What Is Fractional Excretion and How Does Measuring It Help in Clinical Practice?



What Is Fractional Excretion and How Does Measuring It Help in Clinical Practice?





DERIVATION OF THE FORMULA

Fractional excretion is a term that nephrologists frequently use in clinical practice (Schrier, 1988a, b). What exactly does it mean? Forgetting the formulae for now, fractional excretion of a substance is the amount of that substance that is excreted into the urine relative to the amount of that substance that is filtered by the kidney.

Fractional excretion = Amount excreted/Amount filtered

Since fractional excretion is generally expressed as a percentage and not a fraction, it can be defined simply as the percentage of the filtered substance that is excreted into the urine.

Now we will derive the formula. For example, take fractional excretion of sodium (FENa). The amount of sodium excreted is the product of the urine sodium concentration and the urine flow rate, that is, UNa × V. Since sodium is freely filterable, the amount of sodium filtered is the product of the plasma sodium concentration and the glomerular filtration rate (GFR), that is, PNa × GFR. Therefore,

FENa = (UNa × V)/(PNa × GFR)

Substituting creatinine clearance (UCr × V/PCr) for GFR (a pretty good approximation) results in as follows:

FENa = (UNa × V)/[PNa × (UCr × V/PCr0]


The V terms cancel out, resulting in the following equation:

FENa = UNa/(PNa × UCr/PCr)

Multiplying both numerator and denominator by PCr/UCr results in the most commonly used formula:

FENa = (UNa/PNa) × (PCr/UCr) × 100

Do not forget to multiply by 100, to convert from a fraction to a percentage!


WHAT IS THE FRACTIONAL EXCRETION OF SODIUM IN HEALTHY PEOPLE?

Most healthy people on standard American diets have a FENa of approximately equal to 1%. Let us see why.

A typical American diet contains about 6 g of sodium daily. How many millimoles (mmol) or milliequivalents (mEq) is 6 g? First of all, since sodium is univalent (has a single charge, in this case, positive), millimoles = milliequivalents. For the remainder of this discussion, we are going to use millimoles, since milliequivalents cannot be used for nonionic (uncharged) analytes. However, in other chapters, we will use milliequivalents sometimes and millimoles at other times for ion concentrations.

The molecular weight of sodium is 23. Therefore, 1 mmol = 23 mg. (Remember this mantra: A millimole is the molecular weight in milligrams.)

Thus, 6 g or 6,000 mg sodium = 6,000 mg/(23 mg/mmol) = 261 mmol

In the steady state, what is ingested must be excreted. Most of the excretion will be in the urine. For the sake of simplicity, let us assume other losses (stool, sweat) are minimal and that 250 mmol/day of sodium appears in the urine, and that there is 2 L/day of urine excretion. The urine sodium concentration is thus 125 mmol/L.

Remember that daily urine sodium excretion (in this case, 250 mmol) is urine concentration (125 mmol/L) × urine flow rate (2 L/day), and thus the numerator of the FENa formula. Now, how about the denominator of the formula? The normal creatinine clearance is about 180 L/day (125 mL/min). This means that 180 L of plasma is completely cleared of creatinine by the kidneys in 1 day. The normal plasma sodium concentration is 140 mmol/L. Therefore, the amount of sodium filtered equals 180 L/day × 140 mmol/L = 25,200 mmol/day.

FENa = (250 mmol/25,200 mmol) × 100 ≈ 1%



FRACTIONAL EXCRETION OF SODIUM IN PATHOPHYSIOLOGIC STATES


Chronic Kidney Disease

Many patients with chronic kidney disease (CKD) are unaware of their condition and are referred to a nephrologist when the GFR is found to be low in the absence of specific symptoms and signs.



PATIENT 1

A 50-year-old man with moderately severe CKD (estimated GFR by creatinine-based formula of 25 mL/min/1.73 m2) is seen in clinic. His physical examination is normal and he has no peripheral edema.

Q: What is the expected FENa?

A: As stated earlier, the typical American diet contains about 6 g of sodium per day. If a patient with CKD continues to ingest a typical diet (a good assumption for many patients!) and has not developed progressive edema, this means that the daily amount of sodium excreted still approximates the daily dietary intake of sodium. Sodium excreted thus has not changed; however, the amount of filtered sodium must decrease proportional to the GFR. In this instance, if the GFR is 25% of normal, that is, 45 L/day rather than 180 L/day, the amount of sodium filtered will be 45 L/day × 140 mmol/L = 6,300 mmol/day.

FENa = (250 mmol/63,000 mmol) × 100 ≈ 4%

One can see that, assuming no change in dietary sodium intake, the FENa will double for every halving of GFR. With very severe CKD (e.g., GFR 10% of normal), FENa may be as high as 10%. Of course, hopefully such a patient has learned to restrict his or her sodium intake so that it will not actually be that high. Diseased kidneys usually have an impaired ability to excrete ingested sodium, so the development of sodium overload is common as the GFR declines unless the patient restricts sodium intake.

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Sep 7, 2016 | Posted by in NEPHROLOGY | Comments Off on What Is Fractional Excretion and How Does Measuring It Help in Clinical Practice?

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