The effect of protein deprivation on the validity of creatinine and urea in evaluation of renal function. An experimental study in the goat.

Reliability of plasma creatinine and urea concentrations and clearances in evaluation of renal function was studied in four goats on two diets of equal energy content but differing in protein (14% and 2% of dry matter). Glomerular filtration rate (GFR) was measured with [hydroxymethyl-14C] inulin. GFR was 88.6 +/- 8.4 ml/min on the high protein diet and 36.9 +/- 2.9 ml/min on the low protein diet. The endogenous creatinine clearance values agreed with the inulin clearances on both diets. Reduction of protein intake was followed by significant increase in plasma creatinine concentration (from 83.8 +/- 3.6 mumol/l during high protein diet to 124.6 +/- 3.5 mumol/l during low protein diet). Plasma urea concentration, urea clearance and urea excretion fell to a very low level during low protein intake. Endogenous creatinine clearance seems to be suitable for measurement of GFR even during protein deprivation when GFR is decreased.