Calculates the percentage of filtered potassium excreted in urine from paired serum and urine potassium and creatinine values. This premium guide explains physiology, calculation, interpretation, limitations and evidence.
Overview
Calculates the percentage of filtered potassium excreted in urine from paired serum and urine potassium and creatinine values.
Clinical Significance
An adjunctive index of renal potassium handling influenced by kidney function, distal sodium delivery, diuretics and acid–base status.
When to Use
- Evaluate renal potassium handling in hypo- or hyperkalaemia.
- Use paired samples obtained close together.
- Interpret with urine potassium, kidney function and medicines.
How It Is Calculated
- FEK = (urine K × plasma creatinine) ÷ (plasma K × urine creatinine) × 100, with creatinine in matching units.
- MedicalC validates required values and rounds only the displayed result.
Interpretation
- An adjunctive index of renal potassium handling influenced by kidney function, distal sodium delivery, diuretics and acid–base status.
- Review source measurements, collection quality and clinical trajectory as well as the final value.
Worked Example
Enter clinically plausible values in the displayed units. MedicalC applies: FEK = (urine K × plasma creatinine) ÷ (plasma K × urine creatinine) × 100, with creatinine in matching units. Verify the result against the source data before use.
Patient Considerations
- Consider kidney function, diuretics, intravenous fluids, nutrition, body composition and rapidly changing physiology.
- For timed urine calculations, confirm collection duration, volume and completeness.
- For electrolyte prescriptions, include all oral, enteral, intravenous and medication sources.
Limitations
- No universal cut-off applies in every setting.
- Diuretics and changing potassium balance reduce reliability.
- A spot sample may not represent daily excretion.
Clinical Pearls
- Use paired serum and urine values obtained as close together as possible when the formula requires them.
- A derived renal index is only as reliable as the collection and assays used.
- Check whether treatment occurred between sampling and interpretation.
Common Mistakes
- Mixing µmol/L and mmol/L creatinine values.
- Using spot concentrations as daily excretion rates.
- Ignoring incomplete timed urine collection.
- Treating a maintenance estimate as a replacement or emergency-correction regimen.
- Using a derived acid–base value without checking albumin, lactate and the blood gas.
Evidence Base
Principal source: Kamel KS, Halperin ML. Use of urine electrolytes and urine osmolality in clinical diagnosis. Kidney Int Rep. 2021.
Frequently Asked Questions
What does the Fractional Excretion of Potassium calculator do?
Calculates the percentage of filtered potassium excreted in urine from paired serum and urine potassium and creatinine values.
Can this result establish a diagnosis?
No. It is an adjunct that must be combined with clinical assessment and current guidance.
Why are timing and units important?
Derived results can be misleading when values are non-contemporaneous or use incompatible units.
When should the calculation be repeated?
Repeat when treatment, kidney function or the relevant measured values change and a new result could influence care.
Does this replace a specialist protocol?
No. Direct measurement, laboratory interpretation and specialist protocols take priority.
References
- Kamel KS, Halperin ML. Use of urine electrolytes and urine osmolality in clinical diagnosis. Kidney Int Rep. 2021. — Primary publication, guideline or authoritative specialty source.
Reviewed by: MedicalC Clinical Editorial Team
Last reviewed: July 2026