Sir,—With interest we read the article of Jämsä et al. (Acta Orthop 2017; 88:3 70-6) assessing risk factors associated with postoperative acute kidney injury (AKI) in 20,575 arthroplasty patients. By multivariable logistic regression analysis, they show that preoperative estimated glomerular filtration rate, ASA classification, body mass index, and duration of operation are independent risk factors for AKI. Strengths of this study are a large sample and its use of appropriate statistical methods to identify risk factors of postoperative AKI. However, this study is a retrospective analysis, which potentially introduces a number of confounding variables. Other than the limitations described in the discussion, we note other issues of this study making interpretation of their results questionable.