- Choose Conventional (mg/dL) or SI (µmol/L) units to match your lab report.
- Enter serum uric acid and eGFR — these two values are the minimum required for target-check and dose guidance.
- For FEUA, also enter serum creatinine, urine uric acid, and urine creatinine from the same spot urine specimen drawn at the same time as the serum sample.
- Select the patient's gout phenotype (acute flare, chronic on ULT, new diagnosis, or tophaceous) to receive phenotype-specific management guidance.
- Results update live: urate vs target, FEUA classification, febuxostat dose recommendation, a plain-language verdict, and recommended action.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Use this tool when evaluating a patient with gout, hyperuricemia, or suspected uric acid nephropathy. It checks whether the serum urate target is met, calculates the fractional excretion of uric acid (FEUA) to classify the dominant pathophysiologic mechanism, and provides febuxostat dose guidance stratified by eGFR.
Appropriate population
Adults with confirmed or suspected gout (serum urate > 6.8 mg/dL, crystal-proven arthritis, or tophaceous deposits) who are being considered for urate-lowering therapy (ULT), or patients already on ULT in whom you need to verify target attainment. Also useful in CKD patients with hyperuricemia to determine whether the primary mechanism is under-excretion (the most common pattern in reduced GFR) or over-production.
When NOT to use it
Do not use this tool to guide acute flare management — uric acid levels fall during an acute attack and are unreliable. Do not initiate or change ULT dose during an active flare. The FEUA requires a spot urine uric acid and creatinine drawn simultaneously with the serum sample; unpaired samples give unreliable results. The dose guidance applies specifically to febuxostat; allopurinol dosing in CKD is more complex and requires CKD-stage adjustment and HLA-B*58:01 screening before initiation.
Pearls & Pitfalls
Treat-to-target, not symptom-driven
Uric acid is asymptomatic between flares, so patients stop ULT when pain resolves. Sustained suppression below target for 12–24 months is required to dissolve deposits and eliminate future flares. The goal is a low, stable urate level — not just treating attacks.
CKD patients are almost always under-excretors
Over 90% of hyperuricemic CKD patients have FEUA <5.5% because GFR directly determines urate excretion. This means xanthine oxidase inhibition (febuxostat preferred over allopurinol in CKD) is the mechanistically correct approach. Losartan is the only ARB with meaningful uricosuric activity and provides additive urate-lowering (~10–15%) as a secondary benefit in hypertensive CKD patients.
Pitfalls
(1) Never start or change ULT during an acute flare — rapid urate shifts mobilize existing crystals and prolong the attack. (2) Serum uric acid falls 10–40% during an acute attack and the intercritical period may show values within the normal range, masking true hyperuricemia — always measure UA during the quiescent period. (3) HLA-B*58:01 screening before allopurinol is mandatory in Filipino patients — the allele frequency is high (~8%) and carriers face severe cutaneous adverse reactions (DRESS, SJS/TEN). Febuxostat does not carry this risk.
Why Use It
Gout is the most common inflammatory arthritis in adults and is strongly linked to CKD. Hyperuricemia above the saturation threshold of 6.8 mg/dL causes monosodium urate crystal deposition in joints, soft tissue, and renal tubules. Sustained serum urate below target dissolves crystals over 12–24 months, eliminating flares and preventing urate nephropathy. The ACR 2020 guideline mandates a treat-to-target strategy: <6.0 mg/dL for most patients, <5.0 mg/dL for tophaceous gout.
The FEUA distinguishes under-excretors (FEUA <5.5%) from over-producers (FEUA >10%). Over 90% of hyperuricemic CKD patients are under-excretors because reduced GFR directly limits uric acid clearance. This distinction guides drug choice: xanthine oxidase inhibitors (febuxostat, allopurinol) reduce production and work regardless of excretion mechanism; uricosurics (probenecid) are contraindicated when eGFR <30. Identifying the mechanism also explains whether losartan — the only ARB with clinically meaningful uricosuric activity — provides additive benefit.
Uric Acid Calculator — Target Checker, FEUA & ULT Dose Guide
Enter serum uric acid, eGFR, and (optionally) paired urine values to check target attainment, classify the mechanism of hyperuricemia, and review febuxostat dose guidance by kidney function.
⚕ FEUA = (Urine UA × Serum Cr) ÷ (Serum UA × Urine Cr) × 100. FEUA <5.5% = under-excretor (most common in CKD); FEUA >10% = over-producer. ULT preference per Dr. Rivero practice protocol: febuxostat 40–80mg OD preferred over allopurinol in CKD (no dose adjustment needed for eGFR >30). Losartan is the only ARB with clinically meaningful uricosuric effect (Nishida 2014). This tool is educational — ULT initiation and flare management require physician supervision.
Next Steps
Use the result to support — not replace — clinical judgment.
- Interpret the value against the targets shown in the calculator and the Evidence section below, in the context of the full clinical picture.
- Trend serial measurements rather than acting on a single result; confirm abnormal or unexpected values before changing management.
- Apply the relevant KDIGO / specialty-guideline threshold and document the indication.
- Escalate or refer to nephrology when results are out of range, rapidly changing, or discordant with the clinical picture — and discuss the implications with the patient.
Evidence & References
Formula & Equations
| Quantity | Equation / Threshold |
|---|---|
| Fractional Excretion of Uric Acid (FEUA) | FEUA (%) = (Urine UA × Serum Cr) ÷ (Serum UA × Urine Cr) × 100 |
| Under-excretor classification | FEUA < 5.5% — reduced renal urate clearance is the dominant mechanism |
| Over-producer classification | FEUA > 10% — excessive xanthine oxidase-mediated urate synthesis |
| Mixed / normal | FEUA 5.5–10% — both mechanisms contribute, or normal urate handling |
Urate targets (ACR 2020 / EULAR 2017)
| Patient group | Serum urate target |
|---|---|
| Gout — general (first-line target) | < 6.0 mg/dL (< 357 µmol/L) |
| Tophaceous gout | < 5.0 mg/dL (< 297 µmol/L) to accelerate crystal dissolution |
| Saturation threshold (no crystallization below this) | 6.8 mg/dL (405 µmol/L) — physiochemical solubility limit at 37°C |
| Febuxostat starting dose (eGFR ≥ 30) | 40 mg once daily; escalate to 80 mg if UA not at target after 4 weeks |
| Febuxostat in eGFR 15–29 | 40 mg once daily with caution; limited pharmacokinetic data |
| Febuxostat in eGFR < 15 / dialysis | Use under nephrologist guidance; data very limited |
The FEUA formula requires simultaneous serum and spot urine samples. The ratio is dimensionless if all four values are in the same units (mg/dL or µmol/L). Converting between units: 1 mg/dL uric acid = 59.48 µmol/L; 1 mg/dL creatinine = 88.4 µmol/L.
Evidence & References
Urate targets and ULT initiation criteria follow the ACR 2020 Gout Guideline (FitzGerald et al.) and the 2016 updated EULAR recommendations (Richette et al.). The FEUA cut-point of 5.5% for under-excretion derives from studies comparing 24-hour urine uric acid excretion with spot FEUA in gout patients. Febuxostat's CKD dosing profile is based on its predominantly hepatic metabolism (80–90%), which explains why it does not require dose adjustment down to eGFR 30.
- FitzGerald JD, Dalbeth N, Mikuls T, et al. 2020 American College of Rheumatology Guideline for the Management of Gout. Arthritis Care Res. 2020;72(6):744–760.
- Richette P, Doherty M, Pascual E, et al. 2016 updated EULAR evidence-based recommendations for the management of gout. Ann Rheum Dis. 2017;76(1):29–42.
