Hyperchloremic Metabolic Acidosis — The Normal Anion Gap
A patient with weeks of chronic diarrhea presents with a metabolic acidosis — but the anion gap is normal. This case shows how the urine anion gap distinguishes gastrointestinal bicarbonate loss from renal tubular acidosis, and how chloride holds the diagnostic key.
Presentation
A 58-year-old woman presents with three weeks of profuse watery diarrhea, initially attributed to a viral illness but persisting and worsening. She reports fatigue, lightheadedness on standing, and muscle cramps. She has no history of renal disease, diabetes, or autoimmune disease and takes no regular medications. On examination she is volume-contracted (orthostatic BP drop of 18 mmHg systolic) with reduced skin turgor. The metabolic panel shows a metabolic acidosis — but the question this case turns on is whether the anion gap is elevated or normal.
Initial laboratory values
The pattern is unmistakable once the gap is calculated: a low bicarbonate (16) with a markedly elevated chloride (115) and a normal anion gap. The acidosis is real, but no unmeasured anions are accumulating — instead, bicarbonate has been lost directly and chloride has risen to maintain electroneutrality. This is the definition of a hyperchloremic (normal anion gap) metabolic acidosis, also called NAGMA.
Anion gap calculation
Applying AG = Na⁺ − (Cl⁻ + HCO₃⁻): 134 − (115 + 16) = 134 − 131 = 3 mEq/L. Against the conventional normal range of 8–12, this is at the low end or even slightly below normal — certainly not elevated. The acidemia (pH 7.28) with a normal gap confirms that the problem is bicarbonate loss or gain of chloride-rich fluid, not accumulation of unmeasured acids. Verify the arithmetic in the anion gap calculator. The clinical interpretation of a normal-gap acidosis — and the full differential — is laid out on the NAGMA interpretation page.
Urine anion gap — distinguishing GI from renal loss
Once a normal-anion-gap acidosis is confirmed, the next step is to determine whether the kidney is appropriately excreting acid (pointing to GI bicarbonate loss) or failing to do so (pointing to renal tubular acidosis). The urine anion gap (UAG) — a surrogate for urinary ammonium (NH₄⁺) excretion — answers that question.
A spot urine electrolyte panel returns: urine Na⁺ 30, urine K⁺ 40, urine Cl⁻ 90 mEq/L. Apply the formula:
UAG = urine Na⁺ + urine K⁺ − urine Cl⁻ = 30 + 40 − 90 = −20
A negative UAG (here, −20) indicates high urinary NH₄⁺ excretion — the kidney is appropriately ramping up acid excretion to compensate for the systemic acidosis. That rules out renal tubular acidosis (where the UAG would be positive or zero) and points squarely to an extrarenal source of bicarbonate loss. In this clinical context — chronic diarrhea — the diagnosis is clear. The GI tract loses bicarbonate-rich secretions, the kidney compensates appropriately, and the UAG confirms the kidney is doing its job. Run the same urine values through the urine anion gap calculator to reproduce the result.
The urine anion gap converts a single spot urine into a renal physiology test. A negative value means the kidney is excreting ammonium appropriately (GI diarrhea, orotic aciduria, or toluene). A positive value means the kidney is failing to excrete ammonium — the signature of renal tubular acidosis. Always calculate the UAG before attributing a NAGMA to "diarrhea" by history alone.
Diagnosis and treatment
The unifying diagnosis is chronic diarrhea-induced normal anion gap metabolic acidosis, with concurrent hypokalemia (K⁺ 2.9) and volume contraction (elevated BUN, orthostasis). The pathophysiology: diarrheal stool is rich in bicarbonate and potassium, so prolonged losses deplete both. Chloride rises to replace the lost bicarbonate on the anion ledger, holding the anion gap normal. The hypokalemia reflects both GI potassium loss and the kaliuresis driven by bicarbonaturia and secondary hyperaldosteronism. The causes of normal-AG acidosis beyond diarrhea — renal tubular acidosis, saline dilution, ureteroenteric fistula, pancreatic fistula — are catalogued on the causes of normal AG acidosis page.
Treatment is three-pronged: volume repletion with isotonic saline to restore perfusion, potassium repletion (the hypokalemia must be corrected before bicarbonate is given, because alkalinization drives potassium intracellularly and can precipitate dangerous hypokalemia), and oral bicarbonate replacement (sodium bicarbonate or potassium citrate) once potassium is safe. The underlying diarrhea is investigated and treated in parallel — stool studies, colonoscopy, and a search for chronic causes (inflammatory bowel disease, pancreatic insufficiency, surreptitious laxative use). The principles of managing NAGMA across its causes are detailed in the NAGMA guide.
Key lessons from this case
- Calculate the anion gap before assuming an acidosis is "the bad kind." A normal gap with acidemia is hyperchloremic (NAGMA); the differential and workup are completely different from HAGMA.
- Elevated chloride is the fingerprint of NAGMA — chloride rises to replace the lost bicarbonate and holds the gap normal.
- The urine anion gap is the decisive test — negative confirms GI loss (appropriate renal compensation), positive confirms renal tubular acidosis (renal failure to acidify).
- Correct potassium before bicarbonate — alkalinization shifts potassium intracellularly and can provoke life-threatening hypokalemia in a depleted patient.