Which of the following is incorrect about transtubular potassium gradient (TTKG)?
A. It is a measurement which estimates the tubular fluid potassium concentration at the end of the cortical collecting tubule, the site responsible for most of potassium secretion. Numerically it is (urine K/plasma K) divided by (urine osmo/plasma osmo).
B. TTKG = 8 to 9 in normal subject on a normal diet
C. In hyper K, TTKG > 7 means that effective circulating volume is low
D. In hypo K, TTKG > 7 means non-renal loss of K
Answer: D
Simply think of the TTKG as a rough measurement of aldosterone activity in the kidney. As aldosterone’s action is retention of Na with excretion of K, tubular fluid K concentration is high if there is aldosterone action, and so is TTKG. Division by (urine osmo/plasma osmo ) is to adjust the urinary potassium for the concentrating effects that occur in the collecting tubule, where water is removed from the urine. Note that this formula is valid only when Uosm >300 and UNa >25. A high TTKG suggests that the kidney is wasting potassium, which may be appropriate (in the setting of hyperkalemia) or inappropriate (in the setting of hypokalemia). Normally, hypokalemia should produce a TTKG < 3. The expected TTKG in hyperkalemia is > 10.
In hyper K, if the TTKG is high, it means that there is high aldosterone action (appropriate), e.g. from low effective circulating volume in CHF. If TTKG is < 5, it means that aldosterone activity is low, and the reason for hyper K is due to abnormal renal handling of K, e.g. in type IV renal tubular acidosis.
In hypo K, if TTKG is still > 7, it means that there is inapproriately high aldosterone activity despite the hypo K, and thus renal loss of K is the cause of the hypo K, e.g. hyperaldosteronism, Cushing’s syndrome (pseudohyperaldosteronism). If TTKG is < 5, it means that aldosterone activity is appropriately low to conserve K, and thus the hypo K is not due to renal loss, i.e. non-renal loss.
Tool for calculation
1. TTKG calculator from md+calc