In response to Hahn and Wuethrich’s comment on our article we agree that creatinine-based definitions of acute kidney injury (AKI) are imperfectly specific for damage to kidney tissue, and many such cases of AKI may represent adaptive changes in renal hemodynamics in response to hypovolemia.  These cases typically resolve quickly with fluid administration, which corrects hypovolemia, restores the glomerular filtration rate, and allows for a return to the baseline level of urinary clearance of creatinine.

We read with interest the article by Hahn et al. demonstrating that higher urine creatinine concentration was observable preoperatively in some patients who experience postoperative elevated plasma creatinine.  The impact of preoperative fasting on hemodynamic status and fluid responsiveness has been debated, with studies suggesting that an 8-h fast has no or minimal impact on fluid responsiveness.  That being said, as discussed in our review, assessing volume status and considering fluid challenge in those determined to have a high probability of hypovolemia is an important step in the perioperative setting when facing a patient with oliguria.

Regarding potential effects of infusion fluid dilution during anesthesia induction, we agree that fluid infusion can decrease serum creatinine. However, given the volume of distribution of creatinine (about 60% of total body weight, equivalent to total body water) and the rapid extravasation of most of crystalloids in the interstitial compartment, only a large volume of fluids would lead to a decrease in the serum creatinine after the end of the fluid infusion. For instance, a 2-l fluid infusion in a 70-kg man with 42 l expected total body water and serum creatinine of 1.00 mg/dl would be expected to have only a minimal impact on serum creatinine (estimated serum creatinine, 0.95 mg/dl [1.00 × (42/44)]). Therefore, the decrease in plasma creatinine after fluid administration chiefly results from correction of hypovolemia resulting in increased renal clearance of creatinine.