Transtubular potassium concentration gradient: a useful test to estimate renal aldosterone bio-activity in infants and children.

The present investigation was designed to validate the usefulness of transtubular potassium (K) concentration gradient (TTKG) as an indicator of aldosterone bio-activity in infants and children. TTKG was calculated by the formula: [K]urine: (urine/plasma)osmolality/[K]venous blood. We compared this index with fractional K excretion (FEK) and urine K concentration to urine sodium (Na) concentration ratio (UK/UNa) in 473 normal children aged 1 month-15 years. Values of TTKG followed a non-gaussian distribution (median, 6.3; 3rd centile, 4.1; 97th centile, 13.4). TTKG in infants (n = 108; median, 7.8) was significantly higher than in children (n = 365; median, 6.0). TTKG correlated directly with FEK and UK/UNa. Indices of K excretion were also assessed in 13 patients with hypo- and pseudohypoaldosteronism. TTKG values varied between 1.6 and 4.1 and were all below the 3rd percentile established for the age of the subject. We conclude that calculation of TTKG is an easy and sensitive method for the evaluation of mineralocorticoid action in distal and collecting tubules.