The fractional excretion of sodium or FENa refers to the percentage of sodium filtered by the kidney which gets excreted in the urine. You measure this value in terms of urine sodium and plasma. A FENa calculator becomes a useful tool in the assessment of actual kidney failure.
How to use the FENa calculator?
Although computing the FENa value using the FENa formula or the FENa equation can be quite confusing, you can utilize this FENa calculator to perform the calculation for you. This online tool is easy to understand and using it requires only a few steps.
Before using this FENa calculator, you must have specific values which you can acquire from laboratory testing. Here are the steps to follow for this calculator:
- First, enter the value of the Creatinine.
- Then enter the value of the Sodium.
- Next up, enter the value of the Urine Creatinine.
- Finally, enter the value of the Urine Sodium.
- After entering all of these values, the calculator automatically performs the calculation for you to give you the FENa value which is a percentage value.
What does FENa mean?
As stated earlier, FENa refers to the percentage of sodium the kidney filters which eventually gets excreted through the urine. The information derived from FENa is commonly used in differentiating pre-renal disease from ATN or acute tubular necrosis which causes acute kidney injury.
By the way, ATN is a kind of medical condition involving the death of tubular epithelial cells that form the renal tubules of the kidneys. Keep in mind though, that using FENa has multiple limitations:
- You can only use FENa for acute renal failure but not for chronic renal failure.
- You cannot use FENa for patients on diuretic medications. This refers to those who take drugs to increase the passage of urine.
How do you calculate FENa?
To successfully find the value of FENa, you have to know first the results of your urine and blood examinations. Therefore, you must first have a laboratory test performed before using the FENa calculator. Specifically, you must have the following tested:
A laboratory test helps you determine your level of creatinine in mg/dL. The reading indicates if you have some type of kidney condition. You can also use this value for a Model for End-Stage Liver Disease or MELD calculator.
Those who have normal creatinine levels in their blood should have readings approximately from 0.5 to 1.1 mg/dL for females and 0.6 to 1.2 ml/dL for males.
A test for serum sodium which is also known as Na+ test shows you the amount of sodium circulating in the blood. You also need this value for the FENa calculator.
- Urine Creatinine
To determine the creatinine levels in your urine, the doctor may utilize either a 24-hour urine volume test or a random urine sample. The former test, as the name implies, measures the creatinine in your urine that you produce in one day.
- Urine Sodium
This test measures how much sodium your collected urine in one day contains. The normal range of urine sodium for the duration should be 40-220 mmol/liter.
The FENa calculator uses the FENa formula which you can also use for manual calculations. The FENa equation is:
FENa = (PCr * UNa) / (PNa * UCr)
PCr refers to the plasma creatinine
PNa refers to the plasma sodium
UCR refers to the urine creatinine
UNa refers to the urine sodium.
You can also come up with the same results using another method. For this method, you multiply the concentration of plasma sodium by the glomeric filtration rate.
Apart from performing the calculation, you may also want to learn more about how to interpret the result you get from the calculator. Here are the basic interpretations:
- Pre-renal cause when the value of FENa <1%
Pre-renal acute kidney diseases refer to those responsible for the decrease of sufficient flow of blood to the kidneys which causes a reduction in the glomeric filtration rate. These diseases include hypovolemia, sepsis, Renal Artery Stenosis, CHF, and more.
- Intrinsic renal cause when the value of FENa 1-4%
Intrinsic diseases are those which cause damage directly to the kidney. Such diseases include AIN, ATN, Glomerulonephritides and others.
- Post-renal cause when the value of FENa >4%
Post-renal acute diseases refer to those which occur because of an obstruction of the urinary tract. These include bladder stones or bilateral ureter obstruction.
What is normal fractional excretion of sodium?
For normal people, the fractional excretion of sodium is typically lower than 1% but this may vary as it can get elevated when you increase your intake of sodium. For patients who suffer from azotemia, a low value of the fractional excretion of sodium is usually an indication of a pre-renal process that’s volume-repletion responsive.
But a low FENa value with azotemia can also happen due to cardiac or hepatic failure, pigment nephropathy, acute glomerulonephritis, contrast nephrotoxicity or polyuric renal failure, renal transplant rejection, acute obstruction, and on occasion, non-oliguric acute renal failure.
A FENa result of more than 1% in severe azotemic patients is usually an indication of intrinsic renal injury. However, it’s consistent in volume depletion for patients who take diuretic medication or in those who have chronic renal insufficiencies.
In the same fashion, a low value in an acute renal parenchymal injury is generally interpreted as an indication of extensive tubular integrity although it’s still consistent with varying pathophysiologic processes. You should always base the interpretation of FENa results in specific clinical settings and other laboratory information used the management of patients.
When it comes to FENa, here are some points worth considering:
- Not all FENa percentages always accurately suggest “pre-renal disease as there are more factors to consider before drawing a conclusion. These factors include clinical context, history, current medications, and physical examination.
- Repeated FENa or urine studies through a patient’s hospital stay can yield more clinical clues.
- Non-volume depleted states with low urine sodium and a low FENa include cardiorenal syndrome, acute glomerulonephritis, contrast-related nephropathy, hepatorenal syndrome, and rarely, acute obstruction along with early acute interstitial nephritis or acute tubular necrosis.