Which laboratory value is least relevant in assessing the effectiveness of allopurinol therapy in tumor lysis syndrome?

• A. Uric acid level
• B. Serum potassium
• C. Serum phosphate
• D. Hemoglobin level

Answer: (D)

Allopurinol works by blocking xanthine oxidase, which stops the production of uric acid from the breakdown of nucleic acids. Because of that, the most informative measure of how well allopurinol is controlling TLS is the uric acid level—you want it to stay low as uric acid nephropathy risk diminishes. In TLS, potassium and phosphate levels rise due to the rapid release from lysed cells and need ongoing monitoring to guide hydration and electrolyte management, but they don’t directly reflect the effectiveness of uric acid–lowering therapy. Hemoglobin indicates red blood cell status and overall hematologic health, not the activity of uric acid production or response to allopurinol. So the lab value least relevant to assessing allopurinol effectiveness is the hemoglobin level.