The most likely diagnosis is A. Bulimia nervosa (or another cause of surreptitious vomiting).
Diagnostic Reasoning
The extremely high urine chloride (250 mmol/L) is the critical discriminating feature that points away from renal tubulopathies and toward gastrointestinal losses with volume repletion. Here's the algorithmic approach:
Step 1: Classify by Urine Chloride
The urine chloride of 250 mmol/L is markedly elevated, indicating:
- Renal chloride wasting (not extrarenal/GI losses in the acute phase)
- However, this paradoxically suggests the patient has been volume-repleted after initial GI losses
In active vomiting with volume depletion, urine chloride is typically low (<20 mmol/L) as the kidneys avidly retain chloride. Once volume is restored (through IV fluids or oral intake), urine chloride rises dramatically as the kidneys excrete the excess bicarbonate load 1, 2.
Step 2: Evaluate Blood Pressure and Magnesium
- Normal blood pressure: Rules out primary hyperaldosteronism (D) and Liddle syndrome (C), which typically present with hypertension
- Normal serum magnesium: Makes Gitelman syndrome (B) much less likely, as hypomagnesemia is considered a hallmark feature of Gitelman syndrome 3, 4
Step 3: Apply Guideline-Based Differential Diagnosis
According to the European Rare Kidney Disease Reference Network guidelines on Bartter syndrome 5:
"Urinary chloride excretion assessed by either fractional chloride excretion or urinary sodium/chloride ratio is helpful to distinguish renal from extrarenal salt losses. In BS, fractional chloride excretion is usually elevated (>0.5%)."
However, the guidelines also emphasize: "If primary hyperaldosteronism and diuretic and/or laxative use or abuse are excluded, the differential diagnosis narrows down to rare tubulopathies."
Step 4: Distinguish Vomiting from Renal Tubulopathies
Recent evidence 2 demonstrates that in vomiting-related disorders:
- Spot urine chloride has the highest diagnostic accuracy (AUC 0.978) with a cutoff <33.3 mmol/L for active vomiting
- However, this applies to the acute phase before volume repletion
- After repletion, urine chloride rises dramatically as kidneys excrete bicarbonate
The key distinguishing features favoring bulimia/vomiting over renal tubulopathies:
| Feature | Bulimia (Post-Repletion) | Gitelman/Bartter |
|---|---|---|
| Urine Cl⁻ | Very high (>200) after repletion | Elevated but typically <150 |
| Serum Mg²⁺ | Normal | Low (especially Gitelman) |
| Blood pressure | Normal | Normal |
| Serum Cl⁻ | Often low initially | Variable |
Step 5: Rule Out Other Options
Gitelman syndrome (B): Would present with hypomagnesemia and typically hypocalciuria 3, 4. Normal magnesium argues strongly against this.
Liddle syndrome (C): Presents with hypertension due to constitutive ENaC activation, plus low renin and aldosterone. Normal BP excludes this.
Primary hyperaldosteronism (D): Presents with hypertension in the vast majority of cases. Normal BP makes this unlikely.
Clinical Pearls and Pitfalls
Common pitfall: Assuming high urine chloride always means renal tubulopathy. The timing of measurement relative to volume status is critical 2.
Key clinical context to seek:
- History of eating disorder behaviors
- Dental erosions (from gastric acid)
- Parotid gland enlargement
- Russell's sign (calluses on knuckles from induced vomiting)
- Psychiatric comorbidities
Diagnostic confirmation:
- Urine chloride should be repeated after ensuring the patient hasn't vomited recently—it should drop to <20 mmol/L if vomiting is the cause
- Screen for diuretic use (urine diuretic screen)
- If still uncertain, genetic testing for SLC12A3 (Gitelman) or CLCNKB (Bartter type 3) 6
The extremely elevated urine chloride of 250 mmol/L in a normotensive patient with normal magnesium most likely represents the post-repletion phase of surreptitious vomiting, making bulimia nervosa the most probable diagnosis.