Uric acid clearance and fractional excretion: formulas, example and how to read the result
What clearance and FEUA are and what they are used for
Uric acid clearance is the volume of plasma the kidneys clear of urate per minute, in mL/min. The fractional excretion of uric acid (FEUA) is the percentage of urate filtered by the glomeruli that appears in the urine. Both describe the renal handling of uric acid, alongside 24-hour urinary excretion.
These parameters help separate two mechanisms of hyperuricemia: reduced renal excretion or increased production. Perez-Ruiz and colleagues, in Arthritis & Rheumatism in 2002, compared 100 patients with primary gout and 72 controls using 24-hour urine. Patients with gout had lower clearance and FEUA, and the authors concluded that renal underexcretion is the main mechanism of hyperuricemia in primary gout.
Clearance follows the same principle as creatinine clearance: urinary concentration times urine flow, divided by the blood concentration. FEUA does not need volume or time, because it uses creatinine as the reference for filtration.
How to use the calculator: required and optional fields
The calculator asks for four required concentrations, all in mg/dL, from blood and urine collected in the same period:
- Serum uric acid, greater than zero and up to 30 mg/dL.
- Urine uric acid, from 0 to 500 mg/dL.
- Serum creatinine, greater than zero and up to 30 mg/dL.
- Urine creatinine, greater than zero and up to 1,000 mg/dL.
With these data, it shows the FEUA in %, with two decimal places.
The optional fields are for those who have a timed urine collection:
- Collection urine volume, in mL, up to 10,000 mL;
- Collection time, in hours, prefilled with 24 and accepted up to 48.
With the volume, the uric acid clearance in mL/min and the urinary excretion in mg/24h also appear. Values outside the accepted ranges produce a message saying which field to correct. Values in µmol/L convert to mg/dL by dividing by 59.48 (uric acid) or 88.4 (creatinine), factors from the KDIGO 2024 conversion table.
Formulas used by the calculator
The formulas are the same ones used by Perez-Ruiz and colleagues to describe the renal handling of urate.
FEUA (%) = (UAu × Crs) ÷ (UAs × Cru) × 100
Clearance (mL/min) = (UAu × V) ÷ (UAs × t)
Excretion (mg/24h) = UAu × V ÷ 100 × 1,440 ÷ t
- UAu and UAs: urine and serum uric acid, in mg/dL.
- Cru and Crs: urine and serum creatinine, in mg/dL.
- V: collection volume, in mL.
- t: collection time in minutes (1,440 in 24 hours).
- ÷ 100: converts mg/dL × mL to mg.
FEUA has the same structure as the fractional excretion of sodium and the fractional excretion of urea: urine solute times serum creatinine, divided by serum solute times urine creatinine.
Worked example with 24-hour urine
Hypothetical case: serum uric acid of 8.0 mg/dL, urine uric acid of 30 mg/dL, serum creatinine of 1.0 mg/dL, urine creatinine of 100 mg/dL and 1,500 mL of urine in 24 hours.
- FEUA: (30 × 1.0) ÷ (8.0 × 100) × 100 = 30 ÷ 800 × 100 = 3.75%.
- Clearance: (30 × 1,500) ÷ (8.0 × 1,440) = 45,000 ÷ 11,520 = 3.91 mL/min.
- Excretion: 30 × 1,500 ÷ 100 = 450 mg/24h.
The calculator returns exactly these values. Because FEUA was below 5.5%, the tool notes that this is the cutoff used by Ichida and colleagues (2012) to define renal underexcretion, together with excretion of up to 600 mg/day/1.73 m².
Without the volume, the calculator would show only the FEUA of 3.75%. If the same urine had been collected over 12 hours, the clearance would double to 7.81 mL/min, and the estimated 24-hour excretion would become 900 mg.
How to understand the result
The calculator applies a single cutoff with a published source: FEUA below 5.5%. Ichida and colleagues, in Nature Communications in 2012, used this value, together with urinary excretion of up to 600 mg/day/1.73 m², to classify hyperuricemia as renal underexcretion. The criteria were applied on a low-purine diet.
With FEUA of 5.5% or more, the tool reports that the value is above that cutoff, without classifying it as normal: there is no universal normal range verified in a source, and the result varies with diet, medications and kidney function.
Clearance in mL/min and 24-hour excretion are not classified. The excretion in Ichida's cutoff is expressed per 1.73 m² of body surface area; the tool shows the raw value in mg/24h.
Interpreting these data, together with the clinical picture, is up to the physician. Filtration can be estimated with the eGFR by the CKD-EPI 2021 equation, because declining kidney function changes urate excretion.
Common errors, limitations and checklist
Most errors come from the collection and the units, not from the calculation.
- Samples from different times: serum and urine should be from the same period.
- Mixed units: urate in µmol/L and creatinine in mg/dL in the same calculation produce meaningless numbers. Convert everything to mg/dL.
- Incomplete collection or wrong time: they change the clearance and the excretion, but not the FEUA, which uses only concentrations.
- Volume in liters: 1.5 L entered as 1.5 mL lowers the clearance a thousandfold.
- Medications and diet: diuretics, uricosurics, allopurinol and purine intake change the values and should be recorded.
Checklist:
- Are all four values in mg/dL?
- Are blood and urine from the same period?
- Is the collection time entered the actual one?
- Were diet and current medications noted?
- Was the FEUA read together with the 24-hour excretion and kidney function?
Frequently asked questions about uric acid clearance and fractional excretion
What is the difference between uric acid clearance and fractional excretion?
Clearance is the volume of plasma cleared of urate per minute and requires collection volume and time. FEUA is the percentage of filtered urate that leaves in the urine and uses only the uric acid and creatinine concentrations in blood and urine.
How is FEUA calculated?
FEUA = (urine uric acid × serum creatinine) ÷ (serum uric acid × urine creatinine) × 100. With 30 and 8.0 mg/dL of uric acid and 100 and 1.0 mg/dL of creatinine, the FEUA is 3.75%.
What FEUA value indicates underexcretion?
Ichida and colleagues (2012) used FEUA below 5.5%, together with urinary excretion of up to 600 mg/day/1.73 m² and a low-purine diet, to define renal underexcretion of uric acid. The calculator shows this cutoff as a reference.
Do I need a 24-hour urine collection?
For FEUA, no: a blood sample and a urine sample from the same period are enough. For clearance in mL/min and daily excretion, you need to enter the collection volume and time.
Is there a normal range for uric acid clearance in mL/min?
There is no single range with a published source that the tool adopts. The calculator shows clearance and FEUA with the published formulas and uses as a reference only the 5.5% FEUA cutoff from Ichida and colleagues.
References
- Perez-Ruiz F, Calabozo M, Erauskin GG, Ruibal A, Herrero-Beites AM. Renal underexcretion of uric acid is present in patients with apparent high urinary uric acid output. Arthritis Rheum. 2002;47(6):610-3. doi:10.1002/art.10792. View
- Ichida K, Matsuo H, Takada T, et al. Decreased extra-renal urate excretion is a common cause of hyperuricemia. Nat Commun. 2012;3:764. doi:10.1038/ncomms1756. View
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int. 2024;105(4S):S117-S314. doi:10.1016/j.kint.2023.10.018. View
Updated . A support tool for educational purposes; it does not replace clinical assessment.