Metabolic Acidosis (Anion Gap and Non-Gap)
Primary decrease in bicarbonate; differentiated by anion gap into gap (MUDPILES) and non-gap (HARDUP).
Also known as: metabolic acidosis, anion gap acidosis, non-anion gap acidosis, MUDPILES
Overview
Primary decrease in serum bicarbonate (HCO3 <22 mEq/L) with compensatory hyperventilation (decreased PCO2). Classified by anion gap (AG = Na − (Cl + HCO3); normal 8-12) into high-anion-gap (HAGMA) and normal-anion-gap (non-gap / hyperchloremic) metabolic acidosis.
Epidemiology
Common in hospitalized and critically ill patients. Etiology varies: HAGMA dominates in DKA, lactic acidosis, ingestions, and AKI; non-gap acidosis in diarrhea, RTA, and dilutional acidosis.
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Risk factors
- Diabetes (DKA, ketoacidosis)
- Shock states, sepsis, tissue hypoperfusion (lactic acidosis)
- Alcoholism (alcoholic ketoacidosis, methanol or ethylene glycol ingestion)
- CKD/AKI (uremic acidosis)
- Toxic ingestions: methanol, ethylene glycol, salicylates, isoniazid, iron
- GI bicarbonate loss: diarrhea, pancreatic/biliary fistula, ureteral diversion
- Renal tubular acidosis (types 1, 2, 4)
- Metformin, propylene glycol, linezolid (lactic acidosis)
Pathophysiology
Acidosis results from accumulation of acid (organic acids in HAGMA, raising the anion gap; HCl in non-gap, maintaining chloride balance) or loss of bicarbonate (GI or renal). Buffering by extracellular HCO3 lowers serum HCO3; respiratory compensation (Winter's formula: PCO2 = 1.5×HCO3 + 8 ± 2) reduces PCO2. Failure of expected compensation indicates a coexisting respiratory disorder.
Clinical presentation
Symptoms
- Hyperventilation (Kussmaul respirations) — deep, rapid, labored breathing
- Fatigue, weakness, malaise
- Nausea, vomiting, abdominal pain (especially DKA, uremia)
- Confusion, altered mental status
- Specific to cause: polyuria/polydipsia (DKA), neurologic symptoms (toxic ingestions), diarrhea, oliguria
Signs / physical exam
- Tachypnea, Kussmaul breathing
- Tachycardia, hypotension if shock or volume depletion
- Fruity (acetone) breath in DKA
- Altered mental status
- Findings of underlying cause: dehydration, abdominal tenderness, retinal findings (methanol), kidney bruits, etc.
Classic findings
Young patient with hyperventilation, abdominal pain, dehydration, and fruity breath = DKA until proven otherwise.
Differential diagnosis
- DKA — Hyperglycemia + ketones + AG acidosis; diabetic, recent insulin omission or illness
- Lactic acidosis — Elevated lactate >2 mmol/L; sepsis, shock, ischemia, metformin, propylene glycol
- Uremia — AKI/CKD with elevated BUN/Cr; AG from retained organic anions
- Methanol / ethylene glycol — Ingestion history; osmolar gap >10; calcium oxalate crystals (EG); blurred vision/blindness (methanol)
- Salicylate poisoning — Mixed AG metabolic acidosis + respiratory alkalosis; tinnitus, hyperventilation, hyperthermia
- Alcoholic ketoacidosis — Recent binge with poor intake; high anion gap with predominant beta-hydroxybutyrate (may not register on ketone dipstick)
- Diarrhea — Non-gap acidosis with low urine anion gap (negative); recent GI illness
- RTA — Non-gap acidosis with positive urine anion gap; specific tubular defect
Diagnostic workup
Diagnostic criteria
Metabolic acidosis = pH <7.35 and HCO3 <22. Apply anion gap to subdivide: AG >12 = HAGMA (think MUDPILES); AG normal = non-gap (think HARDUP/GOLDMARK). Check expected compensation by Winter's formula.
Labs
- BMP — Na, K, Cl, HCO3 (low), glucose, BUN, Cr; calculate anion gap = Na − (Cl + HCO3)
- ABG — pH (low), PCO2 (low — compensation), HCO3
- Lactate — for any unexplained AG acidosis
- Serum ketones (beta-hydroxybutyrate preferred)
- Serum osmolality + osmolar gap if toxic ingestion suspected (osmolar gap = measured − calculated >10 indicates osmotically active substance — methanol, ethanol, ethylene glycol, propylene glycol)
- Salicylate, methanol, ethylene glycol, ethanol levels
- Urine anion gap (UAG = Una + Uk − Ucl) for non-gap acidosis: negative (<-20) suggests GI loss; positive (>0) suggests RTA
- Urinalysis — calcium oxalate crystals in ethylene glycol; ketones in DKA
Imaging
- Generally not required for diagnosis of acidosis itself, but may be needed for underlying cause (CT for ischemic bowel in unexplained lactic acidosis)
Diagnostic algorithm
| Type | Anion Gap | Mnemonic / Examples | Key Lab Clue |
|---|---|---|---|
| High AG (HAGMA) | >12 | MUDPILES — methanol, uremia, DKA, propylene glycol, INH/iron, lactic, ethylene glycol, salicylate | AG >12, often with osmolar gap |
| Non-gap (HCMA) | Normal | HARDUP — hyperalimentation, acetazolamide/Addison, RTA, diarrhea, ureteral diversion, pancreatic fistula | Hyperchloremia, normal AG |
| Lactic acidosis | High | Sepsis, shock, ischemia, metformin | Lactate >2 mmol/L |
| DKA | High | Diabetic, missed insulin | Glucose >250 + ketones + AG |
| Toxic alcohols | High | Methanol, ethylene glycol | Osmolar gap >10 |
| RTA (1, 2, 4) | Normal | Sjögren (RTA-1), Fanconi (RTA-2), diabetes (RTA-4) | Positive urine AG |
Treatment
First-line
- Treat the underlying cause — the most important and effective therapy
- DKA: IV fluids (normal saline initially), IV insulin infusion (0.1 U/kg/h after K replete), K replacement, address precipitant
- Lactic acidosis: restore perfusion (fluids, vasopressors), treat sepsis, source control
- Methanol/ethylene glycol: fomepizole (alcohol dehydrogenase inhibitor) ± hemodialysis; folate (methanol), thiamine + pyridoxine (ethylene glycol)
- Salicylate poisoning: aggressive IV fluids, urine alkalinization with sodium bicarbonate, hemodialysis if severe
- Uremic acidosis: dialysis if severe; oral sodium bicarbonate for chronic CKD HCO3 <22
- Diarrhea: rehydration, treat infection
- RTA: oral bicarbonate (sodium bicarbonate or potassium citrate)
Second-line / adjunct
- Sodium bicarbonate IV: controversial; consider for severe acidemia (pH <7.1) with hemodynamic compromise; for hyperkalemia with acidosis; for cyclic antidepressant or salicylate overdose
- Hemodialysis indications: severe metabolic acidosis refractory to medical therapy, toxic ingestion (methanol, ethylene glycol, salicylate, metformin lactic acidosis), severe AKI with acidosis
- Tris-hydroxymethyl aminomethane (THAM) — alternative buffer when CO2 retention is a concern (rarely used)
- Address contributing medications (metformin, propylene glycol vehicles)
- Monitor closely with repeat ABG, BMP, anion gap, lactate every 1-4 hours during acute treatment
Complications
- Cardiovascular: decreased contractility, vasodilation, arrhythmias
- Respiratory muscle fatigue from sustained hyperventilation
- Altered mental status, coma
- Hyperkalemia from cellular shift
- Bone demineralization (chronic acidosis in CKD)
- Insulin resistance, catabolism
- Death — particularly in shock, severe DKA, toxic ingestions without prompt treatment
PANCE pearls
- MUDPILES for AG acidosis: Methanol, Uremia, DKA (and AKA, starvation), Propylene glycol/Paraldehyde, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates.
- HARDUP for non-gap acidosis: Hyperalimentation, Acetazolamide/Addison disease, RTA, Diarrhea, Ureteral diversion, Pancreatic fistula.
- Osmolar gap >10 + AG acidosis = consider methanol or ethylene glycol ingestion. Treat empirically with fomepizole while confirming.
- Winter's formula for expected respiratory compensation: PCO2 = 1.5 × HCO3 + 8 ± 2. Higher or lower than expected indicates additional respiratory disorder.
- Urine anion gap (UAG) interprets non-gap acidosis: negative (chloride > Na + K) = appropriate ammonium excretion = GI loss; positive = inadequate ammonium = RTA.
- Delta-delta (ΔAG/ΔHCO3): ratio of rise in AG to fall in HCO3. <1 suggests concurrent non-gap acidosis; >2 suggests concurrent metabolic alkalosis.
- Sodium bicarbonate in DKA is generally NOT recommended unless pH <6.9 — may worsen intracellular acidosis and cerebral edema.
References
- ADA 2024 — ADA Standards of Care in Diabetes 2024 — DKA management
- Berend 2014 — Physiological Approach to Assessment of Acid-Base Disturbances (Berend et al., NEJM 2014)
- Kraut Madias — Treatment of Acute Non-Anion Gap Metabolic Acidosis (Kraut and Madias, CJASN 2012)
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