How to Order FENa
To order a fractional excretion of sodium (FENa) test, you need to obtain simultaneous serum and urine samples for sodium and creatinine measurements, which are then used to calculate FENa using the formula: FENa = (Urine Na × Serum Cr) / (Serum Na × Urine Cr) × 100.
Required Laboratory Orders
You must order the following tests simultaneously:
- Serum sodium
- Serum creatinine
- Urine sodium (spot or random sample)
- Urine creatinine (spot or random sample)
The laboratory will typically calculate FENa automatically if you order these four tests together, or you can calculate it manually using the formula above.
Clinical Context and Timing
Order FENa when evaluating acute kidney injury (AKI) to help differentiate prerenal azotemia from acute tubular necrosis (ATN), particularly in patients with oliguria 1. The test is most useful in oliguric patients without chronic kidney disease or recent diuretic use 2.
Critical Timing Considerations
- Withdraw all diuretics before testing 1 - diuretics significantly affect FENa accuracy and can falsely elevate values
- Obtain samples before initiating volume resuscitation when possible
- In cirrhotic patients, consider ordering after diuretics have been held for an appropriate washout period
Important Caveats and Limitations
When FENa is LESS Reliable
FENa has limited diagnostic value in several clinical scenarios 2:
- Patients on diuretics - sensitivity drops to 80% and specificity to only 54% 2
- Chronic kidney disease patients - pooled sensitivity 83%, specificity only 66% 2
- Cirrhotic patients - FENa <1% has 100% sensitivity but only 14% specificity for prerenal causes 1
- Sodium-avid states - congestive heart failure, nephrotic syndrome, burns 3
- Hepatorenal syndrome - may have low FENa despite intrinsic renal dysfunction 4
Alternative: Consider FEUrea in Cirrhosis
In cirrhotic patients with ascites, fractional excretion of urea (FEUrea) may be superior to FENa 1. FEUrea <28.16% has 75% sensitivity and 83% specificity for separating hepatorenal syndrome from non-HRS causes, and is not affected by diuretic use 1.
To order FEUrea, obtain:
- Serum urea (BUN)
- Serum creatinine
- Urine urea
- Urine creatinine
Interpretation Guidelines
Standard Cutoffs
- FENa <1% suggests prerenal azotemia/volume depletion 1, 5
- FENa >1% (typically >3%) suggests ATN or intrinsic renal disease 5
Optimal Performance
FENa performs best in oliguric patients without CKD or diuretic exposure, achieving 95% sensitivity and 91% specificity 2. In this ideal population, it is a highly reliable test for differentiating prerenal from intrinsic AKI.
Practical Ordering Approach
- Assess patient eligibility: Is the patient oliguric? Any recent diuretic use? History of CKD or cirrhosis?
- Hold diuretics if clinically safe and time permits
- Order as a panel: "Serum sodium, serum creatinine, urine sodium, urine creatinine for FENa calculation"
- In cirrhotic patients: Consider adding urine and serum urea for FEUrea calculation
- Interpret in clinical context: Never rely on FENa alone - integrate with urinalysis, volume status assessment, and response to fluid challenge 4