Fractional Excretion of Potassium (FEK) Calculator
Fractional Excretion of Potassium (FEK) Calculator
In hypokalaemia, separate extrarenal potassium loss from inappropriate renal potassium wasting.
Fractional Excretion of Potassium (FEK)
K clearance ratioUrine K 40 mmol/L, serum K 3.1 mmol/L, urine creatinine 90 mg/dL, serum creatinine 2.4 mg/dL
Formula
- urine K, serum K
- mmol/L, from a single paired sample
- urine creatinine, serum creatinine
- mg/dL
- interpretation
- only meaningful alongside a known serum potassium — this index characterises the renal response to hypokalaemia, it does not diagnose it
Worked example
Urine K 40 mmol/L, serum K 3.1 mmol/L, urine creatinine 90 mg/dL, serum creatinine 2.4 mg/dL
40 × 2.4 = 96
3.1 × 90 = 279
96 ÷ 279 × 100 = 34.4%
With hypokalaemia (3.1 mmol/L) this is well above 6.5% → renal potassium wasting
Interpretation in hypokalaemia
| FEK | Pattern | Consider |
|---|---|---|
| < 6.5% | Extrarenal loss | Diarrhoea, laxative use, poor intake, transcellular shift |
| 6.5 – 20% | Expected range | Less discriminating; correlate with the clinical picture |
| > 20% | Renal potassium wasting | Diuretics, hyperaldosteronism, Bartter or Gitelman syndrome |
Reading FEK alongside serum potassium
The fractional excretion of potassium characterises how the kidney is responding to a given serum potassium, so it only makes sense read alongside that value — a mid-range FEK means something different in a hypokalaemic patient than in a normokalaemic one. In hypokalaemia the physiologically appropriate response is to conserve potassium, so a low FEK is expected and reassuring, pointing towards a non-renal explanation for the low level.
When FEK is inappropriately high in a hypokalaemic patient, the kidney itself is the problem. Loop and thiazide diuretics are the commonest cause in practice, followed by primary or secondary hyperaldosteronism and inherited tubulopathies such as Bartter and Gitelman syndrome, which mimic the effect of diuretics without one being taken.
FEK is used less often at the bedside than FENa or FEUrea; the transtubular potassium gradient and a timed urine potassium collection are more established alternatives in many renal units, though FEK avoids the assumptions the transtubular gradient makes about medullary osmolality. As with the sodium and urea indices, a single paired sample is enough to calculate it, but the result should always be read against the serum potassium that prompted the test.
Frequently asked questions
What FEK suggests extrarenal potassium loss?
In a hypokalaemic patient, a FEK below about 6.5% suggests the kidney is appropriately conserving potassium and the loss is extrarenal — most often gastrointestinal.
What FEK suggests renal potassium wasting?
In a hypokalaemic patient, a FEK above roughly 6.5%, and particularly above 20%, indicates the kidney is inappropriately wasting potassium — think diuretics, hyperaldosteronism or a tubulopathy.
Does FEK need to be interpreted with serum potassium?
Yes. The index reflects the renal response to a given potassium level, so the same FEK value means something different depending on whether serum potassium is low, normal or high.
Is FEK the standard test for hypokalaemia work-up?
It is one option. Many renal units prefer the transtubular potassium gradient or a timed urine potassium collection, though FEK avoids some of the gradient’s assumptions about urine concentration.
Related calculators
References
- Lin SH, Lin YF, Chen DT et al. Laboratory tests to determine the cause of hypokalemia and paralysis. Arch Intern Med. 2004;164(14):1561–6.
- Kamel KS, Halperin ML. Intrarenal urea recycling and the urine concentrating mechanism. J Am Soc Nephrol. 2011;22(4):605–7.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.