Factors affecting urine volume in chronic renal failure.

Twenty-four-hour urine volume was studied in 80 determinations from 12 patients with chronic renal failure (creatinine clearances between 7.1 and 38.9 L/d) hospitalized on a metabolic ward. Urine volume ranged from 0.91 to 4.51 L/d. Various components of urine that might affect urine output, ie, osmolar excretion, sodium excretion, urea nitrogen excretion, free water clearance, and potassium excretion, were correlated with urine volume to determine their relative effects. Total osmolar excretion correlated highly with urine output (r = .92, P less than .001), while correlation with free water clearance was weak. Of the three osmotic components, total urea nitrogen excretion correlated best with urine volume (r = .86, P less than .001), while the correlation with sodium excretion was less pronounced (r = .75, P less than .001). The relatively greater impact of urea excretion on urine volume was confirmed by multiple regression analysis. However, total cation excretion (sodium plus potassium) gave nearly as good a regression with urine volume (r = .83, P less than .001). Our findings confirm that total osmolar excretion is a major determinant of urine volume in chronic renal failure and suggest that urea excretion may play the most important role in determining output.