FENa Calculator (Fractional Excretion of Sodium)

FENa is the percentage of the sodium filtered by the kidney, which is excreted in the urine. This value is widely used to help differentiate pre-renal disease (decreased renal perfusion) from acute tubular necrosis (ATN) as the cause of acute kidney injury (AKI).

There are multiple limitations to the use of FENa, including:

• FENa is only useful in acute renal failure, not chronic renal failure.
• FENa cannot be used in patients on diuretics.

If you want to calculate your FENa, you need to know the result of your blood and urine tests:

• Creatinine — First of all, you need to know your creatinine level in mg/dL. This is an indicator of kidney malfunction that is also used by our MELD calculator. Normal levels of creatinine in blood are approximately 0.6 to 1.2 mg/dL in adult males and 0.5 to 1.1 mg/dL in adult females.
• Sodium — The serum sodium test, also referred to as Na+ test, tells you the level of sodium in your blood.
• Urine creatinine — Your doctor may use a random urine sample or a urine 24-hour volume test to test for creatinine. As the name suggests, this creatinine urine test measures the amount of urine produced in a day.
• Urine sodium — This is the amount of sodium in all urine collected during 24 hours. The reference range for urine sodium is 100-260 mmol/L per 24 hours.

The equation used by our FENa calculator is:

Fractional Excretion of Sodium (FENa) = (PCr * UNa) / (PNa * UCr)

where:

• PCr — Plasma creatinine;
• PNa — Plasma sodium;
• UCr — Urine creatinine; and
• UNa — Urine sodium.

You can find the same number using a different method — multiplying the plasma sodium concentration by the glomerular filtration rate (to be found in the GFR calculator).

• Prerenal cause: FENa <1%
• Intrinsic Renal cause: FENa 1-4%
• Postrenal cause: FENa >4%

Prerenal acute kidney diseases are those that decrease effective blood flow to the kidney and cause a decrease in the GFR (glomerular filtration rate). They include hypovolemia, CHF, Renal artery stenosis, sepsis, etc.

Intrinsic AKI refers to disease processes which directly damage the kidney itself. These include ATN, AIN, Glomerulonephritides and other.

Postrenal acute kidney diseases most often occur as a consequence of urinary tract obstruction — conditions such as BPH, bladder stone or bilateral ureter obstruction.