Fractional Excretion of Potassium
Fractional excretion of sodium (FEK+)
100 * ((Urinary K+) x (Serum creatinine))
((Serum K+) x (Urinary creatinine))
Urinary potassium (Urine k):
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
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
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.
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
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
Porush JG. New concepts in acute renal failure. Am Fam Physician 1986