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Difficulty: HardInterpreting Laboratory Results

A 45-year-old patient presents with vague symptoms including fatigue and muscle weakness. During the workup, various acid-base and electrolyte profiles are considered. Match each comprehensive laboratory profile and clinical parameter to its most likely underlying diagnosis.

  • pH 7.52, PaCO2PaCO_2 46 mmHg, HCO3−HCO_3^- 36 mEq/L; Na+Na^+ 140 mEq/L, K+K^+ 2.6 mEq/L, Cl−Cl^- 92 mEq/L; Urine Cl−Cl^- 8 mEq/L; Blood pressure 110/70 mmHg.Surreptitious vomiting
  • pH 7.51, PaCO2PaCO_2 45 mmHg, HCO3−HCO_3^- 35 mEq/L; Na+Na^+ 144 mEq/L, K+K^+ 2.8 mEq/L, Cl−Cl^- 95 mEq/L; Urine Cl−Cl^- 58 mEq/L; Blood pressure 168/105 mmHg.Primary hyperaldosteronism
  • pH 7.28, PaCO2PaCO_2 30 mmHg, HCO3−HCO_3^- 14 mEq/L; Na+Na^+ 138 mEq/L, K+K^+ 3.1 mEq/L, Cl−Cl^- 114 mEq/L; Urine Anion Gap −25-25 mEq/L; Blood pressure 115/75 mmHg.Chronic laxative abuse
  • pH 7.30, PaCO2PaCO_2 31 mmHg, HCO3−HCO_3^- 15 mEq/L; Na+Na^+ 139 mEq/L, K+K^+ 5.7 mEq/L, Cl−Cl^- 113 mEq/L; Urine Anion Gap +18+18 mEq/L; Blood pressure 120/80 mmHg.Type 4 renal tubular acidosis

Answer

Profile 1 matches Surreptitious vomiting; Profile 2 matches Primary hyperaldosteronism; Profile 3 matches Chronic laxative abuse; Profile 4 matches Type 4 renal tubular acidosis.
The correct pairings rely on a systematic approach to acid-base and electrolyte abnormalities. First, the pH, PaCO2, and bicarbonate determine the primary acid-base disorder. Next, the anion gap correctly classifies metabolic acidosis, while the urine chloride differentiates the volume status in metabolic alkalosis. Finally, the urine anion gap differentiates renal from extrarenal causes of non-anion gap metabolic acidosis.

Step-by-Step Solution

1
Calculate the serum anion gap and assess the primary acid-base disturbance for each laboratory profile.
Profiles 1 and 2 exhibit metabolic alkalosis (pH > 7.45, high bicarbonate). Profiles 3 and 4 exhibit normal anion gap (hyperchloremic) metabolic acidosis (pH < 7.35, low bicarbonate).
Establishing the primary disorder and the serum anion gap immediately narrows down the diagnostic categories into distinct pathophysiologic mechanisms.
2
Differentiate the two metabolic alkalosis etiologies using urine chloride.
Profile 1 has a low urine chloride (saline-responsive), correctly matching surreptitious vomiting. Profile 2 has a high urine chloride alongside hypertension (saline-resistant), correctly matching primary hyperaldosteronism.
Urine chloride reflects volume status; low urinary chloride implies a volume-contracted state driving the alkalosis, whereas high urinary chloride reflects mineralocorticoid-induced volume expansion.
3
Differentiate the two non-anion gap metabolic acidosis etiologies using serum potassium and the urine anion gap (UAG).
Profile 3 has hypokalemia and a negative UAG (gastrointestinal bicarbonate loss), matching laxative abuse. Profile 4 has hyperkalemia and a positive UAG (impaired renal ammonium excretion), matching Type 4 renal tubular acidosis.
The UAG indirectly measures urinary ammonium. A negative UAG means the kidneys are responding normally to the acidosis, pointing to an extrarenal etiology.

Key Concept

Interpretation of secondary diagnostic indices (urine chloride and urine anion gap) in complex acid-base disorders.
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Interpreting Laboratory Results

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