Fractional Excretion of Sodium Calculator

Enter paired serum, urine, sodium, and creatinine values now. Compare fractional excretion with practical interpretation. Save outputs for reports, audits, and chemistry study notes.

Calculator Form

mmol/L or mEq/L
mmol/L or mEq/L
mL, for timed clearance only
hours, for timed clearance only

Formula Used

FENa % = (Urine Na × Serum Creatinine) ÷ (Serum Na × Urine Creatinine) × 100

The calculator converts creatinine values to the same unit before applying the formula. Sodium in mmol/L and mEq/L is numerically equivalent for this calculation.

Sodium clearance = (Urine Na ÷ Serum Na) × Urine flow

Creatinine clearance = (Urine Creatinine ÷ Serum Creatinine) × Urine flow

Clearance based FENa % = Sodium clearance ÷ Creatinine clearance × 100

How to Use This Calculator

  1. Enter serum sodium and urine sodium from paired samples.
  2. Enter serum creatinine and urine creatinine.
  3. Select the correct creatinine unit for each value.
  4. Add urine volume and collection hours only for timed clearance outputs.
  5. Select decimal precision for the displayed result.
  6. Press calculate to view results above the form.
  7. Use CSV or PDF export for records and study notes.

Example Data Table

Case Serum Na Urine Na Serum Cr Urine Cr FENa Typical Reading
A 140 mmol/L 20 mmol/L 1.5 mg/dL 120 mg/dL 0.18% Low sodium excretion pattern
B 138 mmol/L 70 mmol/L 2.0 mg/dL 80 mg/dL 1.27% Borderline pattern
C 142 mmol/L 95 mmol/L 3.0 mg/dL 65 mg/dL 3.09% High sodium excretion pattern

Chemistry Context

Fractional excretion of sodium, often called FENa, describes sodium loss compared with creatinine handling. It uses paired urine and blood measurements. The value is a percentage. It estimates how much filtered sodium appears in urine. Chemistry students use it to connect concentration, clearance, and renal transport.

Why The Calculation Matters

Sodium is the main extracellular cation. The kidneys filter it freely, then reclaim most of it. Creatinine is used as a reference marker because it reflects filtration more than tubular sodium transport. By comparing sodium to creatinine ratios, FENa reduces the effect of simple urine concentration. A dilute specimen and a concentrated specimen can still be compared more fairly.

Interpreting Results Carefully

A low percentage often means strong sodium retention. Many references use values below one percent as a pattern seen with reduced kidney perfusion. Higher values may suggest tubular sodium loss. Values between one and two percent need caution. Diuretics, chronic kidney disease, contrast exposure, sepsis, and mixed injury can change the meaning. The result should never replace clinical review.

Good Data Practices

Use blood and urine samples collected close together. Enter creatinine units correctly. Serum creatinine and urine creatinine must be compared after unit conversion. Sodium values are commonly reported as mmol/L or mEq/L. For sodium, these values are numerically the same. Check for transcription errors before using the output.

Advanced Options

This calculator can also estimate sodium clearance and creatinine clearance when urine volume and collection time are supplied. These extra outputs help explain the clearance form of the formula. They are useful in laboratory teaching, audit notes, and case discussions. Keep notes about medications, hydration, and sample timing with every calculation.

Limits And Safety

FENa is a screening calculation. It is not a diagnosis. It should be interpreted with history, examination, urine sediment, and trends. Very low muscle mass can affect creatinine. Recent saline, osmotic diuresis, or bicarbonate therapy can also shift sodium excretion.

Reporting Tips

Report the final percentage with the entered units. Include the formula line. State whether a spot sample or timed collection was used. Save the CSV or document output for review. Clear records make repeated chemistry checks easier to compare. They support clear laboratory communication over time.

FAQs

What is fractional excretion of sodium?

It is the percentage of filtered sodium that appears in urine. It compares urine sodium, serum sodium, urine creatinine, and serum creatinine in one ratio.

What formula does this calculator use?

It uses FENa % = (urine sodium × serum creatinine) ÷ (serum sodium × urine creatinine) × 100.

Can sodium be entered in mEq/L?

Yes. Sodium has a charge of one. For sodium, mmol/L and mEq/L have the same numeric value in this calculation.

Why are creatinine units important?

Serum and urine creatinine must be compared in matching units. This tool converts selected creatinine units to mg/dL before calculating the final percentage.

What does a low FENa suggest?

A value below one percent often shows strong sodium retention. It may fit reduced kidney perfusion patterns, but clinical review is always needed.

What does a high FENa suggest?

A value above two percent may show greater tubular sodium loss. Diuretics, kidney disease, and mixed injury can change interpretation.

Do I need urine volume?

No. FENa can be calculated from spot samples. Urine volume and collection time are only needed for clearance outputs.

Is this a diagnostic tool?

No. It is an educational and calculation aid. Use results with clinical findings, laboratory trends, medication history, and professional judgment.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.