Фракционная экскреция калия (FEK) (венозная кровь, суточная моча)\FEK, Fractional Excretion of Potassium (blood and urine)

Potassium, 24‑Hour Urine (urine potassium; K+, 24‑hour urine) measures daily urinary potassium excretion to assess overall potassium balance and fluid–electrolyte status. The test assists in distinguishing renal from extrarenal potassium losses, supports evaluation of kidney and adrenal cortical function, and helps monitor the effect of potassium supplementation or therapies that influence renal electrolyte handling.

Potassium is the principal intracellular cation, while sodium predominates in the extracellular space. The distribution of these ions maintains osmotic pressure, intravascular volume, transmembrane transport, and neuromuscular excitability. Whole‑body potassium homeostasis depends on gastrointestinal and renal reabsorption and secretion, as well as active and passive transport across cell membranes. In the kidney, potassium is secreted in the distal nephron and collecting ducts in exchange for sodium and is reabsorbed primarily in the proximal tubule; proximal tubular injury can therefore deplete body potassium stores. Renal potassium loss is influenced by distal sodium delivery, the hydrogen ion concentration in the distal tubule, and circulating aldosterone. Urinary potassium excretion increases during alkalosis, typically accompanied by a fall in serum potassium. Excessive renal potassium loss may reflect enhanced distal sodium–potassium exchange under the effect of mineralocorticoids or glucocorticoids, inhibition of the sodium pump, reduced tubular reabsorption from tubular dysfunction, or Fanconi syndrome associated with hypercalcemia. Low serum potassium may coexist with high urinary potassium excretion, whereas elevated serum potassium often accompanies reduced urinary excretion. For differential diagnosis, daily urinary potassium greater than 30 mmol/day (or >15 mmol/L) indicates a renal or adrenal cause of loss, while values less than 25 mmol/day (or <15 mmol/L) favor extrarenal losses such as diarrhea, villous adenoma, or laxative overuse. Results should be interpreted alongside the clinical context and concurrent measurements of serum potassium and other key electrolytes.