Electrolyte excretion in 12-hour urine and in spot urine. Relationship to plasma renin activity and aldosterone concentration in older children.

Plasma renin activity (PRA) and urinary electrolyte excretion were measured in 137 healthy children aged 6 to 14 years. Plasma aldosterone concentration (PAC) was measured in 52 of the children. Nocturnal 12-hour urine was collected in 110 of the children. Spot urine was collected on two occasions, once just before lying down, once after 90-min supine rest in another 27. Na/K ratio and fractional Na excretion rate (FENa) in 12-hour urine showed a significant inverse correlation with PRA or PAC. Na excretion (mmol/min, mmol/mmol creatinine), Na/K ratio and FENa in the spot urine following 90 min in a supine position showed a significant inverse correlation with PRA or PAC, but they failed to show a significant relationship to PRA or PAC in the spot urine preceding supine rest. A spot urine after 90 min in the supine position is collected easily and hence most appropriate to study the relationship between Na excretion and PRA or PAC clinically.