Renal · Year 2 · from Renal
Case 2: Type 2 (Proximal) Renal Tubular Acidosis in Multiple Myeloma
Patient Presentation
A 67-year-old male recently diagnosed with multiple myeloma presents with bone pain, fatigue, and laboratory abnormalities detected during oncology workup.
History of Present Illness
- Diagnosed with IgG kappa multiple myeloma 2 weeks ago
- Progressive fatigue and lower back pain
- 12-pound weight loss over 3 months
- No vomiting or diarrhea
- No diuretic or acetazolamide use
Physical Examination
- Blood pressure: 132/78 mmHg
- Pallor
- Point tenderness over lumbar spine
- No hepatosplenomegaly
Workup
Laboratory Studies:
- Serum potassium: 3.1 mEq/L (low)
- Serum bicarbonate: 16 mEq/L (low)
- Anion gap: 11 (normal)
- Serum phosphorus: 2.0 mg/dL (low)
- Serum uric acid: 2.2 mg/dL (low)
- Serum glucose: 98 mg/dL (normal)
- Urine pH: 5.2 (acidic - can acidify once threshold reached)
- Urine glucose: 2+ (despite normal serum glucose)
- Urine amino acids: Elevated (generalized aminoaciduria)
- Fractional excretion of bicarbonate (after bicarbonate load): 18%
- Serum free light chains: Kappa markedly elevated
Diagnosis
Type 2 (Proximal) Renal Tubular Acidosis with Fanconi Syndrome Secondary to Light Chain Deposition
Discussion
This case illustrates proximal RTA and Fanconi syndrome:
- Reduced Bicarbonate Threshold: The lecture describes how proximal RTA results from impaired bicarbonate reabsorption. Plasma bicarbonate falls to a new, lower threshold (typically 15-18 mEq/L) where the reduced filtered load can finally be reclaimed.
- Acidic Urine pH: Unlike distal RTA, the urine can be appropriately acidified once the bicarbonate threshold is reached. This patient's urine pH of 5.2 demonstrates intact distal acidification.
- Fanconi Syndrome Components: Light chain deposition damages the proximal tubule globally, causing glucosuria at normal glucose levels, aminoaciduria, phosphaturia (hypophosphatemia), and uricosuria (hypouricemia) in addition to bicarbonaturia.
- Fractional Excretion of Bicarbonate: The elevated FE-HCO3 >15% (after raising serum bicarbonate above the threshold) confirms proximal tubule bicarbonate wasting.
- Hypokalemia Mechanism: Bicarbonaturia creates a non-reabsorbable anion in the collecting duct that enhances potassium secretion.
Treatment
- Chemotherapy for underlying myeloma (bortezomib-based regimen)
- High-dose bicarbonate replacement (10-15 mEq/kg/day needed because most is lost in urine)
- Potassium supplementation
- Phosphate supplementation for hypophosphatemia
- Thiazide diuretics may help by inducing mild volume contraction that enhances proximal reabsorption
Clinical Pearl
Proximal RTA requires much higher alkali doses than distal RTA because administered bicarbonate is lost in the urine until levels exceed the lowered threshold. The condition is self-limiting - bicarbonate stabilizes at 15-18 mEq/L without treatment. Associated Fanconi syndrome features (glucosuria with normal glucose, hypophosphatemia, hypouricemia) help identify the proximal tubule as the problem site.