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Difficulty: HardNephrology

Match each clinical vignette describing a patient with a normal anion gap metabolic acidosis to the most likely corresponding serum and urine laboratory profile.

  • A 35-year-old woman with Sjogren syndrome who is evaluated for recurrent calcium phosphate nephrolithiasis.Serum potassium 2.8 mEq/L, urine pH 6.8, urine anion gap +22 mEq/L
  • A 50-year-old man with multiple myeloma who presents with bone pain, osteomalacia, and generalized aminoaciduria.Serum potassium 3.2 mEq/L, urine pH 5.2, urine anion gap +14 mEq/L, urine dipstick positive for glucose
  • A 65-year-old man with advanced diabetic nephropathy currently taking a high-dose angiotensin-converting enzyme inhibitor.Serum potassium 5.7 mEq/L, urine pH 5.3, urine anion gap +28 mEq/L
  • A 28-year-old woman with chronic, profuse watery bowel movements and severe generalized fatigue.Serum potassium 2.9 mEq/L, urine pH 5.0, urine anion gap -35 mEq/L

Answer

Sjogren syndrome with nephrolithiasis matches to the profile with high urine pH and positive UAG (Type 1 RTA). Multiple myeloma with aminoaciduria matches to positive urine glucose, positive UAG, and low potassium (Type 2 RTA). Diabetic nephropathy on an ACE inhibitor matches to hyperkalemia and positive UAG (Type 4 RTA). Chronic diarrhea matches to negative UAG and low potassium.
The differentiation of normal anion gap metabolic acidosis relies heavily on the urine anion gap (UAG) and serum potassium levels. A negative UAG indicates intact renal ammonium excretion, typical of extrarenal bicarbonate loss like diarrhea. A positive UAG indicates impaired renal ammonium excretion, which is diagnostic of renal tubular acidosis (RTA). Among the RTAs, Type 4 is distinct for hyperkalemia. Type 1 and Type 2 both present with hypokalemia, but Type 1 is characterized by an inability to acidify urine (pH >5.5> 5.5) leading to calcium phosphate stones, while Type 2 often occurs as part of Fanconi syndrome with glycosuria and allows for an acidic urine pH when systemic acidemia is severe enough to overwhelm the proximal tubule defect.

Step-by-Step Solution

1
Analyze the patient with Sjogren syndrome and nephrolithiasis.
Identified as having Type 1 (distal) Renal Tubular Acidosis.
Type 1 RTA involves a defect in distal H+H^+ secretion, resulting in a positive urine anion gap, hypokalemia, and an inability to acidify urine (pH >5.5> 5.5), which strongly promotes calcium phosphate precipitation.
2
Analyze the patient with multiple myeloma, osteomalacia, and aminoaciduria.
Identified as having Fanconi syndrome causing Type 2 (proximal) Renal Tubular Acidosis.
This defect in proximal bicarbonate reabsorption is accompanied by glycosuria (positive urine dipstick), aminoaciduria, and phosphaturia. Distal acidification is intact, allowing urine pH <5.5< 5.5 when systemically acidemic.
3
Analyze the patient with diabetic nephropathy taking an ACE inhibitor.
Identified as having Type 4 (hyperkalemic) Renal Tubular Acidosis.
Hypoaldosteronism impairs distal sodium reabsorption and potassium/hydrogen secretion. It is uniquely characterized by hyperkalemia alongside a positive urine anion gap.
4
Analyze the patient with chronic profuse watery diarrhea.
Identified as having gastrointestinal bicarbonate loss.
The kidneys respond appropriately by excreting acid as ammonium chloride, resulting in a negative urine anion gap. Potassium is lost in the stool, causing profound hypokalemia.

Key Concept

Normal anion gap metabolic acidosis (NAGMA) differentiation using serum potassium and urine anion gap.
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Topics

Nephrology

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