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Fractional Excretion of Potassium



Fractional excretion of sodium (FEK+) =

100 * ((Urinary K+) x  (Serum creatinine)) 
   ((Serum K+)  x  (Urinary creatinine))


Urinary potassium (Urinek):   

Serum Creatinine: 

Serum potassium:   

Urinary Creatinine: 

Background Info

Fractional excretion

The fractional excretion is a process which measures the percent of filtered salts that are excreted in the urine. In simpler words, the definition of fractional excretion would be “Fractional excretion of sodium is the amount of salt that leaves the body through urine compared to the amount filtered and reabsorbed by the kidney”. This calculation is widely used to help differentiate pre-renal disease such as, the decreased renal perfusion from acute tubular necrosis or used to find the causes of acute kidney injuries. Fractional extraction of sodium is widely used for diagnosis and prognosis of epidemics but presently, fractional excretion of Potassium is also determined.
Check out our urinary indices calculator.

Fractional excretion of Potassium (FEK)

The measurement of urinary potassium fractional excretion is very helpful in the differential diagnosis of both hypokalemia and hyperkalemia, which are the most common electrolyte abnormalities in hospitalized patients. These are usually defined as serum potassium of less than 3.5 mmol/l (hypokalemia) and serum potassium greater than 5.0 mmol/l (hyperkalemia), and may cause many generalized weaknesses and ultimately can lead to cardiac arrest. So FEK is applied to diagnose these health concerns by usually urinary samples, since the kidneys are mainly responsible for Potassium homeostasis.

Methodology of FEK

To determine urinary potassium excretion it is preferable to collect a 24-hour urine sample. However, this is not feasible in many cases. Random measurement of the urinary potassium concentration is simple to perform but may be less accurate than a 24-hour collection, since it is influenced by two independent factors: K+ secretion and water reabsorption in the medulla. Therefore, the fractional excretion of potassium (FEK+) has been proposed as a useful diagnostic tool.

Significance of FEK

The fractional excretion of potassium (FEK) is directly related to the severity, intensity and duration of acute kidney problems and injuries. The increase of potassium in urine as acute kidney injury gorws is possibly a result of potential decrease in the glomerular filtration rate and the aldosterone activation which is an attempt to maintain homeostasis of potassium ions (K+). It is noticed to be more useful than typically measured fractional excretions of sodium and urea. The primary data available on fractional excretion of potassium indicate that the increases in its normal value may be a sign of a decrease in the glomerular filtration rate, even before a rise in serum “Creatinine”. This is an intriguing line of research that deserves prompt additional studies.

Studies have shown that the FEK value in hypokalemic patients of extra-renal origin generally ranges from 1.5 to 6.4%. Whereas, FEK in patients with hypokalemia of renal origin ranges from 9.5 to 24% and in patients of hypokalaemia, value of FEK is noted to be more than 6.5% and indicates inappropriate kaliuresis.


Patients with hyperkalemia:

FEK less than 10 percent indicates renal origin
FEK greater than 10 percent indicates extrarenal

Patients with hypokalemia:

FEK greater than ~ 9 - underlying cause is of renal origin.
FEK less than ~ 6.5 to 9 suggests extrarenal origin.

Additional diagnostic information regarding urinary potassium loss can be evaluated using our Transtubular Potassium Gradient (TTKG) calculator.
Transtubular potassium gradient = (urine k/serum k)/(urine osm/serum osm))

Fractional excretion of Potassium (FEK) is a useful diagnostic tool that may aid in the differential diagnosis of potassium inbalances.


  1. Carvounis CP, Nisar S, Guro-Razuman S. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int. 2002;62(6):2223-9.
    Durakovic Z, Durakovic A, Durakovic S. The lack of clinical value of laboratory parameters in predicting outcome in acute renal failure. Ren Fail 1989-90;11(4):213-9.

  2. M Elisaf, KC Siamopoulos. Fractional excretion of potassium in normal subjects and in patients with hypokalaemia. Postgrad MedJ 1995; 71: 211-212 C) The Fellowship of Postgraduate Medicine, 1995
  3. Porush JG. New concepts in acute renal failure. Am Fam Physician 1986 Mar;33(3):109-18.
Fractional Excretion of Potassium