To the Editor:

In a prospective, randomized, and placebo-controlled trial, Lee et al.1  recently showed that administration of 20% exogenous albumin immediately before surgery increases urine output during surgery and reduces the risk of acute kidney injury (AKI) after off-pump coronary artery bypass surgery in patients with a preoperative serum albumin level of less than 4.0 g/dl. Many things in this study were done correctly. But to differentiate the effects of one factor on study endpoints, all the other factors have to be standardized in the study design. Several important factors in this study were not addressed well.

First, contrast angiography or ventriculography was not included in data analysis. Actually, preoperative contrast angiography or ventriculography was not rare among patients undergoing cardiac surgery. Contrast-induced nephropathy is independently associated with increased risks of postoperative adverse renal events.2 

Second, perioperative hemoglobin levels were not provided. Preoperative anemia is not rare among patients undergoing cardiac surgery and is an important risk factor for postoperative AKI. Anemic patients undergoing cardiac surgery are more susceptible to transfusion-related AKI than nonanemic patients are.3 

Third, as shown in table 1,1  mean volumes of blood transfusion and fluids during surgery and the postoperative period were not significantly different between the two groups. But total input volumes at all observed points had highly variable ranges. It was unclear whether serum creatinine levels applied in diagnosis of postoperative AKI had been adjusted based on the perioperative fluid balance of patients. Moore et al.4  showed that using Acute Kidney Injury Network criteria for diagnosis of AKI, without adjusting serum creatinine levels for fluid balance, can underestimate the incidence and severity of AKI after cardiac surgery.

Finally, when assessing the association of albumin with postoperative AKI by multivariable analysis, only preoperative and intraoperative covariates were adjusted in this study. It has been shown that postoperative complications including low cardiac output syndrome, hypoalbuminemia, anemia, and sepsis are independent risk factors of AKI after cardiac surgery.5  We argue that not taking postoperative covariates into account would have biased the true effect of exogenous albumin administration on the occurrence of postoperative AKI in this study.

Competing Interests

The authors declare no competing interests.

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