A 52-year-old man with newly diagnosed multiple myeloma is being evaluated for chemotherapy. His baseline renal function shows creatinine 1.8 mg/dL and eGFR 42 mL/min/1.73m². Before initiating melphalan-based therapy, which investigation is most important to assess the risk of severe alkylating agent toxicity?

## Assessment of Renal Function Before Alkylating Agent Therapy **Key Point:** Accurate measurement of glomerular filtration rate (GFR) is essential before administering melphalan and other alkylating agents, as these drugs and their metabolites are renally cleared and accumulate in renal impairment. ### Why 24-Hour Urine Creatinine Clearance is the Gold Standard 1. **Direct measurement of GFR**: The 24-hour urine creatinine clearance provides the most accurate assessment of true renal function compared to estimated GFR formulas, which can be unreliable in elderly patients and those with muscle wasting (common in myeloma). 2. **Dose adjustment guidance**: Melphalan and other alkylating agents require dose reduction in renal impairment. A creatinine clearance <50 mL/min mandates dose reduction to prevent severe toxicity (myelosuppression, mucositis, hemorrhagic cystitis in case of cyclophosphamide). 3. **Baseline documentation**: This serves as a baseline to monitor for drug-induced nephrotoxicity during treatment. **High-Yield:** Melphalan is primarily renally eliminated; in patients with CrCl <30 mL/min, dose should be reduced by 50%. Serum creatinine alone is unreliable in elderly, cachectic patients. ### Why Other Investigations Are Secondary | Investigation | Role | Timing | |---|---|---| | Serum uric acid & LDH | Assess tumor burden and risk of tumor lysis syndrome | Prognostic, not dose-limiting | | Bone marrow biopsy | Confirm diagnosis, assess cytogenetics | Already done for diagnosis | | CT abdomen/pelvis | Assess disease extent | Staging, not renal function | **Clinical Pearl:** In this patient with eGFR 42, the discrepancy between estimated and measured creatinine clearance could be significant. A 24-hour collection is mandatory before committing to melphalan dosing.