To the Editor: In a retrospective matched cohort study, Dr Wald and colleagues1 concluded that acute kidney injury (AKI) requiring dialysis was associated with an increased risk of end-stage renal disease but not all-cause mortality. This finding contradicts the conclusions of recent studies demonstrating a significant association between AKI and long-term mortality.2 - 4 The selection criteria for both the AKI exposure and control groups in their study might have contributed to this difference, rendering questionable the external validity of the study.
The authors identified 15 028 patients with International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes for AKI and dialysis during index hospitalization in a 10-year time period (1996-2006). Although 8855 of these patients survived to discharge, only 3769 were included in the final exposure group. Among 4783 excluded patients with AKI were those with no renal recovery at discharge (3481/8855, 39%), early postdischarge deaths (387/8855, 4%), and readmissions (915/8855, 10%). This approach would have excluded the sickest patients from the exposure population, artificially reducing the mortality rate of this group.
In addition, patients without AKI in the control group were identified by the absence of an ICD-9-CM code for AKI. However, an ICD-9-CM code for AKI in an administrative database may be present in only 16% of patients with AKI defined by measured changes in serum creatinine.2 Thus, their method for identifying patients in the control group likely led to the inclusion of patients with AKI not requiring dialysis, a group that comprises more than 75% of all patients with AKI and as much as 15% of all hospitalized patients.5
Primary findings included all-cause mortality of 10.10 per 100 patient-years for the AKI group and 10.83 per 100 patient-years for the control group. In contrast, in a study of 10 518 patients discharged after major surgery, all-cause mortality for patients with AKI vs patients without AKI was 8.5 per 100 patient-years (95% confidence interval [CI], 8.13-8.94) vs 4.43 per 100 patient-years (95% CI, 4.26-4.62; P < .001).2 These rates are comparable with those reported in a meta-analysis of studies published before 2008 (8.9/100 patient-years for AKI group vs 4.3/100 patient-years for no AKI group; relative risk, 2.59; 95% CI, 1.99-3.42).3 Another large study of patients with acute myocardial infarction found an increase in long-term mortality associated with AKI.4 Overall, the statistically significant adjusted hazard ratio for mortality associated with AKI ranged between 1.24 and 2.62.2 - 4
Unless the authors can account for the apparent selection bias, we believe that no conclusions can reasonably be reached regarding the long-term mortality risk associated with AKI on the basis of this study.
Financial Disclosures: Dr Bihorac reported receiving grant support from the National Center for Research Resources. No other disclosures were reported.
Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
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