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Difficulty: HardInterpreting Laboratory Results

A 66-year-old man presents to the clinic for a follow-up evaluation of acute kidney injury. Over the past two months, he has experienced worsening fatigue and a dull, constant ache in his mid-to-lower back that is not relieved by rest. He has no significant past medical history and takes no medications. Vital signs are temperature 37.1°C (98.8°F), blood pressure 138/86 mmHg, pulse 82/min, and respirations 14/min. Physical examination reveals conjunctival pallor and mild tenderness to palpation over the lumbar spine. Laboratory studies are obtained and shown below:

TestResultReference Range
Hemoglobin9.2 g/dL13.5–17.5 g/dL
Mean corpuscular volume89 fL80–100 fL
Blood urea nitrogen42 mg/dL8–20 mg/dL
Serum creatinine3.4 mg/dL0.7–1.3 mg/dL
Calcium11.6 mg/dL8.4–10.2 mg/dL
Total protein9.4 g/dL6.0–7.8 g/dL
Albumin3.6 g/dL3.5–5.0 g/dL

Urinalysis
Color: Yellow
Specific gravity: 1.012
pH: 6.0
Protein: Trace
Blood: Negative
Glucose: Negative
Microscopy: 1-2 WBCs/hpf, 0-1 RBCs/hpf, occasional granular casts

A 24-hour urine collection is subsequently ordered and reveals a total protein excretion of 4.1 g/day. Based on these laboratory findings, which of the following is the most likely underlying mechanism of this patient's renal failure?

  1. Intratubular obstruction by precipitated monoclonal proteinsAnswer
  2. B
    Glomerular deposition of light chain-derived amyloid fibrils
  3. C
    Renal hypoperfusion secondary to hypercalcemia-induced diuresis
  4. D
    Nephrotoxic injury to proximal tubule cells by analgesic use
  5. E
    Deposition of calcium phosphate crystals in the renal interstitium

Answer

Intratubular obstruction by precipitated monoclonal proteins (Myeloma cast nephropathy)
The correct answer describes myeloma cast nephropathy. This patient's laboratory results demonstrate a significantly elevated serum protein gap (Total protein 9.4 - Albumin 3.6 = 5.8 g/dL), hypercalcemia, and normocytic anemia, which strongly suggest multiple myeloma. Furthermore, there is a striking discrepancy between the urine dipstick ('Trace' protein) and the 24-hour urine collection (4.1 g/day of protein). Because standard urine dipsticks primarily detect albumin, this discrepancy indicates the massive excretion of a non-albumin protein—specifically, monoclonal free light chains (Bence Jones proteins). In multiple myeloma, these freely filtered light chains precipitate with Tamm-Horsfall mucoprotein in the distal tubules, causing intratubular obstruction and toxic injury (cast nephropathy).

Step-by-Step Solution

1
Calculate the serum protein gap.
Total protein (9.4 g/dL) - Albumin (3.6 g/dL) = 5.8 g/dL (Normal is < 4 g/dL).
An elevated protein gap indicates the presence of excess non-albumin proteins in the serum, highly suggestive of a monoclonal gammopathy (paraproteinemia).
2
Compare the urine dipstick protein result with the 24-hour urine protein result.
Dipstick shows 'Trace' protein, but the 24-hour collection shows 4.1 g/day (massive proteinuria).
Urine dipsticks utilize a colorimetric assay that primarily detects albumin. A 24-hour urine collection or spot protein-to-creatinine ratio measures all proteins. A large discrepancy indicates heavy excretion of non-albumin proteins, such as free light chains (Bence Jones proteins).
3
Synthesize the laboratory findings with the clinical presentation.
The combination of an elevated protein gap, non-albumin proteinuria, hypercalcemia, normocytic anemia, bone pain, and acute kidney injury points to multiple myeloma.
These are the classic 'CRAB' criteria for multiple myeloma (Calcium elevation, Renal insufficiency, Anemia, Bone lesions).
4
Determine the specific mechanism of renal failure based on the isolated non-albumin proteinuria.
Myeloma cast nephropathy (intratubular obstruction by precipitated monoclonal proteins).
Free light chains are freely filtered at the glomerulus and precipitate with Tamm-Horsfall proteins in the distal tubules, forming obstructing casts. This causes renal failure without significant albuminuria.

Key Concept

Interpreting discrepancies between urine dipstick and quantitative protein tests to identify non-albumin proteinuria (e.g., Bence Jones proteins) in the setting of multiple myeloma.
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Topics

Interpreting Laboratory Results

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