Normal Anion Gap Metabolic Acidosis (NAGMA)
In NAGMA the anion gap stays in the normal 8–12 mEq/L range while bicarbonate falls — a hyperchloremic pattern caused by bicarbonate loss (GI) or failure to excrete acid (renal). The urine anion gap tells the two apart.
What is NAGMA?
Normal anion gap metabolic acidosis (NAGMA) — also called hyperchloremic metabolic acidosis — is a metabolic acidosis in which the anion gap remains in the normal 8–12 mEq/L range despite a falling serum bicarbonate. The key physiological insight: in NAGMA the body is losing bicarbonate (or failing to regenerate it) faster than it is gaining unmeasured anions. To preserve electroneutrality, chloride rises to fill the gap left by the missing HCO₃⁻ — hence the alternative name "hyperchloremic."
Contrast this with high anion gap metabolic acidosis (HAGMA), where a new acid (lactate, ketones, toxin) is added to the blood and its anion widens the gap. In NAGMA no new anion is added; instead chloride simply replaces bicarbonate. The formula Na⁺ − (Cl⁻ + HCO₃⁻) therefore stays roughly constant: as HCO₃⁻ drops, Cl⁻ rises by the same amount. This is why a low bicarbonate with a rising chloride and a normal gap is the biochemical fingerprint of NAGMA.
Causes of normal anion gap acidosis
The causes split cleanly into three buckets: GI bicarbonate loss, renal acid excretion failure (the renal tubular acidoses), and iatrogenic dilution. The full differential is on our dedicated causes of normal anion gap acidosis page; the table below is the working summary.
| Category | Cause | Mechanism |
|---|---|---|
| GI bicarbonate loss | Diarrhea | Loss of HCO₃⁻-rich pancreatic secretions; Cl⁻ retained. |
| GI fistulas / ostomies | Drainage of small-bowel or pancreatic secretions. | |
| Ureterosigmoidostomy | Colon reabsorbs Cl⁻ in exchange for secreting HCO₃⁻ into urine. | |
| Renal (RTA) | Distal (Type I) RTA | Failure to excrete H⁺ → low NH₄⁺; urine pH > 5.5, K⁺ low. |
| Proximal (Type II) RTA | Wasted HCO₃⁻ reabsorption; urine pH rises once above threshold. | |
| Type IV RTA | Hypoaldosteronism → reduced NH₃ buffering; urine pH < 5.5, K⁺ high. | |
| Iatrogenic | Normal saline dilution | Large-volume 0.9% NaCl lowers serum HCO₃⁻ by dilution; SID falls. |
| Acetazolamide, ammonium chloride | Carbonic anhydrase inhibition → urinary HCO₃⁻ loss; acid load. |
Diarrhea is overwhelmingly the most common cause of NAGMA in clinical practice — and the easiest bedside clue is that hypokalemia almost always accompanies it. Renal tubular acidosis is rarer but should be suspected whenever NAGMA appears without an obvious GI source, especially with an abnormal potassium: distal (Type I) and proximal (Type II) RTA present with hypokalemia, whereas Type IV RTA presents with hyperkalemia from hypoaldosteronism (Kraut & Madias, CJASN; LITFL Acid-Base). For the full workup and management of each, see our NAGMA full guide.
Distinguishing GI from renal causes: the urine anion gap
The single most useful test for separating GI-driven NAGMA from renal-driven NAGMA is the urine anion gap (UAG): (Urine Na⁺ + Urine K⁺) − Urine Cl⁻. The UAG is a surrogate for urinary ammonium (NH₄⁺) excretion — the kidney's main route for eliminating acid (Kraut & Madias, StatPearls 'Serum Anion Gap'). Because NH₄⁺ is hard to measure directly, the UAG estimates it indirectly: chloride accompanies NH₄⁺ into the urine, so a high urinary Cl⁻ (with Na⁺ + K⁺ relatively low) means the kidney is excreting NH₄⁺ appropriately.
| Urine anion gap | Interpretation | Typical cause |
|---|---|---|
| Negative (≈ −20 to −50) | Appropriate ↑ NH₄⁺ excretion — kidney working fine | GI bicarbonate loss (diarrhea) |
| Positive (≈ +20 to +50) | Failure to excrete NH₄⁺ — kidney is the problem | Renal tubular acidosis (RTA Types I, II, IV) |
The rule is simple: a negative UAG points to a GI cause (the kidney is appropriately dumping NH₄⁺ to compensate), while a positive UAG points to a renal cause (the kidney cannot excrete NH₄⁺). Run your own numbers with our urine anion gap calculator, and read the derivation in the urine anion gap formula reference. One caveat: the UAG is unreliable when urine sodium is very low (<25 mEq/L, i.e. poor distal Na⁺ delivery) or when unmeasured anions (ketoacids, hippurate in toluene intoxication) are present — in those cases use the urine osmolar gap instead.
A normal-AG acidosis can hide behind a high-AG acidosis. If the bicarbonate falls more than the anion gap rises, a concurrent NAGMA is present alongside HAGMA — a mixed metabolic disorder that is easy to miss without the delta-delta analysis. Common combinations: sepsis (lactic HAGMA) with diarrhea, or DKA (ketoacidosis) with saline-induced NAGMA. Calculate the delta ratio and review the full differential in our guide to mixed acid–base disorders.
Practical interpretation summary
- Pattern: low HCO₃⁻, high Cl⁻, normal anion gap (8–12 mEq/L) = NAGMA.
- First branch point: is there an obvious GI cause (diarrhea)? If yes, treat volume and potassium — the UAG will be negative.
- If no GI cause: check the UAG. A positive gap means renal tubular acidosis — let the serum potassium subtype it (low K⁺ → Type I or II; high K⁺ → Type IV).
- Iatrogenic: large-volume normal saline and acetazolamide are common, easily-overlooked causes of hospital-acquired NAGMA.
- Always exclude a mixed disorder with the delta ratio when the clinical picture does not fit a single cause.
Sort GI from renal in seconds.
Calculate the urine anion gap, then check the anion gap and delta ratio to exclude a mixed disorder — all free.
Open the urine anion gap calculator