A patient with tumor lysis syndrome is taking allopurinol. Which laboratory value should be monitored to determine the effectiveness of the medication?

• A. Uric acid level
• B. Serum potassium
• C. Serum phosphate
• D. Blood urea nitrogen (BUN)

Answer: (A)

In tumor lysis syndrome, rapid cell breakdown floods the body with purines that are converted to uric acid. Allopurinol blocks xanthine oxidase, reducing the formation of new uric acid from these purines. Because of that, the most informative lab to monitor is the uric acid level—seeing it decrease indicates the drug is effectively lowering uric acid production and helps prevent uric acid crystal deposition in the kidneys. Other labs reflect electrolyte disturbances or kidney function from TLS, but they don’t measure the drug’s specific effect on uric acid production. If uric acid remains high, additional interventions (like rasburicase) may be needed.