In a 75-year-old woman attempting weight loss who reports taking over‑the‑counter pills of unknown name, with serum sodium 134 mmol/L, serum potassium 3.2 mmol/L, serum chloride 90 mmol/L, serum bicarbonate 22 mmol/L, urine potassium 15 mmol/L, and urine chloride 13 mmol/L, what is the most likely cause of her hypokalemia?

Answer: A. Laxative abuse

The most likely cause of this patient's hypokalemia is laxative abuse, as evidenced by the combination of hypokalemia with low urine potassium (15 mmol/L) and low urine chloride (13 mmol/L), indicating extrarenal potassium losses.

Diagnostic Reasoning

The key to solving this case lies in interpreting the urine potassium and urine chloride values in the context of hypokalemia:

Urine Potassium Interpretation

• Urine potassium <15-20 mmol/L indicates appropriate renal conservation of potassium, suggesting extrarenal losses 1, 2, 3
• This patient's urine potassium of 15 mmol/L sits at the threshold, consistent with extrarenal potassium wasting
• In renal potassium wasting (diuretics, RTA), urine potassium would typically be >20-30 mmol/L despite hypokalemia 1, 3

Urine Chloride as the Discriminator

The urine chloride of 13 mmol/L is critically low and serves as the definitive clue:

• Urine chloride <20-25 mmol/L indicates volume depletion with appropriate renal chloride conservation 4
• This pattern is characteristic of gastrointestinal losses (vomiting or laxative abuse) 4
• Laxative abuse causes colonic potassium and fluid losses, leading to volume depletion and secondary hyperaldosteronism

Why Not the Other Options?

B. Diuretic abuse - Would show:

• High urine potassium (>20-30 mmol/L) due to ongoing renal losses 5, 6, 7
• High urine chloride (>40 mmol/L) from diuretic-induced natriuresis and chloruresis
• Loop and thiazide diuretics cause hypokalaemia through renal mechanisms 5

C. Licorice ingestion - Would show:

• High urine potassium from mineralocorticoid excess
• Typically presents with hypertension and metabolic alkalosis
• Not consistent with low urine potassium

D. Renal tubular acidosis - Would show:

• High urine potassium (>20 mmol/L) indicating renal potassium wasting 3
• Type 1 (distal) RTA presents with metabolic acidosis, not the relatively normal bicarbonate (22 mmol/L) seen here
• The bicarbonate of 22 mmol/L is at the lower end of normal, not frankly acidotic

Clinical Context

This 75-year-old woman taking over-the-counter weight loss pills strongly suggests:

• Laxative abuse is common in individuals attempting weight loss
• The pattern of low urine potassium and chloride with hypokalemia is pathognomonic for surreptitious laxative use 3
• Patients often deny laxative use, making the laboratory pattern essential for diagnosis

Common Pitfalls

1. Don't rely solely on urine potassium - The urine chloride provides critical additional information to distinguish renal from extrarenal losses 4
2. Borderline urine potassium values (around 15 mmol/L) require careful interpretation with other parameters
3. Consider the clinical context - Over-the-counter "weight loss" products frequently contain stimulant laxatives
4. Metabolic alkalosis may be mild or absent in chronic laxative abuse, unlike acute vomiting

The combination of hypokalemia, low urine potassium (<20 mmol/L), and markedly low urine chloride (<20 mmol/L) in a patient seeking weight loss makes laxative abuse the most likely diagnosis 3, 4.