A 27-year-old nurse presents to the emergency department with a 30-minute history of palpitations and chest pain. She has been experiencing nausea, vomiting, and diarrhea after eating leftovers 2 days ago. Her husband tells the physician that he is concerned that the patient has been increasingly obsessed about her weight recently. She has a history of major depressive disorder for which she takes escitalopram. Her temperature is 98.6°F (37.0°C), blood pressure is 110/72 mmHg, pulse is 110/min, and respirations are 11/min. Physical examination is notable for dry mucous membranes and tachycardia with regular rhythm. Laboratory testing shows the following results:

Serum:
Na+: 135 mEq/L
Cl-: 96 mEq/L
K+: 2.9 mEq/L
HCO3-: 36 mEq/L

Venous blood gas shows a pH of 7.52. Which of the following tests is most appropriate in differentiating between different etiologies of this patient's lab abnormalities?
Serum glucose
Complete blood count
Urine chloride
Arterial blood gas
Urine chloride
This patient who presents with hypokalemia and metabolic alkalosis may either be vomiting or engaging in diuretic abuse. A urine chloride test can be used to distinguish between these possibilities because urine chloride will be high in diuretic abuse and will be low/normal in surreptitious vomiting.

Metabolic alkalosis is an acid-base disorder that is due to an increased level of serum bicarbonate. This increase can be secondary to increased bicarbonate intake or loss of hydrogen ions. Etiologies consistent with increased intake include antacid use or milk-alkali syndrome. Etiologies consistent with hydrogen ion loss include gastrointestinal losses such as vomiting or renal losses such as hyperaldosteronism, Bartter syndrome, or diuretic abuse. Urine chloride can distinguish between gastrointestinal and renal losses because urine chloride levels will be high in patients with renal tubule dysfunction or diuretic abuse. This is due to increased urinary excretion of ammonium chloride in cases of diarrhea. On the other hand, urine chloride will generally be low in hyperaldosteronism due to loss of chloride with vomiting.

Wu et al. studied the causes of chronic hypokalemia as well as laboratory findings that can be used to distinguish between different etiologies. They show how urine sodium and chloride can be helpful for identifying the surreptitious use of diuretics. The authors recommend the use of the sodium-to-chloride excretion ratio to help diagnose patients with chronic normotensive hypokalemia.

Incorrect Answers:
Answer A: Arterial blood gas is useful for determining the acid-base state of a patient because it simultaneously measures carbon dioxide and bicarbonate levels. This test is not as useful in determining different causes of metabolic alkalosis because all of these etiologies will have similar findings (high bicarbonate and compensatory high carbon dioxide), especially in patient who has already received a venous blood gas.

Answer B: Complete blood count is useful for determining whether a patient is anemic resulting in decreased oxygen delivery to end organs due to decreased carrying capacity for oxygen. In cases of severe anemia and hypoperfusion, peripheral tissues produce lactic acid which will result in an anion gap metabolic acidosis rather than metabolic alkalosis.

Answer C: Serum glucose elevation can result in increased diuresis as well as pseudohyponatremia (decreased measured sodium due to increased glucose). Alternatively, acidosis may also be caused by insulin deficiency in diabetic ketoacidosis. Neither of these findings is consistent with the lab results seen in this patient.

Answer E: Urine toxicology can be used to determine whether the patient is taking any substances of abuse. There is no pathognomonic acid-base finding that can aid in diagnosing overdose and/or substance abuse. However, most standard urine toxicology screens do not detect the use of diuretics.

Bullet Summary:
Urine chloride can distinguish between gastrointestinal and renal causes of metabolic alkalosis.