Potassium excretion indices in the diagnostic approach to hypokalaemia

Abstract

Sir, Measurement of urinary potassium excretion is very helpful in the differential diagnosis of both hypokalaemia and hyperkalaemia. 1,2 To determine urinary potassium excretion, it is preferable to collect a 24-h urine sample. 3 However, this is not feasible in many cases. Random measurement of the urinary potassium concentration is simple to perform but may be less accurate than a 24-h collection, since it is influenced by two independent factors; potassium secretion and water reabsorption in the medulla. 1,4,5 Therefore, the fractional excretion of potassium (FEK +) has been proposed as a useful marker of potassium excretion. 6 This index relates the amount of potassium excreted to the amount filtered.

Hypokalaemia in alcoholic patients

“…In 12 of these 16 patients inappropriate kaliuresis evidenced by increased FEK + > 6.4% and TTKG > 2 was observed [7,8]. Serum potassium levels were significantly lower in alcoholic patients compared to 150 age-and sex-matched controls (3.8 ± 1.1 mmol/l vs. 4.6 ± 0.9 mmol/l, p < 0.0001).…”

“…A value more than 2 in hypokalaemic patients is highly suggestive of inappropriate kaliuresis [6,7]. A value more than 2 in hypokalaemic patients is highly suggestive of inappropriate kaliuresis [6,7].…”

Electrolyte Disturbances in SARS-CoV-2 Infection

“…The urine to plasma osmolality ratio adjusts for the degree of medullary water reabsorption which increases urine potassium concentration as more water is reabsorbed. Therefore, TTKG is relatively accurate providing the urine is not dilute, that the osmolality of urine is greater or equal to the osmolality of plasma (because vasopressin is required for optimal potassium excretion in the distal nephron) and that urine sodium concentration is >25mmol/l so that distal sodium delivery is not limiting 41 . The main premise underlying the TTKG is the absence of significant solute transport in the collecting duct so that any change in urinary potassium concentration only occurs due to medullary water reabsorption and the TTKG does not overestimate the gradient for collecting duct potassium secretion.…”

Case Report: Renal potassium wasting in SARS-CoV-2 infection

“…It is easy to measure the TTKG and measurement only requires serum and urine osmolality and potassium (urine potassium/serum potassium)/(urine osmolality/ serum osmolality). Estimation is accurate providing urine is not dilute and that urine sodium concentration is >25mmol/l so that distal sodium delivery is not a limiting factor 17 . A urine potassium: creatinine ratio can also be used and is preferred over a spot urine potassium or 24-hour urinary potassium collection as, because creatinine is theoretically secreted in a constant state (in the absence of an GFR drop), a urine potassium: creatinine ratio corrects for urine volume variations.…”