We have read the article by Tallarico et al. on perioperative oliguria and congratulate the authors on their well-articulated exploration of this complex topic. We would, however, like to highlight some commonly overlooked aspects.

We believe that the diagnostic criteria for acute kidney injury (AKI) might be met in the absence of organic kidney damage by a combination of strong preoperative water retention and additional sodium and fluid build-up resulting from decreased arterial pressure induced by anesthesia.  Preoperative water conservation usually stems from variations in habitual water intake and can be identified by biomarkers such as elevated urine osmolality or creatinine levels.  Healthy individuals can be near their maximum renal capacity for urine concentration. 

Our studies show an inverse correlation between urine flow during surgery and the subsequent rise in postoperative plasma creatinine.  Patients with high urine creatinine before surgery were unable to further concentrate their urine during the operation. This resulted in a postoperative increase in plasma creatinine that was 2 to 3 times higher than in other patients, but these patients still concentrated the urine like before the surgery or even slightly better in the first postoperative morning. By contrast, those with low urinary creatinine before surgery exhibited an expected increase in renal water concentration during the operation. Notably, patients who met the grade I AKI criteria after surgery even had an increase in plasma creatinine from admission to the start of surgery. Conversely, others experienced a decrease in plasma creatinine during the preoperative phase, which is the expected course due to infusion fluid dilution during anesthesia induction. 

Perioperative oliguria and postoperative AKI may be diagnosed due to inadequate daily water intake and chronic but unnoticed renal constraints in concentrating the urine. Further research is needed to distinguish these scenarios from those leading to organic harm to the kidneys.