To the Editor In an Original Investigation by Haaland and colleagues1 recently published in an issue of JAMA Internal Medicine, the authors reported that use of warfarin is associated with lower incidence of cancer. We believe this study is subject to several methodological flaws that undermine its validity.
First, the authors1 did not adjust for any confounders but sex and age. Norway has several nationwide registries that could have provided information on potential confounders including comorbidities and medical treatment. Furthermore, they did not exclude or adjust for previous cancer before 2006, even though the Cancer Registry has collected data since 1951. The claim that incident cancer was studied is misleading. The lack of confounder adjustment is not properly addressed, and the reader is left to speculate on the magnitude and direction of these issues.
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Svendsen K, Karlstad Ø, Småbrekke L. Lower Cancer Incidence—Warfarin Effect or Immortal Time Bias?. JAMA Intern Med. 2018;178(4):585. doi:10.1001/jamainternmed.2018.0370
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