A 66-year-old man has fatigue, bone pain, and serum creatinine of 5.8 mg/dL. Urine dipstick shows trace protein, yet 24-hour urine protein quantification is 4.2 g/day. Serum protein electrophoresis shows an M spike and free light chain assay is markedly elevated. Why is the urine dipstick falsely reassuring in this condition?
- A Light chains are too small to be filtered by the glomerulus
- B The dipstick reagent detects albumin almost exclusively and does not react with free light chains ✓
- C Tubular reabsorption destroys all immunoglobulin fragments before they reach the urine
- D High concentrations of light chains saturate the buffer and produce a false negative result
Correct answer: B. The dipstick reagent detects albumin almost exclusively and does not react with free light chains
Explanation
In myeloma cast nephropathy, monoclonal free light chains are filtered freely and precipitate with Tamm-Horsfall protein in distal tubules, causing obstruction and interstitial injury. Standard urine dipsticks use sulfosalicylic-type indicators that detect albumin, so massive light chain proteinuria can show only trace positivity. This discordance between a near-negative dipstick and heavy proteinuria on quantification is a classic clue to paraprotein-related kidney disease and mandates urine electrophoresis.
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.