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Potassium Urine

In healthy individuals, nearly all potassium filtered by the kidney is reabsorbed.  Potassium excretion reflects distal tubule secretion of potassium, which is stimulated by aldosterone and the rate of potassium entry into the plasma from the diet and from cells. Urine potassium levels are generally helpful only in evaluation of patients with unexplained hypokalemia.  Urine potassium levels between 0 and 15 mEq/L suggest the GI tract is the source of potassium loss, while levels >15 mEq/L suggest renal potassium loss. 

The most common causes of potassium loss in the GI tract are vomiting and diarrhea. Renal potassium loss can be caused by drugs such as diuretics, gentamycin and mineralocorticoids; endocrine disorders associated with excess secretion of mineralocorticoids or glucocorticoids; and renal diseases such as renal tubular acidosis, renal artery stenosis, and rare genetic diseases.

Reference range is 25 - 123 mEq/24 hr. The most accurate measurement of urine potassium is obtained with a 24 hour urine collection, collected in a container without preservative. Specimen should be refrigerated during and after the collection.

Sometimes, collection of a 24 hour urine specimen is not feasible. An alternative method to assess urine potassium excretion is to collect a random urine sample and calculate the fractional excretion of potassium (FEK). This calculation reflects the relationship of potassium excreted to the amount that has been filtered by the kidneys and requires measurement of potassium and creatinine in both urine and plasma. The formula is:

FEK = (urine potassium x plasma creatinine x 100)/plasma potassium x urine creatinine

In patients with hyperkalemia, an FEK of less than10% suggests that hyperkalemia is due to renal disease. A value greater than 10% suggests that hyperkalemia is due to extrarenal causes.

In patients with hypokalemia, an FEK <10% suggests that hypokalemia is due to extrarenal causes, while an FEK value greater than 10% suggests that the underlying cause is of renal origin.

 Urine potassium loss can also be evaluated by calculating the transtubular potassium gradient (TTKG) using the following formula:

TTKG = urine potassium/(plasma osmolality/urine osmolality)/serum potassium

For this formula to be accurate urine osmolality should be higher than plasma osmolality and urine sodium should be greater than 25 mEq/L.

Individuals with hyperkalemia or high potassium intake and normal renal function should excrete potassium into the urine resulting in a TTKG above 10. Values below 7 are consistent with mineralcorticoid deficiency, especially if accompanied by hyponatremia and high urine sodium concentration.

Individuals with hypokalemia should have TTKG values below 2. Higher values are consistent with inappropriate stimulation of potassium secretion. 

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