Free-water clearance patterns as predictors and therapeutic guides in acute renal failure.

Sequential changes in renal function were studied in a series of 114 postoperative patients who developed acute renal failure. The loss of concentrating ability, manifest by a change from strongly negative free-water clearances to values near zero, occurred 24 to 48 hours before the onset of blood urea nitrogen (BUN) and creatinine elevations. In 15 patients a transient period of positive free-water clearance was documented just before these values approached zero. Criteria were proposed for early diagnosis of acute renal failure (ARF) based upon description of the temporal pattern of free-water clearance values. The loss of concentration ability occurred with decreased urinary Na+ concentration unless patients were given large amounts of saline solution prior to the development of ARF. This was followed by gradually increasing urinary Na+ concentrations. Changes in K+ concentrations were not significant until the late stage of renal failure. Recovery patterns in 46 of these patients who survived demonstrated an early return of negative free-water clearance followed by gradually decreasing BUN and serum creatinine values. During this period recovery of the ability of Na reabsorption and excretion of K+ was indicated by decreased urinary Na+ concentrations and increased urinary K+ concentrations.