Prerenal Acute Renal Failure with High FENa (Fractional Excretion of Sodium)

Author: V. Dimov, M.D., Department of Hospital Medicine, Cleveland Clinic

A 58-year-old African American male who is a nursing home resident due to advanced Parkinson's disease is admitted to the hospital with chief complaint of generalized weakness. His laboratory results show acute renal failure (ARF). The patient has had poor PO intake of fluids and food for the last 3-4 days.

PMH:
HTN, Parkinson's disease, hypothyroidism

Medications:
Carbidopa-Levodopa (Sinemet), levothyroxine (Synthroid), HCTZ

Physical examination:
VSS
HEENT: Dry MM
Chest: CTA (B)
CVS: Clear S1S2
Abdomen: Soft, NT, ND, +BS
Extremities: no c/c/e, cogwheel rigidity and spasticity

Laboratory results:
BMP shows acute renal failure. BUN/Cr were normal several months ago.


BUN/Cr in prerenal ARF


Creatinine improvement with IV hydration in prerenal ARF

What is the most likely diagnosis?
Prerenal ARF due to dehydration

How to confirm the diagnosis?
UA
Urinary sodium and creatinine to calculate the fractional excretion of sodium (FENA)

What other tests would you order?
BMP in 6 and 24 hours
Renal ultrasound to rule out urinary obstruction and nephrolithiasis

What treatment would you start for this patient?
Foley catheter
Strict I/O
NS at 150 cc/hr x 2 L, then 125 cc/hr, adjust the rate of IVF according to I/O, avoid fluid overload.

What happened?
FENA is 2.16%.
Renal U/S ruled out urinary obstruction.
The patient had good urine output with IVF and was in a positive fluid balance.
There was a downward trend in BUN and creatinine.


FENa greater than 1% due to diuretics in prerenal ARF. MDCalc.com calculator of FENa (right).

Does FENa higher than 1% rules out prerenal ARF?
No. Patients who receive diuretics may have prerenal ARF with FENa greater than 1% due to diuretic-induced sodium excretion.

Final diagnosis:

Pre-renal acute renal failure due to dehydration. FENa is higher than 1% due to diuretics.

What did we learn from this case?
The fractional excretion of sodium (FENa) is useful in diagnosing prerenal ARF. FENa is less than 1 % in most patients with pre-renal ARF.

Patients who receive diuretics may have prerenal acute renal failure but the fractional excretion of sodium may be increased by diuretic-induced sodium excretion. Therefore, these patients may have FENa greater than 1% in the presence of prerenal ARF.

In patients who are receiving diuretics, a fractional excretion of urea (FEUrea) can be useful since urea transport is not affected by diuretics. FEUrea of less than 35% is suggestive of a pre-renal state.

References:
Acute Renal Failure. M. Agrawal, R. Swartz. American Family Physician, April 1, 2000.
Acute Renal Failure. eMedicine, 09/2007.
MDCalc: FEUrea and FENa.

Created: 08/27/2006
Updated: 02/15/2008

5 comments:

  1. Very good case in showing a frequently overlooked issue of diuretics. Remember, there is almost never a reason to give both fluids and diuretics...make up your mind. IV fluids are the #1 method to try in oliguric pts NOT Lasix.

    Do not agree with Foley cath placement if patient is able to urinate and can check creatinine to know pt is improving. Any catheter is a foreign body and increases infection risk.It is also very uncomfortable for patients.

    Not sure why U/S of kidney needed right away either unless the patient has chronic kidney disease or does not improve with fluids.

    Unnecessary tests add to the expense of healthcare which all of us pay for. This increases insurance costs, medicaid costs, etc so much it can put companies (and gov't in the future?) out of business.

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  2. Foley catheter relieves mechanical obstruction, if present, and can be both diagnostic and therapeutic in that case.

    Renal U/S rules out hydronephrosis, distended bladded and kidney stones.

    Both are indicated, in my opinion.

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  3. I agree with PGY3. a simple bladder scan would be suffice, instead of USG to rule out urine retention.

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  4. Bladder scans though sensitive, do not give you the full picture... esp with elderly. I guess if bladder scan shows 0 ml or close to than, yes. But often in olyguric patients, and esp in demented patient which this patients most likely is, foley catheter r/o post renal picture.

    Yes, foley catheters are not comfortable but in this case is indicated.

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  5. "olyguric" is correctly spelled "oliguric". And yes, Foley catheter is indicated with the presentation above.

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