What Is FE Uric Acid (FEUA)?
FE uric acid, commonly written as FEUA, means fractional excretion of uric acid. It is the percentage of filtered uric acid that is ultimately excreted into urine rather than reabsorbed by the kidney tubules. In daily practice, FEUA is often used as a supportive laboratory marker when evaluating patients with electrolyte and volume disorders, especially hyponatremia.
The concept is similar to other fractional excretion indices: you compare urinary and serum levels of a solute while correcting for water handling with creatinine. This gives a ratio that better reflects tubular handling than isolated serum or urine numbers alone. As a result, FEUA can provide useful context when clinical signs are ambiguous.
Why FEUA Matters in Clinical Practice
In many patients, hyponatremia has overlapping causes and non-specific physical findings. FEUA can help refine differential diagnosis when interpreted with history, examination, osmolality, urine sodium, and response to treatment. It is particularly discussed in settings where the clinician is trying to distinguish SIADH from hypovolemic states, including patients who may be exposed to diuretics.
FEUA is not a standalone diagnostic test, but it can improve pattern recognition. A higher FEUA may support inappropriate uric acid wasting patterns seen in SIADH physiology, while lower FEUA tends to align more with uric acid retention in effective volume depletion. This framework is most valuable when the blood and urine samples are collected in a coherent clinical time window and interpreted with all major confounders in mind.
FEUA Formula and Step-by-Step Calculation
The calculator above uses the standard FEUA equation:
Inputs required
- Serum uric acid
- Urine uric acid
- Serum creatinine
- Urine creatinine
Unit rules
- Use the same uric acid unit for serum and urine uric acid.
- Use the same creatinine unit for serum and urine creatinine.
- If those conditions are met, the ratio remains valid regardless of specific unit system.
Because FEUA is a ratio-based percentage, careful unit consistency matters more than any one preferred unit convention. The calculator accepts mg/dL, µmol/L, and mmol/L labels for convenience, but mathematically it assumes matching units within each analyte pair.
How to Interpret FEUA Results
Published thresholds vary somewhat by population and clinical context. A practical bedside interpretation framework used by many clinicians is:
| FEUA Result | General Pattern | Clinical Meaning (Context Required) |
|---|---|---|
| < 10% | Lower uric acid excretion | Can be seen with effective volume depletion or uric acid reabsorption states. |
| 10% to 12% | Borderline/indeterminate | Interpret carefully with osmolality, urine sodium, medications, and trajectory. |
| > 12% | Higher uric acid excretion | May support SIADH-like physiology in the right hyponatremia context. |
A strong interpretation combines FEUA with plasma osmolality, urine osmolality, urine sodium, thyroid/adrenal review when indicated, and medication history. Trends over time can be more informative than a single isolated value.
FEUA in Hyponatremia: SIADH vs Volume Depletion
FEUA is often discussed in the hyponatremia workup because both SIADH and hypovolemia can present with non-specific findings. In SIADH physiology, uric acid handling often shifts toward greater excretion, pushing FEUA upward. In hypovolemia, compensatory reabsorption mechanisms can reduce uric acid excretion, contributing to lower FEUA.
One practical use is when diuretics blur urine sodium interpretation. In such scenarios, FEUA may add a secondary data point to support reasoning, though it should never override direct assessment of perfusion and volume status. For example, hypotension, tachycardia, dry mucosa, or obvious fluid losses still carry high diagnostic importance.
Clinical integration checklist
- Confirm true hypotonic hyponatremia before subtype analysis.
- Review medications: thiazides, loop diuretics, urate-lowering drugs, SGLT2 inhibitors, and others.
- Assess kidney function; severe AKI/CKD can distort tubular indices.
- Use timing consistency: collect serum and urine samples close together.
- Pair FEUA with clinical response to isotonic saline or fluid restriction when safe and indicated.
Worked Calculation Examples
Example 1: Higher FEUA pattern
Suppose values are: urine uric acid 40, serum uric acid 4.0, serum creatinine 1.0, urine creatinine 80 (with matched units for each analyte pair).
A result around 12.5% sits in a higher range and may support SIADH physiology if the full clinical picture is compatible.
Example 2: Lower FEUA pattern
Suppose values are: urine uric acid 20, serum uric acid 6.0, serum creatinine 1.2, urine creatinine 120.
A lower FEUA may fit reduced uric acid excretion patterns that can occur with effective volume depletion, but interpretation depends on exam findings and parallel labs.
Common Pitfalls and Limitations
FEUA is useful, but overreliance can mislead. Keep these limitations in mind:
- Unit mismatch: If serum and urine units are mixed incorrectly, results become meaningless.
- Timing mismatch: Serum and urine values drawn far apart may reflect different physiologic states.
- Medication effects: Diuretics, uricosuric agents, and recent treatment changes can alter urate handling.
- Renal dysfunction: Advanced kidney disease can reduce reliability of tubular indices.
- Single-point bias: One number should not replace serial monitoring and clinical trajectory.
Frequently Asked Questions
Is FEUA the same as fractional excretion of sodium (FENa)?
No. FEUA evaluates uric acid handling, while FENa measures sodium handling. They answer different physiologic questions and can diverge in the same patient.
Can I use spot urine for FEUA?
Yes, spot urine is commonly used in bedside practice as long as serum sampling is temporally close and interpretation is context-aware.
What FEUA value suggests SIADH?
Many clinical references consider FEUA values above approximately 12% as supportive of SIADH-like physiology in the right setting, but no cutoff should be treated as absolute.
Does FEUA diagnose SIADH by itself?
No. SIADH remains a clinical-laboratory diagnosis requiring exclusion of other causes and integration of osmolar status, endocrine evaluation when appropriate, medications, and volume assessment.
When should FEUA be avoided?
Be cautious when units are uncertain, sampling is asynchronous, or there is severe kidney dysfunction likely to invalidate tubular interpretation.
Key Takeaways
The FE uric acid calculator gives a fast, standardized FEUA percentage from four routinely obtained values. This can improve clarity in challenging hyponatremia evaluations, particularly when trying to distinguish SIADH-like physiology from effective volume depletion patterns. Still, the strongest decisions come from synthesis: bedside assessment, medication review, osmolality data, urine indices, renal function, and response over time.
Use this calculator for speed and consistency, then apply clinical judgment for final diagnosis and treatment planning.