Bit · Pharm/Tox
Anion gap metabolic acidosis (MUDPILES)
When metabolic acidosis is paired with a high anion gap, an unmeasured anion is in the blood. MUDPILES is the differential.
Mechanism
The anion gap = Na⁺ − (Cl⁻ + HCO₃⁻); normal is ~8–12 mEq/L. A wide gap means the acidosis is driven by accumulation of an unmeasured anion (not Cl⁻). Each MUDPILES letter corresponds to a different accumulated species:
- M — Methanol → formic acid; visual disturbance, retinal toxicity. Treat with fomepizole.
- U — Uremia → accumulation of organic acids (sulfates, phosphates) in renal failure.
- D — DKA (and starvation/alcoholic ketoacidosis) → β-hydroxybutyrate, acetoacetate.
- P — Propylene glycol (carrier in IV lorazepam, phenobarbital) → lactic acidosis.
- I — Iron / Isoniazid → iron causes lactic acidosis from cellular toxicity; INH causes lactic acidosis ± seizures from B6 antagonism (treat with pyridoxine).
- L — Lactic acidosis → sepsis, shock, mesenteric ischemia, metformin (in renal failure), seizures, mitochondrial toxins (cyanide, CO).
- E — Ethylene glycol → glycolate, oxalate; calcium oxalate crystals in urine, AKI. Treat with fomepizole.
- S — Salicylates (aspirin) → mixed respiratory alkalosis (early stimulation of medullary respiratory centre) AND anion-gap metabolic acidosis (uncoupling of oxidative phosphorylation). Classic on Step 1.
Differentiator Table
| Letter | Cause | Accumulating species | Distinguishing clue | Antidote / treatment |
|---|---|---|---|---|
| M | Methanol | Formic acid | Visual loss, retinal hemorrhage; windshield-washer fluid ingestion | Fomepizole; folate; dialysis if severe |
| U | Uremia | Organic acids (sulfates, phosphates) | AKI / CKD with very high BUN/Cr | Treat underlying renal failure; dialysis |
| D | DKA / starvation / alcoholic ketoacidosis | β-hydroxybutyrate, acetoacetate | Hyperglycemia in DKA; normoglycemia in alcoholic | IV fluids + insulin (DKA); glucose + thiamine (alcoholic) |
| P | Propylene glycol | Lactic acidosis (from metabolism by ADH/ALDH) | High-dose IV lorazepam or phenobarbital infusion | Stop offending infusion; fomepizole in severe cases |
| I | Iron / Isoniazid | Lactate (iron: cellular toxicity; INH: NAD+ depletion) | Iron: child with toy or supplement ingestion. INH: TB treatment + seizures | Iron: deferoxamine. INH: pyridoxine (B6) + treat seizures |
| L | Lactic acidosis | Lactate | Hypoperfusion, sepsis, metformin in CKD, seizures, CO/CN poisoning | Treat cause; bicarbonate controversial |
| E | Ethylene glycol | Glycolate, oxalate (Ca²⁺-oxalate crystals) | AKI, calcium oxalate crystalluria, often kids/intoxicated adult | Fomepizole; dialysis; calcium for hypocalcemia |
| S | Salicylates | Salicylate (early mixed picture) | MIXED — respiratory alkalosis + anion-gap acidosis; tinnitus, fever, tachypnea | Urinary alkalinisation with bicarbonate; dialysis for severe |
The Pivot
Two steps:
- Calculate anion gap. Na⁺ − (Cl⁻ + HCO₃⁻). > 12 → high gap.
- Pattern-match to a MUDPILES cause: visual loss → methanol; AKI + oxalate crystals → ethylene glycol; tinnitus + mixed acid-base → salicylate; hyperglycemia + ketones → DKA; AKI alone → uremia; sepsis or shock → lactic acidosis; toddler with iron supplements → iron; TB patient with seizures → INH.
For methanol and ethylene glycol, also check the osmolar gap — it's elevated early (parent alcohol still present) and falls as the alcohol is metabolized to the acid.
NBME-Style Stem
A 22-year-old man is brought to the ED after his roommate found him obtunded next to an empty bottle of antifreeze. Blood gas shows pH 7.18, HCO₃⁻ 8 mEq/L, anion gap 26 mEq/L. Urinalysis shows envelope-shaped crystals and microscopic hematuria. Serum creatinine is 2.8 mg/dL. Which of the following is the most appropriate first-line therapy?
Concept Anchor
MUDPILES is a checklist for the unmeasured anion in your patient's plasma — every letter has a different molecule pile-up and a different specific antidote. Pattern-match the clinical clue to the letter.