1. Factors influencing the total body and renal clearances of inulin were investigated in a total of 37 healthy adult volunteers and 10 patients with stable chronic renal failure after the single intravenous injection of a dose of 70 mg/kg given over 5 min.

2. The elimination of inulin was highly concentration-dependent, and in healthy volunteers the renal clearance fell from 103.7 ± 14.4 ml min−1 1.73 m−2 during the first hour after administration to 49.1 ± 20.9 ml min−1 1.73 m−2 over the period 6-8 h. In the patients with renal failure the renal clearance fell correspondingly from 39.7 ± 16.5 to 26.6 ± 8.6 ml min−1 1.73 m−2. There were no changes in the simultaneously measured clearances of creatinine.

3. The values obtained for the total body clearance of inulin after a single injection depend critically on dose, the number and timing of blood samples, the choice of pharmacokinetic model, the number of data points chosen for estimation of the slope of the terminal elimination phase for analysis by the methods of residuals, and the weighting used for curve fitting by non-linear regression analysis.

4. With standardized conditions of sampling from 0 to 2 h and weighted non-linear regression analysis of the plasma concentration-time data, the total body and renal clearances of inulin were almost identical in subjects with normal renal function at 105.2 ± 10.2 and 102.9 ± 13.0 ml min−1 1.73 m−2. In the patients with chronic renal failure sampling was continued for 3 h and the corresponding clearances were 40.4 ± 15.3 and 38.9 ± 15.7 ml min−1 1.73 m−2.

5. The 0-2 h total body and renal clearances of inulin were measured by the single injection method and the renal clearance was measured by the standard constant infusion method on different occasions in 10 healthy volunteers. The respective clearances were similar at 101.4 ± 6.6, 94.9 ± 11.9 and 88.4 ± 12.1 ml min−1 1.73 m−2.

6. The reproducibility of the single injection and constant infusion methods was compared by measuring the inulin clearance with both techniques on three occasions in separate groups of eight and nine healthy volunteers. The mean coefficient of variation for the total body clearance with the single injection method was only 3.9% compared with 9.5% for the renal clearance determined the same way and 12.0% for the renal clearance during constant infusion.

7. Urine collection and fluid loading are not required for the single injection technique, and it is more reproducible and less demanding than the constant infusion method. With simple precautions, the single injection method with inulin is suitable for routine estimation of the glomerular filtration rate.

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