How to Calculate Fractional Excretion of Potassium (FEK)
The fractional excretion of potassium (FEK) is calculated using the formula: FEK (%) = (Urine K+ × Serum Creatinine) / (Serum K+ × Urine Creatinine) × 100.
The Formula and Required Values
To calculate FEK, you need four laboratory values from simultaneous blood and spot urine samples:
- Urine potassium (UK+) in mEq/L or mmol/L
- Serum potassium (SK+) in mEq/L or mmol/L
- Urine creatinine (UCr) in mg/dL or mmol/L
- Serum creatinine (SCr) in mg/dL or mmol/L
FEK (%) = (UK+ / SK+) / (UCr / SCr) × 100
Or equivalently: FEK (%) = (UK+ × SCr) / (SK+ × UCr) × 100
Interpretation in Hypokalemia
In patients with persistent hypokalemia, an FEK >9-10% indicates inappropriate renal potassium wasting, while FEK <6-7% suggests extrarenal losses 1, 2.
Specific Cutoff Values:
- FEK <2.8-6.4%: Suggests hypokalemia from extrarenal losses (GI losses, inadequate intake, transcellular shifts) 1
- FEK >9.3%: Indicates renal potassium wasting with 80.6% sensitivity and 85.7% specificity 2
- FEK 9.5-24%: Strongly suggests renal tubular disorders, diuretic use, or mineralocorticoid excess as the cause 1
Clinical Context and Pitfalls
Important Considerations:
Recent diuretic use can falsely elevate FEK, making interpretation difficult. The test is most reliable when obtained before initiating potassium supplementation or after discontinuing diuretics for at least 24-48 hours.
In normal subjects, mean FEK is approximately 8% (range 4-16%) 1. FEK correlates positively with serum potassium levels (r = 0.74) and inversely with serum creatinine 1.
Alternative Markers:
If you cannot calculate FEK immediately, the urine potassium-to-creatinine ratio (UK/UCr) from a spot urine sample provides similar diagnostic information and correlates strongly with FEK (r = 0.831) 2. A UK/UCr ratio can be rapidly obtained and interpreted alongside FEK.
Practical Application Algorithm
- Obtain simultaneous samples: Collect spot urine and serum samples at the same time, ideally before treatment initiation
- Calculate FEK using the formula above
- Interpret based on clinical context:
- FEK <7% + low urine K/Cr ratio → Extrarenal losses (vomiting, diarrhea, poor intake)
- FEK >9-10% + normal/high urine K/Cr ratio → Renal wasting (diuretics, tubular disorders, hyperaldosteronism)
- Consider confounders: Recent diuretics, acid-base status, and volume status all affect interpretation
In patients receiving potassium supplementation, FEK remains the most reliable spot urine parameter for distinguishing renal from extrarenal losses 2, making it superior to isolated urine potassium measurements or the transtubular potassium gradient (TTKG) in this setting.