Renal function studies in man with advanced renal insufficiency.

In subjects with advanced renal insufficiency, the average of the creatinine and urea clearances ([Ccr + Cur]/2) is frequently used as an indicator of glomerular filtration rate (GFR). To test the accuracy of this method, 31 subjects with inulin clearance (Cin) less than 20 ml/min/1.73 m2 underwent simultaneously timed creatinine, urea, and inulin clearances. The results indicated that: (1) Ccr correlated positively with Cin; mean fractional excretion of creatinine (Ccr/Cin) was 1.5; (2) Cur correlated positively with Cin; in contrast to creatinine, the fractional excretion of urea (Cur/Cin) correlated negatively with Cin; (3) in 10 of 12 subjects with Cin less than 8 ml/min 1.73 m2, the Cur/Cin was greater than 1, suggesting that tubular secretion may have occurred; and (4) the (Ccr + Cur)/2 correlated positively with Cin and was the best clinical indicator of GFR.