Renal length and inulin clearance in the radiologically normal single kidney.

Compensatory hypertrophy of a single functioning kidney is well described and has been shown to occur in utero. The long-term effects of hypertrophy and hyperfiltration in this situation are unknown. This study defined the growth parameters for single kidneys during childhood and correlated them with inulin clearance. Patients were those who had a radiologically "normal" single kidney, where the contralateral kidney was known to be non-functioning from infancy. Data were obtained from 74 children (40 boys and 34 girls) drawn from a registry of cases with a single kidney, and in whom simultaneous measurements of inulin clearance and renal length had been made at around 5, 10, and 16 years of age. Renal length was taken as the maximal bipolar measurement using real-time ultrasound scan. Inulin clearance was by continuous infusion technique. Nomograms for single kidney growth were determined against age, height, weight, and body surface area. Renal growth was correlated with inulin clearance. Renal length was found to correlate best with body surface area ( r=0.85, P<0.001), but this was not significantly superior to correlations with age, height, or weight separately. Inulin clearance per body surface area correlated positively with standardized renal length, i.e., Z score for renal length normalized for body surface area ( r=0.53, P<0.001). The larger kidneys have a higher glomerular filtration rate. Provided that the nephron number in the single kidney is similar to that in a paired kidney, single kidneys are hypertrophied and the single nephron glomerular filtration rate is likely to be abnormally high in these children.