A 68-year-old man has renal failure with a serum creatinine of 5.8 mg/dL and heavy proteinuria. Urine dipstick is negative despite elevated total urinary protein on a 24-hour collection. Renal biopsy shows laminated, fracture-prone eosinophilic casts within dilated tubules surrounded by multinucleated giant cells. What explains the negative dipstick?

• A The filtered protein is albumin, which binds the reagent poorly at low pH
• B The casts trap protein within tubules so none reaches the urine
• C Tubular damage prevents delivery of protein to the dipstick pad
• D The excreted free light chains are not detected by the standard dipstick, which reacts mainly with albumin ✓

Explanation

This is cast nephropathy in multiple myeloma. Bence Jones proteins precipitate with Tamm-Horsfall protein to form brittle waxy casts that obstruct and injure tubules. The sulfosalicylic acid test detects light chains but the routine dipstick uses albumin-sensitive indicators such as bromphenol blue, so a strongly positive SSA test with a negative dipstick points to light chains.

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.