Fractional Excretion of Sodium (FENa)

id

60

full_title

Fractional Excretion of Sodium (FENa)

short_title

FENa

med_description

Determines if renal failure is due to prerenal or intrinsic pathology.

description

The Fractional Excretion of Sodium (FENa) determines if renal failure is due to pre-renal, intrinsic, or post-renal pathology.

keywords

Fractional Excretion of Sodium (FENa), fena, fractional excretion, FENa, FE sodium, sodium excretion, renal failure measurement, renal failure calc, prerenal calc, post renal calc, intrinsic renal failure, elevated creatinine, fena calc, fe Na, fractional urine, fractional urine sodium, fractional sodium, fractional Na

complaint

[ "Dizziness" ]

formula

Fractional Excretion of Sodium (FENa), % = 100 × (SCr× UNa) / (SNa× UCr) SCr, serum creatinine; UNa, urine sodium; SNa, serum sodium; UCr, urine creatinine. How the equation is derived: FENa is a measure of tubular resorption of Na. FENa = (Na excreted/Na filtered) x 100. Na excreted = UNa× urine volume. Therefore, Na filtered = PNa× (UCr× urine volume)/PCr.

evidence

Espinel in 1976 originally described a FENa cutoff of <1% for pre-renal azotemia in a study study of 17 adults with oliguria, defined as urine output <20mL/hr.  Patients with urinary obstruction, acute glomerulonephritis, CKD, and diuretic use were excluded.

A FENa cutoff of <1% for pre-renal azotemia was clinically validated in a prospective study, though there was overlap between patients with true prerenal azotemia and acute tubular necrosis.  Specificity for prerenal azotemia (with a cutoff of <1%) was decreased in nonoliguric acute renal failure (Miller 1978).

At a normal GFR of 180 L/day and sodium concentration of 140 mEq/L, the filtered sodium load is 25,200 mEq/day (=140 × 180). A FENa of 1% in this setting represents a sodium excretion of 252 mEq/day.  This is higher than the average sodium intake of 80 to 250 mEq/day. This is the physiologic basis for hypothesizing that patients with normal GFR have a FENa below 1%.

A recent 2022 review article provided validation by pooling studies that showed a sensitivity and specificity for FENa of 95% and 91% in oliguric patients without CKD. Sensitivity and specificity decreased to 83% and 66% when CKD patients and those on diuretics were included.

measurements

[ { "name": "Creatinine", "unit": "cr", "error_min": "0.01", "error_max": "40", "warn_min": "0", "warn_max": "10", "conversion": "88.4", "normal_max_si": "115", "normal_max_us": "1.3", "normal_min_si": "62", "normal_min_us": "0.7", "units_si": "µmol/L", "units_us": "mg/dL" }, { "name": "Urine Creatinine", "unit": "cr_urine", "error_min": "0", "error_max": "700", "warn_min": "1", "warn_max": "450", "conversion": "88.4", "normal_max_si": "32708", "normal_max_us": "370", "normal_min_si": "1768", "normal_min_us": "20", "units_si": "µmol/L", "units_us": "mg/dL" }, { "name": "Urine Sodium", "unit": "na_urine", "error_min": "0", "error_max": "500", "warn_min": "1", "warn_max": "260", "conversion": "1", "normal_max_si": "260", "normal_max_us": "260", "normal_min_si": "100", "normal_min_us": "100", "units_si": "mmol/L", "units_us": "mEq/L" }, { "name": "Sodium", "unit": "na", "error_min": "100", "error_max": "200", "warn_min": "0", "warn_max": "170", "conversion": "1", "normal_max_si": "145", "normal_max_us": "145", "normal_min_si": "136", "normal_min_us": "136", "units_si": "mmol/L", "units_us": "mEq/L" } ]

information

Prerenal: Anything causing decreased effective renal perfusion: hypovolemia, heart failure, renal artery stenosis, sepsis, etc. Remember, contrast-induced nephropathy will often look pre-renal.

Intrinsic: ATN, AIN, glomerulonephritides, etc.

refrences

{ "Clinical Practice Guidelines": [ { "href": "", "text": "" } ], "Manufacturer Website": [ { "href": "", "text": "" } ], "Original/Primary Reference": [ { "href": "https://www.ncbi.nlm.nih.gov/pubmed/947239", "text": "Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-81." } ], "Other References": [ { "href": "https://www.ncbi.nlm.nih.gov/pubmed/6486145", "text": "Steiner R. Interpreting the fractional excretion of sodium. Am J Med 1984; 77. (4): 699-702. doi:10.1016/0002-9343(84)90368-1. PMID 6486145" }, { "href": "https://www.ncbi.nlm.nih.gov/pubmed/26689284 ", "text": "Pahwa AK, Sperati J. Urinary fractional excretion indices in the evaluation of acute kidney injury. J Hosp Med. 2016;11(1): 77-80." }, { "href": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9269665/", "text": "Seethapathy H, Fenves AZ. Fractional Excretion of Sodium (FENa): An Imperfect Tool for a Flawed Question. Clin J Am Soc Nephrol. 2022 Jun;17(6):777-778. " }, { "href": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5143430/", "text": "Alsaad AA, Wadei HM. Fractional excretion of sodium in hepatorenal syndrome: Clinical and pathological correlation. World J Hepatol. 2016 Dec 8;8(34):1497-1501." } ], "Outcomes": [ { "href": "", "text": "" } ], "Validation": [ { "href": "https://www.ncbi.nlm.nih.gov/pubmed/666184", "text": "Miller TR, Henrich WH, Schrier RW. Urinary diagnostic indices in acute renal failure. Ann Int Med. 1978; 89:47-50" } ], "Validations": [] }

pearls

• FENa is only clinically validated in patients with oliguric acute kidney injury WITHOUT any of the following: diuretic use, chronic kidney disease (CKD), urinary tract obstruction, or acute glomerular disease.
• Using urine sodium concentration alone is less accurate because it does not account for urine volume and the kidney’s water handling by antidiuretic hormone (ADH).
• Single measurements of serum creatinine are “snapshots” in time and do not reflect the true glomerular filtration rate (GFR). The most accurate measurement of GFR is the average of the 24-hr creatinine clearance and urea clearance.
• The etiology of renal dysfunction cannot be differentiated in those with a FENa <1% who have liver disease.

usecase

Patients with oliguria and/or acute kidney injury of unclear etiology.

reasons

May give an additional data point in patients whose volume status is difficult to assess. Provides a more accurate assessment of kidney function than urine sodium alone (for example, a severely hypovolemic patient may have a relatively high urine sodium as a fraction of total urine volume, despite having little sodium in the urine).

next_advice

• No absolute FENa percentage indicates true “prerenal” disease. Always consider the history, clinical context, physical exam, and current medications.
• Obtaining repeat FENa or urine studies throughout a patient’s hospital course can give more clinical clues.
• Non-volume-depleted states with low urine sodium (and consequently low FENa) include: acute glomerulonephritis, cardiorenal syndrome, hepatorenal syndrome, contrast-related nephropathy, and rarely, acute obstruction and early acute interstitial nephritis (AIN) or acute tubular necrosis (ATN).

next_actions

next_management

• In patients with acute kidney and suspected obstructive uropathy, consider bedside ultrasound to evaluate for hydronephrosis, bladder distention, etc.
• Empiric Foley catheter placement may also be valid in some settings, though it may also be associated with complications and does not rule out ureteral obstructions above the bladder.

diseases

[ "Acid-Base Disturbance", "Acute Kidney Injury", "Renal Failure" ]

instructions

• Do not use in patients taking diuretics or with known chronic kidney disease, urinary tract obstruction, or acute glomerular disease. Use FEUrea in patients on diuretics.
• Remember that FENa <1% cannot differentiate hepatorenal syndrome (HRS) from other causes of renal disease.

published

2022-04-21T20:29:43.846Z

purpose

[ "Diagnosis", "Formula" ]

search_terms

[ "renal", "aki", "arf", "fena", "pitaliou", "feurea", "sodium", "pre renal", "pre-renal", "prerenal", "AKI", "ARF", "acute renal failure", "kidney injury", "acute kidney injury", "oliguria", "ATN", "acute tubular necrosis", "Fractional Excretion of Sodium (FENa)" ]

seo

{ "keywords_en": "Fractional Excretion of Sodium (FENa), fena, fractional excretion, FENa, FE sodium, sodium excretion, renal failure measurement, renal failure calc, prerenal calc, post renal calc, intrinsic renal failure, elevated creatinine, fena calc, fe Na, fractional urine, fractional urine sodium, fractional sodium, fractional Na", "meta_description_en": "The Fractional Excretion of Sodium (FENa) determines if renal failure is due to pre-renal, intrinsic, or post-renal pathology." }

specialty

[ "Critical Care", "Emergency Medicine", "Family Practice", "Geriatrics", "Hospitalist Medicine", "Internal Medicine", "Nephrology", "Primary Care", "Surgery (General)" ]

departments

[ "Renal" ]

tags

[]

version_number

1

versions

[]

related

[ { "calcId": 62, "short_title_en": "FEUrea", "slug": "fractional-excretion-urea-feurea" }, { "calcId": 10063, "short_title_en": "Licurse Score", "slug": "licurse-score-renal-ultrasound" }, { "calcId": 10018, "short_title_en": "AKIN Classification", "slug": "akin-classification-acute-kidney-injury-aki" } ]

ismed

true

section

[ "whenToUseViewed", "pearlsPitfallsViewed", "whyUseViewed", "nextStepsViewed", "evidenceViewed" ]

cleaned_departments

[ "urology" ]

cleaned_use

[ "Patients with oliguria and/or acute kidney injury of unclear etiology." ]

pub

true