Assessment of tubular reabsorption of sodium, glucose, phosphate and amino acids based on spot urine samples.

Reference values for tubular transport of sodium, phosphate, glucose and amino acids are generally based on inulin or creatinine short-term clearances, which are difficult to obtain in children. Hence, quantitative assessment of tubular transport capacities is rarely performed. For a simplified procedure, reference values for fractional sodium excretion, phosphate reabsorption related to glomerular filtration rate, percent glucose and percent amino acid reabsorption were established in 62 children from spot urine and simultaneously obtained blood samples. Sodium excretion, and glucose and amino acid reabsorption were significantly lower in infants than children, whereas phosphate reabsorption decreased during the first year of life. Results using the proposed protocol and those obtained from timed urine specimens correlated well; the phenomenon of renal adaptation during childhood could equally well be demonstrated. Renal tubular dysfunction can be diagnosed without timed urine specimens.