A 62-year-old man with back pain and anaemia has total urinary protein of 3.5 g/day on 24-hour collection, yet the urine dipstick reads only trace positive. Serum electrophoresis shows an M spike and bone marrow has 28% plasma cells. What explains the discrepancy between the dipstick result and the measured proteinuria?

• A The dipstick reagent is inhibited by calcium in myeloma kidney
• B Tamm-Horsfall protein binds the dipstick indicator dye
• C The 24-hour collection was contaminated with vaginal secretions
• D The dipstick primarily detects albumin, while the proteinuria is predominantly free monoclonal light chains ✓

Explanation

Standard urine dipsticks use a pH indicator that binds albumin avidly but reacts poorly with immunoglobulin light chains. In cast nephropathy of multiple myeloma, filtered free light chains account for most of the proteinuria, so the dipstick underestimates it dramatically. The sulfosalicylic acid test precipitates all proteins including light chains and reveals the true protein load. This dissociation between dipstick and quantified protein is a classic clue to myeloma kidney.

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.