A 62-year-old man with back pain and normocytic anaemia develops AKI with creatinine 6.8 mg/dL. Urine dipstick shows trace protein, but the 24-hour urine protein quantifies at 3.5 g. Urinalysis shows no cells or casts. Which test confirms the nature of the proteinuria?
- A Repeat urine dipstick on an alkalised specimen
- B Sulfosalicylic acid precipitation test on the urine sample ✓
- C Urine sodium and fractional excretion of sodium
- D Serum and urine electrophoresis with immunofixation
Explanation
The dipstick uses albumin-specific dye binding and misses monoclonal immunoglobulin light chains. The sulfosalicylic acid test precipitates all proteins, so a strongly positive SSA test with a negative dipstick indicates non-albumin protein such as Bence Jones protein, the hallmark of myeloma cast nephropathy. Electrophoresis with immunofixation characterises the monoclonal band and confirms the diagnosis, but the question asks for the test that confirms the discordant protein is non-albumin, which is the SSA test. Light chains obstruct tubules and form laminated casts with giant cell reaction on biopsy.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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