Residual kidney function and plasma urea concentration in patients with chronic renal failure

The relationships between the plasma levels of urea (P(urea)), renal clearance of urea (C(urea)) and creatinine (Ccr) at an intake of 0.5 g protein/kg body weight/day were followed in 10 patients with chronic renal failure (CRF) under balance conditions. Under these conditions, P(urea) attained a value of 30 mmol/l when C(urea) had decreased below 3.8 ml/min. By contrast, no correlation could be demonstrated between P(urea) and Ccr under these conditions. The same relationships were followed in another group of 30 outpatients with CRF. Even in patients not followed under balance conditions, C(urea) determination makes it possible to establish whether the high increase in P(urea) is due to the decrease in residual renal function below the critical level or whether extrarenal factors are involved. Likewise, no significant correlation between P(urea) and Ccr could be demonstrated under these conditions. The findings suggest that C(urea) measurement in CRF patients helps to assess residual renal function in terms of P(urea) regulation and provides information that cannot be obtained by Ccr measurement.