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Difficulty: Very hardNephrology

Match each set of laboratory findings with the most likely underlying etiology. Assume all patients are normotensive adults who are not currently taking any prescription medications.

  • Arterial pH 7.28, pCO2pCO_2 30 mmHg; Serum: Na+Na^+ 140, K+K^+ 2.2, Cl−Cl^- 115, HCO3−HCO_3^- 15 mEq/L; Urine: Na+Na^+ 40, K+K^+ 30, Cl−Cl^- 25 mEq/L, pH 6.5Distal (Type 1) renal tubular acidosis
  • Arterial pH 7.31, pCO2pCO_2 38 mmHg; Serum: Na+Na^+ 139, K+K^+ 5.9, Cl−Cl^- 112, HCO3−HCO_3^- 19 mEq/L; Urine: Na+Na^+ 35, K+K^+ 20, Cl−Cl^- 35 mEq/L, pH 5.1Hyporeninemic hypoaldosteronism (Type 4 RTA)
  • Arterial pH 7.52, pCO2pCO_2 46 mmHg; Serum: Na+Na^+ 138, K+K^+ 2.6, Cl−Cl^- 92, HCO3−HCO_3^- 36 mEq/L; Urine: Na+Na^+ 45, K+K^+ 30, Cl−Cl^- 8 mEq/L, pH 7.5Surreptitious vomiting
  • Arterial pH 7.50, pCO2pCO_2 43 mmHg; Serum: Na+Na^+ 136, K+K^+ 2.1, Cl−Cl^- 96, HCO3−HCO_3^- 32 mEq/L; Urine: Na+Na^+ 85, K+K^+ 60, Cl−Cl^- 110 mEq/L, elevated urinary calcium excretionBartter syndrome

Answer

Profile 1 matches with Distal (Type 1) RTA, Profile 2 matches with Hyporeninemic hypoaldosteronism (Type 4 RTA), Profile 3 matches with Surreptitious vomiting, and Profile 4 matches with Bartter syndrome.
Correct matching is achieved by systematically applying the diagnostic algorithms for acid-base disturbances. For normal anion gap metabolic acidosis (NAGMA), a positive urine anion gap indicates a renal acidification defect (RTA). A urine pH > 5.5 in the presence of acidemia points to a distal H+H^+ secretion failure (Type 1 RTA), whereas a pH < 5.5 with hyperkalemia indicates defective ammoniagenesis due to aldosterone deficiency or resistance (Type 4 RTA). For metabolic alkalosis, urine chloride < 20 mEq/L defines volume-contracted (saline-responsive) states like vomiting, while urine chloride > 20 mEq/L in a normotensive patient with hypercalciuria points to a thick ascending limb defect, such as Bartter syndrome.

Step-by-Step Solution

1
Calculate the serum anion gap (Na+−[Cl−+HCO3−]Na^+ - [Cl^- + HCO_3^-]) for each profile.
Profile 1 (AG=10) and Profile 2 (AG=8) are normal anion gap metabolic acidoses (NAGMA). Profile 3 and Profile 4 are metabolic alkaloses.
This establishes the primary acid-base disturbance and narrows the differential diagnosis.
2
Evaluate the NAGMA profiles (1 and 2) using serum K+K^+, urine anion gap (Na++K+−Cl−Na^+ + K^+ - Cl^-), and urine pH.
Profile 1 has low K+K^+, a positive UAG (+45), and high urine pH (6.5), diagnostic of Type 1 RTA. Profile 2 has high K+K^+, a positive UAG (+20), and low urine pH (5.1), diagnostic of Type 4 RTA.
UAG distinguishes GI vs renal bicarbonate loss, and urine pH combined with K+K^+ differentiates the RTA subtypes.
3
Evaluate the metabolic alkalosis profiles (3 and 4) using spot urine chloride.
Profile 3 has low urine Cl−Cl^- (8 mEq/L), indicating a saline-responsive state. Profile 4 has high urine Cl−Cl^- (110 mEq/L), indicating a saline-resistant state.
Urine chloride is the most accurate marker of volume status and pathophysiology in metabolic alkalosis.
4
Distinguish the specific etiologies for the alkalotic profiles based on associated findings.
Profile 3's high urine Na+Na^+ despite low Cl−Cl^- suggests active vomiting. Profile 4's high Cl−Cl^- and elevated urine calcium in a normotensive patient points to Bartter syndrome.
Active vomiting obligates Na+Na^+ excretion with bicarbonate, while Bartter syndrome mimics loop diuretic effects, leading to hypercalciuria.

Key Concept

Diagnosis of metabolic acid-base disorders requires systematic integration of serum anion gap, serum potassium, urine anion gap, and urine electrolytes.
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Topics

Nephrology

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