warfarin?

More generally, I wonder if the authors should be reporting number needed to harm at all. There is a lack of data on crucial confounders such as smoking, no comparison with a control group or with the baseline pre-warfarin hazard of the study participants themselves and those at highest risk will not have been started on warfarin in the first place. The authors correctly never make an outright causal claim as this could not be sustained. However, doesn’t calculating a number needed to harm implicitly involve make a strong causal assumption?

Yours faithfully,

Ben Williams

Competing interests: No competing interests

25 February 2015
Benjamin D Williams
PhD Student
University of Manchester
G45 Humanities Bridgeford Street Building, Oxford Road, Manchester