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Difficulty: HardNephrology

Match each clinical presentation and associated laboratory profile to the most likely underlying pathophysiologic mechanism.

  • A 42-year-old woman with recurrent calcium phosphate kidney stones. Labs show Na+Na^+ 140140 mEq/L, K+K^+ 2.72.7 mEq/L, Cl−Cl^- 115115 mEq/L, HCO3−HCO_3^- 1313 mEq/L. Urine studies reveal pH 6.86.8, Na+Na^+ 4545 mEq/L, K+K^+ 2020 mEq/L, Cl−Cl^- 3535 mEq/L.Impaired distal tubular secretion of hydrogen ions
  • A 58-year-old man with long-standing type 2 diabetes mellitus. Labs show Na+Na^+ 138138 mEq/L, K+K^+ 5.65.6 mEq/L, Cl−Cl^- 110110 mEq/L, HCO3−HCO_3^- 1616 mEq/L. Urine studies reveal pH 5.25.2, Na+Na^+ 5050 mEq/L, K+K^+ 2525 mEq/L, Cl−Cl^- 6565 mEq/L.Decreased production or effect of aldosterone
  • A 24-year-old woman evaluated for episodic severe muscle weakness. Labs show Na+Na^+ 136136 mEq/L, K+K^+ 2.92.9 mEq/L, Cl−Cl^- 114114 mEq/L, HCO3−HCO_3^- 1212 mEq/L. Urine studies reveal pH 5.05.0, Na+Na^+ 3030 mEq/L, K+K^+ 1515 mEq/L, Cl−Cl^- 7575 mEq/L.Profound gastrointestinal bicarbonate loss
  • A 32-year-old woman with lethargy, poor dentition, and muscle cramps. Labs show Na+Na^+ 139139 mEq/L, K+K^+ 2.42.4 mEq/L, Cl−Cl^- 9292 mEq/L, HCO3−HCO_3^- 3535 mEq/L. Urine studies reveal pH 7.27.2, Na+Na^+ 4040 mEq/L, K+K^+ 3535 mEq/L, Cl−Cl^- 88 mEq/L.Gastric volume and hydrochloric acid depletion

Answer

Patient 1 matches impaired distal tubular secretion of hydrogen ions (Type 1 RTA). Patient 2 matches decreased production or effect of aldosterone (Type 4 RTA). Patient 3 matches profound gastrointestinal bicarbonate loss. Patient 4 matches gastric volume and hydrochloric acid depletion.
Each laboratory profile requires multi-step analysis: calculating the serum anion gap and, if normal, calculating the urine anion gap (UAG=Na++K+−Cl−\text{UAG} = Na^+ + K^+ - Cl^-). A negative UAG indicates intact renal ammonium excretion (e.g., gastrointestinal bicarbonate loss), whereas a positive UAG indicates impaired renal acidification (e.g., renal tubular acidosis). The profiles are further differentiated by serum potassium levels and urine pH. For metabolic alkalosis, the urine chloride level successfully differentiates saline-responsive from saline-resistant forms.

Step-by-Step Solution

1
Calculate the serum anion gap for each acid-base disturbance using the formula AG=Na+−(Cl−+HCO3−)\text{AG} = Na^+ - (Cl^- + HCO_3^-).
Patients 1, 2, and 3 have a normal serum anion gap (between 10−1210-12 mEq/L), identifying normal anion gap metabolic acidosis (NAGMA). Patient 4 has metabolic alkalosis (HCO3−HCO_3^- of 3535 mEq/L).
Differentiates between high anion gap metabolic acidosis (HAGMA), NAGMA, and metabolic alkalosis to establish the primary disorder.
2
For patients with NAGMA (1, 2, and 3), calculate the urine anion gap using the formula UAG=Na++K+−Cl−\text{UAG} = Na^+ + K^+ - Cl^-.
Patient 1: UAG=45+20−35=+30\text{UAG} = 45 + 20 - 35 = +30. Patient 2: UAG=50+25−65=+10\text{UAG} = 50 + 25 - 65 = +10. Patient 3: UAG=30+15−75=−30\text{UAG} = 30 + 15 - 75 = -30.
A negative UAG indicates appropriate renal ammonium excretion (e.g., GI loss of HCO3−HCO_3^-), while a positive UAG indicates impaired renal acid excretion (e.g., Renal Tubular Acidosis).
3
Evaluate serum potassium and urine pH for patients with positive UAG to differentiate RTA types.
Patient 1 has hypokalemia and a urine pH > 5.55.5, indicating Type 1 RTA. Patient 2 has hyperkalemia and a urine pH < 5.55.5, indicating Type 4 RTA.
Type 1 RTA involves a distal H+H^+ secretion defect (high urine pH, hypokalemia). Type 4 involves aldosterone deficiency or resistance (low urine pH, hyperkalemia).
4
Evaluate the metabolic alkalosis in Patient 4 using the urine chloride level.
Patient 4 has a urine chloride of 88 mEq/L (which is < 2020 mEq/L).
Low urine chloride signifies a chloride-responsive metabolic alkalosis, commonly due to gastric fluid loss (vomiting), supporting the clinical clues of lethargy and poor dentition.

Key Concept

Differentiation of normal anion gap metabolic acidosis (NAGMA) using the urine anion gap, serum potassium, and urine pH, alongside identifying chloride-responsive metabolic alkalosis via urine chloride levels.
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Topics

Nephrology

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