Potassium is excreted primarily by the kidneys; 90% to 95% of ingested potassium is excreted in the urine.10,11 Potassium is filtered by the glomerulus, with 70% reabsorbed in the proximal tubule, an additional 10% to 20% reabsorbed
in the ascending loop of Henle, and a final 10% to 20% delivered to the distal tubule for final determination of urinary potassium
Serum potassium concentration can vary depending on whether the kidney failure is acute or chronic. In patients with acute
oliguric or anuric kidney failure, the potassium concentration is typically elevated because of decreased excretion and may
occur for several reasons. First, there is insufficient time for the kidneys to adapt to nephron loss, which occurs in CKD
but not in acute kidney disease. Second, effective urinary excretion of potassium is altered because of decreased distal tubule
blood flow in states of severely decreased GFR. Third, metabolic acidosis and release of potassium from tissues during a catabolic
state also contribute to hyperkalemia.
In cats with CKD, hypokalemia is attributed to chronic potassium wasting, whereas most dogs with CKD have normal potassium
concentrations. Determining the fractional excretion of potassium (FEK+ ) may help differentiate renal and nonrenal sources of potassium loss.
This index relates the amount of potassium excreted to the amount filtered.11 Because urine potassium and creatinine concentrations are typically much higher than serum concentrations, these values
are usually determined by sending samples to a commercial laboratory instead of by using cage-side analyzers. Random measurement
of the urinary potassium concentration is simple to perform but may be less accurate than measurement from a 24-hour urine
collection, since it is influenced by two independent factors: potassium secretion and water reabsorption in the medulla.
The FEK+ should be < 4% for nonrenal sources of loss. Values > 4% may indicate inappropriate renal loss.10,11