We have some concerns about the methodology of their study. The authors retrospectively reviewed their 2002 to 2005 experience in cardiac surgery and investigated the effect of preoperative renal function on intermediate-term survival. The study is retrospective, and patients' data were collected retrospectively. Could the authors obtain the data of all the patients undergoing either type of surgery and include them in the study? If they could not do so, wouldn't this be a bias in patient selection? We are wondering about the method of how the authors replaced the missing variables at the time of data collection. Although sometimes unavoidable, the missing information reduces the analytic possibilities and quality of analysis. How was the follow-up of patients performed? Were patients followed until the date of last report or was the anniversary method or common closing date method used? The percentage of patients with complete follow-up should be stated in the methodology.
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