Effect of mineralocorticoid activity on transtubular potassium gradient, urinary [K]/[Na] ratio, and fractional excretion of potassium.

Clinical assessment of potassium derangements may require evaluation of mineralocorticoid status. Several indirect indices of mineralocorticoid activity based on renal electrolyte excretion have been proposed and include the transtubular potassium gradient, urinary [K]/[Na] ratio, and renal fractional excretion of potassium. We studied the impact of high mineralocorticoid activity versus blocked mineralocorticoid activity on these indices in otherwise normal subjects who ingested a defined diet. Eight normal subjects received either fludrocortisone or spironolactone for 4 days. After a washout period of > or = 2 weeks, each subject then received the opposite regimen. Subjects ingested an identical high-potassium diet during both experimental periods. The renal fractional excretion of potassium and transtubular potassium gradient were calculated using standard formulas. Fludrocortisone caused an increase in body weight and no significant reduction in serum potassium concentration, while spironolactone decreased body weight and increased plasma potassium concentration. After 1 or 2 days of treatment with fludrocortisone, the average values for all urinary indices of mineralocorticoid activity were significantly higher than after 1 or 2 days of treatment with spironolactone. However, the differences between these indices in the fludrocortisone and spironolactone test periods diminished by day 3 and were nonexistent by day 4. In conclusion, the transtubular potassium gradient, [K]/[Na] ratio, and renal fractional excretion of potassium reflect acute changes in mineralocorticoid activity. However, these indices do not discriminate between states of high and low mineralocorticoid activity lasting longer than 2 to 3 days.