A 64-year-old man with back pain and anaemia is found to have creatinine 4.8 mg/dL. Total urine protein on 24-hour collection is 4.5 g/day, but the urine dipstick test for protein is trace positive. Serum free light chains show kappa predominance. What explains the discrepancy between quantified proteinuria and the dipstick result?

• A Heavy proteinuria saturates the dipstick reagent strip
• B Light chains are too large to be filtered by the glomerulus
• C The dipstick reacts with Bence Jones protein producing a false negative colour change
• D The dipstick detects only albumin and misses free monoclonal light chains ✓

Explanation

The urine dipstick uses a tetrabromophenol blue indicator that binds primarily to albumin and is relatively insensitive to free immunoglobulin light chains. In myeloma cast nephropathy, filtered monoclonal light chains obstruct tubules and are toxic to tubular epithelium, producing substantial 24-hour proteinuria with a near-negative dipstick. This discordance should always raise suspicion for paraprotein-related kidney disease, confirmed by urine electrophoresis and immunofixation. Light chains are small enough to be freely filtered.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.