In Reply to ‘The Pathogenesis of Acute Kidney Injury May Differ Between Elderly and Younger Patients’

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We agree with Dr Schiffl¹ that our results may have been confounded by a greater prevalence of chronic kidney disease or other comorbid conditions in the older population, as noted in our discussion. We also acknowledge that the phenotype of acute kidney injury (AKI) may be different in elderly compared with younger patients, and we agree that AKI should not be viewed as a reversible syndrome, but that chronic kidney disease caused by AKI is a predictor of long-term mortality.

Addressing the differences in data from Dr Schiffl and our own results, it is important to note that we had approximately 3,000 patients in our meta-analysis compared with only 226 surviving patients in the cohort of Dr Schiffl.², ³, ⁴ Although the mean age of recruitment in the cohort of Dr Schiffl was 65 years, the survivors were younger (mean ages, 63 and 59 years). Thus, it appears that the death of very old patients may have added bias to the study. Dr Schiffl also states that age was not an independent predictor of nonrecovery in his analysis, but there are several reasons why this may be the case: lack of power with 100 events of partial recovery (outcome variable) and 9 independent variables along with several that have multiple subcategories, multicollinearity, homoscedasticity, or overfitting (information not provided for the latter 3). The multivariate analysis did not specifically examine the association of age with recovery, and age was one of the covariates in the model. Thus, the model-building techniques and other attributes of the multivariate model become important to consider before we can conclude that age is not related to nonrecovery. If the lack of association between age and recovery was caused by lack of statistical power, combining several studies in a meta-analysis approach can recover this effect.

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Acknowledgements 

Financial Disclosure: None.

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References 

1. Schiffl H: The pathogenesis of acute kidney injury may differ between elderly and younger patients. Am J Kidney Dis 52:1198-1199, 2009
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2. Schmitt R, Coca S, Kanab M, et al. Recovery of kidney function after acute kidney injury in the elderly: A systematic review and meta-analysis. Am J Kidney Dis. 2008;52:262–271
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3. Schiffl H. Renal recovery from acute tubular necrosis requiring renal replacement therapy: A prospective study in critically ill patients. Nephrol Dial Transplant. 2006;21:1248–1252
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4. Schiffl H, Fischer R. Five-year outcomes of severe acute kidney injury requiring renal replacement therapy. Nephrol Dial Transplant. 2008;23:2235–2241
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