Fractional Excretion of Potassium (FEK)
Fractional excretion of potassium (FEK) = (urine K x plasma creatinine) / (plasma K x urine creatinine) x 100.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Urine potassium
- 40 mEq/L
- Plasma/serum potassium
- 4.0 mEq/L
- Urine creatinine
- 100 mg/dL
- Plasma/serum creatinine
- 1.0 mg/dL
Result: FEK 10% — read alongside the serum potassium (interpretation is context-dependent).
The tool opens with these values already filled in. Replace them with your own.
What the result means
- Hypokalemia
- < ~10% suggests extrarenal loss; > ~20% suggests renal potassium wasting.
- Hyperkalemia
- A low FEK (< ~10%) with preserved renal function suggests impaired renal excretion.
What you enter
- Urine potassium (mEq/L)
- Plasma/serum potassium (mEq/L)
- Urine creatinine (mg/dL)
- Plasma/serum creatinine (mg/dL)
How this is calculated
- Formula
- FEK % = (urine K x plasma creatinine) / (plasma K x urine creatinine) x 100. Units cancel. Typical diet averages ~8% (4-16%).
Lin SH, Lin YF, Chen DT, et al. Laboratory tests to determine the cause of hypokalemia and paralysis. Arch Intern Med. 2004;164(14):1561-1566. (FEK; and standard renal physiology references.) Read the source ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.