FENa shows how much of the sodium your kidneys filter ends up in your urine. Physicians use it when kidney function drops suddenly, to tell whether the cause is low blood flow to the kidneys or injury inside them. Enter four lab values from blood and urine collected at the same time.
Enter a serum sodium between 100 and 180 mmol/L.
Enter a urine sodium between 1 and 300 mmol/L.
Enter a serum creatinine between 0.2 and 20 mg/dL (18 to 1,768 µmol/L).
Enter a urine creatinine between 5 and 500 mg/dL (442 to 44,200 µmol/L).
Serum sodium and creatinine are included in every Insyght draw. Urine sodium and urine creatinine come from a separate urine test your physician orders.
FENa is meant for sudden drops in kidney function and should be read by a physician. It isn't useful when kidney function is normal, and it isn't a diagnosis.
FENa ranges at a glance
| FENa | What it suggests | Common causes |
|---|---|---|
| Below 1% | Prerenal: low blood flow to the kidneys | Dehydration, vomiting or diarrhea, blood loss, heart failure, cirrhosis |
| 1% to 2% | Indeterminate | Early or mixed causes; needs clinical context |
| Above 2% | Intrinsic: injury within the kidney | Acute tubular necrosis, some medications, prolonged low blood flow |
What can affect your FENa
FENa only works when nothing else is pushing sodium into the urine or holding it back. It is most reliable when urine output has dropped.
- Can read high without kidney injury: diuretics (water pills) taken in the past 24 hours, chronic kidney disease, IV saline and a very high-salt diet.
- Can read low despite kidney injury: contrast dye injury, muscle breakdown (rhabdomyolysis), sepsis, early urinary blockage and some forms of glomerulonephritis.
- Timing matters: blood and urine should be collected at about the same time.
If you take a diuretic, your physician may use fractional excretion of urea (FEUrea) instead. Below 35% points to low blood flow.
Frequently asked questions
What is a normal FENa?
What does a FENa above 2% mean?
Can I use FENa if I take a diuretic?
Does Insyght test urine sodium?
What's the difference between FENa and eGFR?
Insyght tracks creatinine, eGFR, BUN and sodium every year, reviewed by a physician, so slow changes show up as a trend. $199 a year.
Related tools and tests
- eGFR calculator
- 24-hour urinary sodium and salt intake calculator
- Water intake calculator
- Kidney and electrolyte tests
