A 64-year-old man with back pain and anaemia develops AKI, creatinine 5.1 mg/dL. Total urine protein measured by 24-hour collection is 3.4 g/day, yet the routine urine dipstick reads only trace positive. Urine microscopy shows few coarse waxy casts. Which property of the urinary protein explains the discordance between the dipstick and the measured protein?
- A High urine pH neutralises the indicator dye of the dipstick
- B Light chains are too large to pass the glomerular filtration barrier
- C Free monoclonal light chains are not detected by the albumin-based dipstick reagent ✓
- D Waxy casts bind the protein and prevent its reaction with the pad
Explanation
The standard urine dipstick uses tetrabromophenol blue, which binds primarily albumin and detects it down to low concentrations but reacts poorly with globulins and free light chains. In multiple myeloma with cast nephropathy, the filtered Bence Jones light chains constitute most of the protein load, so the dipstick underestimates while quantitative methods such as sulfosalicylic acid precipitation, urine protein electrophoresis, or immunofixation detect the full amount. Light chains are small enough to filter freely, and neither urine pH nor casts explain the discrepancy.
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.