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Urate Saturation Target Basis — SCE Rheumatology MCQ

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ModerateCrystal ArthropathyUrate Saturation Target BasisSCE Rheumatology

A 55-year-old man with recurrent gout flares has been established on allopurinol. His agreed target serum urate is below 360 micromol/L, and he asks what this equates to in the units quoted in the patient information he has read (mg/dL) and why that particular number was chosen. Which statement gives the correct approximate equivalent concentration and the correct physiological rationale for the target?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: A — 6.0 mg/dL; it lies below the saturation concentration of monosodium urate in body fluids, favouring dissolution of established crystal deposits.

Explanation lettering: E = shown as A · D = shown as B · B = shown as C · C = shown as D · A = shown as E

E is correct. NICE NG219 sets a treat-to-target serum urate below 360 micromol/L, which it expresses as 6 mg/dL (arithmetically 6.05 mg/dL using urate's molar mass of 168.1). The rationale is physicochemical: at body temperature and pH, monosodium urate approaches saturation at roughly 6.8 mg/dL (about 400 micromol/L), so sustaining urate below the target keeps body fluids undersaturated, halting new crystal formation and creating the gradient needed for dissolution of tophi and joint deposits. A is a tenfold conversion error and misdescribes renal handling. B gives the equivalent of the lower 300 micromol/L target NICE advises considering for frequent flares or tophi, not the saturation concentration. C has the right conversion but denies the crystal-kinetic basis of treat-to-target. D is a gross conversion error and is well above, not below, saturation.

Reference: NICE. Gout: diagnosis and management (NG219), Evidence review K: The best serum urate level target to use when treating-to-target in gout, 2022. https://www.nice.org.uk/guidance/ng219/evidence/k-the-best-serum-urate-level-target-to-use-when-treatingtotarget-in-gout-pdf-398898767702