Risk factors associated with acute kidney injury in a cohort of 20,575 arthroplasty patients

© 2017 The Author(s). Published by Taylor & Francis on behalf of the Nordic Orthopedic Federation. This is an Open Access article distributed under the terms of the Creative Commons Attribution-Non-Commercial License (https://creativecommons.org/licenses/by-nc/3.0) DOI 10.1080/17453674.2017. 1375799 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 fi ltration rate, ASA classifi cation, 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. First, the mean age of patients included in this study was 69 years, but perioperative albumin levels were not included in patients’ data. The available evidence indicates that preoperative hypoalbuminemia is associated with AKI after noncardiac surgery (Kim et al. 2013). Furthermore, early postoperative albumin level < 3.0 g/dL is an independent risk factor for AKI in patients undergoing total knee arthroplasty (Kim et al. 2016). Second, the readers were not provided with intraoperative risk factors affecting the occurrence of postoperative AKI. It has been shown that both vast blood loss and massive blood transfusions during surgery are independent risk factors of AKI after noncardiac surgery (Kheterpal et al. 2007). Furthermore, intraoperative hypotension, use of noradrenaline and low urine output are independently associated with increased risk of postoperative AKI (Sun et al. 2015). Actually, even short duration of an intraoperative mean arterial pressure < 55 mmHg can result in postoperative AKI, with an independent graded relationship between duration of intraoperative hypotension and postoperative AKI (Walsh et al. 2013a). Especially, combination of intraoperative anemia, transfusion and hypotension can synergistically act to increase the risk of postoperative AKI (Sun et al. 2015). Third, other than preoperative anemia, postoperative anemia is also strongly associated with AKI after noncardiac surgery. Compared with patients who do not have a decrease in postoperative hemoglobin, a decrement of 1.1 to 2.0 g/dL is associated with an adjusted hazard ratio of 1.5 (95% CI, 1.2–2.0), and a decrement of > 4.0 g/dL with an hazard ratio of 4.7 (95% CI, 3.6–6.2) for AKI (Walsh et al. 2013b). It must be emphasized that a limitation of multivariable logistic regression analysis is the assumption of a particular mathematical relation between intervention and measured outcome. To obtain the true inferences of multivariable logistic regression analysis for adjusted hazard ratios of measured outcome, all of known risk factors affecting measured outcome must be taken into account within the model. If an important risk factor is missed, the multivariate adjustment for hazard ratios of measured outcome can be biased and even a spurious association between intervention and outcome of interest may be obtained (Xue et al. 2017). Thus, we argue that not taking the above intraoperative and postoperative risk factors into account would have tampered with the inferences of multivariable logistic regression analysis for risk factors of postoperative AKI.