Fractional Excretion of Sodium (FENa)

Acute renal failure (also known as acute kidney injury), a condition characterized by a rapid deterioration in kidney function, is a medical emergency. In a person who has acute renal failure, rapidly determining its cause is the key to effective treatment.

Whenever they are attempting to quickly evaluate the underlying cause in a patient with acute renal failure, doctors usually measure the fractional excretion of sodium (FENa). FENa is a quick method to help them evaluate the general type of problem that is producing acute renal failure.

Causes of Acute Renal Failure

The causes of acute renal failure can be divided up into three general categories: Prerenal disease, intrinsic renal disease, and postrenal disease.

In prerenal disease, renal failure is caused by a major reduction in blood flow to the kidneys. While the kidneys themselves may be entirely normal (at least at first), because of the drop in blood flow the kidneys are no longer able to filter toxins from the blood effectively. As a result, urine volume decreases and toxic substances accumulate in the blood.

Acute renal failure caused by prerenal disease can result from several conditions. One cause is depletion in blood volume due to dehydration, hemorrhage, vomiting, or diarrhea. Other causes of prerenal disease include congestive heart failure and cirrhosis of the liver. 

The effective treatment of prerenal kidney failure requires reversing or improving the underlying cause, thereby restoring blood flow to the kidneys.

Intrinsic renal disease, that is, disease directly affecting the kidneys themselves, can also produce acute renal failure. The intrinsic disorder that most commonly causes acute renal failure is a condition called acute tubular necrosis (ATN). ATN occurs when the epithelial cells lining the tubules of the kidneys become damaged. This damage may result from a dramatic drop in blood flow to the kidneys (even if it is a very transient drop), sepsis, or various toxic substances (including several antibiotics, cisplatin, contrast media used during X-ray procedures, mannitol, heme pigments that can accumulate in the blood with hemolytic anemias, and synthetic cannabinoids .

Other types of intrinsic renal diseases that can produce acute renal failure include acute glomerulonephritis (a type of disorder that causes inflammation of the glomeruli of the kidneys), vasculitis, acute interstitial nephritis, or renal emboli (blood clots that lodge in the kidneys). 

While all of these disorders must be considered in a person with acute renal failure due to intrinsic renal disease, ATN remains by far the most common intrinsic cause of acute renal failure.

Postrenal disorders can produce acute renal failure by obstructing the flow of urine that has been produced by the kidneys. This obstruction can occur due to blockage in both ureters, the bladder, or the urethra, and can result from kidney stones, tumors, hemorrhage, or trauma. Postrenal conditions are responsible for acute renal failure in fewer than 10% of cases, and because these conditions are usually accompanied by severe pain or discomfort as well as greatly diminished urine flow, they are not usually difficult to diagnose.

How Can Measuring FENa Help?

It should be plain from this discussion that, in most instances, diagnosing the cause of acute renal failure requires the doctor to differentiate between prerenal disease and ATN. 

The FENa calculation is often most helpful in making this differentiation. 

The FENa calculation estimates the percentage of sodium filtered by the kidneys that ends up being excreted into the urine. (The acronym FENa derives from “fractional excretion” and “Na." Na is the chemical symbol for sodium.) 

Sodium is an electrolyte that is critical to all the cells in the body, and maintaining a normal concentration of sodium in all bodily fluids is vital to life. The kidneys play an extremely important role in maintaining normal sodium balance.

As the kidneys filter the blood, a large amount of sodium enters the renal tubules. This allows the kidneys to excrete large amounts of sodium in conditions where that is necessary in order to maintain sodium balance. However, in most conditions only relatively small amounts of sodium need to be excreted into the urine, so the renal tubules reabsorb most of the filtered sodium back into the bloodstream. The reabsorption of sodium is one of the most important jobs of the renal tubules.

In people without kidney disease, typically only 1% to 2 % of the sodium filtered by their kidneys ends up being excreted in the urine; the rest is reabsorbed by the renal tubules. 

In a person with acute renal failure caused by prerenal disorders, typically less than 1% of the filtered sodium is excreted. This is because the volume of blood filtered by the kidneys is greatly diminished, so the renal tubules (which are functionally normal) are able to reabsorb a very large proportion of the sodium that is presented to them.

In contrast, in a person whose acute renal failure is caused by ATN, a disorder of the renal tubules, more than 2% of the filtered sodium is typically excreted. This excess excretion of sodium occurs because the renal tubules themselves are damaged in ATN, and are unable to reabsorb sodium efficiently. In fact, the excessive loss of sodium, which leads to a decrease in blood volume and other serious problems, is itself one of the clinical issues that must be addressed in a person experiencing ATN.

Measuring FENa (an estimate of the amount of filtered sodium that is excreted in the urine) can give an important clue as to the type of problem (either prerenal or tubular necrosis) that is responsible for acute renal failure.

How Is FENa Measured?

FENa, simply, is the quantity of sodium excreted into the urine, divided by the quantity of sodium filtered by the kidneys, times 100. 

It turns out that this ratio can be accurately estimated by dividing the product of the urinary sodium times the serum creatinine, by the product of serum sodium times the urinary creatinine.

Here is an on-line FENa calculator, provided by Cornell University, that uses these four measurements to give the FENa percentage: Cornell FENa calculator.

When Is Measuring FENa Useful?

Any time a doctor is evaluating a patient with acute renal failure, and (as is usually the case) the problem comes down to distinguishing between prerenal disease and acute tubular necrosis, the FENa calculation can be very helpful in differentiating between the two.

A FENa of less than 1% strongly suggests prerenal disease. A FENa above 2% strongly suggests ATN. A FENa between 1% and 2% could be either disorder. With the results of the FENa calculation in hand, the doctor often has a very good idea as to the cause of acute renal failure.

Limitations of the FENa Calculation

There are several limitations to the FENa calculation. 

There may be an overlap between prerenal and intrinsic renal failure, especially in cases where the condition producing prerenal disease becomes severe enough to produce a significant drop in blood pressure. If the drop in blood pressure is severe enough, it can produce damage to the renal tubules. In such cases, both prerenal disease and ATN may be present, making the FENa result difficult to interpret.

In addition, urinary sodium levels can become quite variable from hour to hour with acute renal failure, especially during the early stages of the disorder. So a single measurement of FENa can give a misleading answer. This limitation can usually be circumvented by measuring FENa several times over a period of several hours, until the measurement stabilizes.

In people with chronic underlying kidney disease (such as chronic glomerulonephritis), a superimposed acute prerenal condition can result in an elevated value of FENa, which can mislead doctors to think that ATN has occurred. So interpreting FENa in the setting of chronic kidney disease must be done with caution.

Finally, it is not possible to reliably interpret the FENa measurement in people taking diuretic therapy, which increases urinary sodium levels.

Nonetheless, as long as the doctor keeps these limitations in mind, the FENa calculation can be quite helpful in determining the type of condition that is producing acute renal failure, and thus, can be helpful in steering the medical team to the most appropriate type of treatment.