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Rasburicase interference with uric acid measurement — ABIM Board MCQ

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HardUric AcidRasburicase interference with uric acid measurementABIM Board

A 46-year-old man with newly diagnosed Burkitt lymphoma begins cytotoxic chemotherapy. Before treatment, the serum uric acid level is 12.8 mg/dL, potassium is 4.6 mEq/L, phosphorus is 4.2 mg/dL, calcium is 8.7 mg/dL, and creatinine is 1.7 mg/dL. Glucose-6-phosphate dehydrogenase activity is normal. He receives intravenous fluids and rasburicase. Six hours later, potassium is 5.8 mEq/L, phosphorus is 6.9 mg/dL, calcium is 7.2 mg/dL, and creatinine is 2.3 mg/dL. The reported uric acid level is less than 0.2 mg/dL. The patient has no arrhythmia, seizure, pulmonary edema, or uremic symptoms. The laboratory confirms that the uric acid specimen was collected in a serum separator tube, transported at room temperature, and assayed 2.5 hours after collection. Which of the following is the most appropriate next step regarding urate management?

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

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Correct answer: D — Repeat plasma uric acid now with cold heparinized specimen handling

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

This patient has ongoing laboratory tumor lysis after chemotherapy, but the extremely low uric acid result is unreliable because the post-rasburicase specimen was transported at room temperature. Rasburicase remains enzymatically active ex vivo and continues converting uric acid to allantoin, producing a spuriously low measured concentration. Plasma uric acid should be repeated using a prechilled heparin-containing tube, immediately maintained in an ice-water bath, and assayed within 4 hours. Further rasburicase dosing should be guided by an accurately measured uric acid level and the overall clinical course. A is premature because the true uric acid concentration is unknown; worsening phosphorus and creatinine do not independently establish persistent hyperuricemia. B is inappropriate because retaining the invalid result risks undertreatment, and allopurinol prevents formation of new uric acid rather than rapidly removing uric acid already present. D is not yet indicated solely by these biochemical abnormalities; dialysis is considered for refractory hyperkalemia, severe hyperphosphatemia or symptomatic hypocalcemia, volume overload, uremic complications, or progressive kidney failure unresponsive to medical management. E incorrectly assumes that urate has normalized and could permit rebound or persistent hyperuricemia to go untreated.

Reference: Elitek (rasburicase) Prescribing Information (2019) — https://www.accessdata.fda.gov/drugsatfda_docs/label/2019/103946s5103lbl.pdf The Tumor Lysis Syndrome (2011) — https://pmc.ncbi.nlm.nih.gov/articles/PMC3437249/