A 65-year-old man with multiple myeloma has creatinine 4.5 mg/dL. Urinalysis shows trace protein on dipstick, yet 24-hour urine protein is 4.2 g/day. Urine microscopy shows waxy casts. Which additional test best confirms the renal lesion?
- A Serum free light chain assay with kappa-lambda ratio ✓
- B Serum anti-PLA2R antibody
- C Urine sodium and fractional excretion of sodium
- D Serum C3 and C4 complement levels
Correct answer: A. Serum free light chain assay with kappa-lambda ratio
Explanation
The dipstick detects albumin but not immunoglobulin light chains, explaining the discordance between trace dipstick protein and heavy 24-hour proteinuria. Monoclonal light chains precipitate with Tamm-Horsfall protein forming casts that obstruct and damage tubules, the lesion behind these findings. Serum free light chains with an abnormal kappa-lambda ratio confirm clonal production. Complement is normal in cast nephropathy and PLA2R relates to membranous nephropathy, a different entity.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.