Renal/Urology · PANCE / PANRE

Metabolic Acidosis (Anion Gap and Non-Gap)

Written & clinically reviewed by Ryan Busch, PA-C · Updated August 2026 · Report an error

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.

Try two board-style Metabolic Acidosis questions

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Question 1RenalMedium
A patient has pH 7.24, bicarbonate 12 mEq/L, and an anion gap of 26. Which of the following is the most likely mechanism?
  • ALoss of bicarbonate through diarrhea
  • BHyperventilation lowering carbon dioxide
  • CAccumulation of unmeasured organic acids
  • DRenal tubular failure to secrete hydrogen ions
Reveal answer & full explanation
Correct answer: C — Accumulation of unmeasured organic acids
  • ALoss of bicarbonate through diarrhea
  • BHyperventilation lowering carbon dioxide
  • CAccumulation of unmeasured organic acids✓
  • DRenal tubular failure to secrete hydrogen ions

Why Accumulation of unmeasured organic acids is correct

  • A markedly elevated anion gap of 26 with low pH and low bicarbonate defines high-anion-gap metabolic acidosis.
  • The gap rises because unmeasured anions such as lactate, ketoacids, or toxic-alcohol metabolites consume bicarbonate.
  • Common causes are lactic acidosis, ketoacidosis, uremia, and toxic ingestions.

Why the others are wrong

  • Loss of bicarbonate through diarrhea — GI bicarbonate loss produces a NORMAL anion gap (hyperchloremic) acidosis; the gap of 26 excludes it.
  • Renal tubular failure to secrete hydrogen ions — Distal RTA also gives a normal-gap acidosis with hyperchloremia, not the elevated gap measured here.
  • Hyperventilation lowering carbon dioxide — That causes respiratory alkalosis with a HIGH pH; it is the opposite acid-base disturbance from this acidemia.
Question 2RenalEasy
An 18-year-old man with type 1 diabetes is brought to the emergency department after two days of vomiting and abdominal pain during a viral illness, during which he stopped his insulin. He is lethargic with deep, rapid respirations and a fruity odor on his breath. Vital signs show heart rate 124/min, blood pressure 98/60 mm Hg, and dry mucous membranes. Laboratory studies show glucose 540 mg/dL, sodium 132 mEq/L, chloride 95 mEq/L, bicarbonate 8 mEq/L, and arterial pH 7.12, with serum and urine ketones strongly positive. Which of the following is the most likely diagnosis?
  • ASalicylate intoxication
  • BDiabetic ketoacidosis
  • CType A lactic acidosis
  • DMethanol intoxication
Reveal answer & full explanation
Correct answer: B — Diabetic ketoacidosis
  • ASalicylate intoxication
  • BDiabetic ketoacidosis✓
  • CType A lactic acidosis
  • DMethanol intoxication

Why Diabetic ketoacidosis is correct

  • A young patient with type 1 diabetes who omits insulin during an illness develops hyperglycemia (glucose 540 mg/dL), an elevated anion gap (132 - [95 + 8] = 29), and strongly positive ketones - the classic triad of DKA.
  • Kussmaul respirations, fruity (acetone) breath, dehydration, abdominal pain, and vomiting are hallmark findings; the high-anion-gap metabolic acidosis is driven by beta-hydroxybutyrate and acetoacetate.
  • Management is IV normal saline, an insulin infusion (after potassium is replete), potassium replacement, and treating the precipitant; bicarbonate is reserved for pH <6.9.

Why the others are wrong

  • Type A lactic acidosis arises from tissue hypoperfusion in shock, sepsis, or ischemia with an elevated lactate; it does not cause marked hyperglycemia with strongly positive ketones after insulin omission.
  • Salicylate intoxication produces a mixed anion-gap metabolic acidosis with respiratory alkalosis plus tinnitus, hyperthermia, and hyperventilation, rather than hyperglycemia with ketosis.
  • Methanol intoxication causes an anion-gap acidosis with an elevated osmolar gap and visual disturbance after ingestion, not insulin omission with positive ketones.
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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

TypeAnion GapMnemonic / ExamplesKey Lab Clue
High AG (HAGMA)>12MUDPILES — methanol, uremia, DKA, propylene glycol, INH/iron, lactic, ethylene glycol, salicylateAG >12, often with osmolar gap
Non-gap (HCMA)NormalHARDUP — hyperalimentation, acetazolamide/Addison, RTA, diarrhea, ureteral diversion, pancreatic fistulaHyperchloremia, normal AG
Lactic acidosisHighSepsis, shock, ischemia, metforminLactate >2 mmol/L
DKAHighDiabetic, missed insulinGlucose >250 + ketones + AG
Toxic alcoholsHighMethanol, ethylene glycolOsmolar gap >10
RTA (1, 2, 4)NormalSjögren (RTA-1), Fanconi (RTA-2), diabetes (RTA-4)Positive urine AG
Metabolic acidosis differential by anion gap — MUDPILES (high AG) vs HARDUP (non-gap).

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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Differentials & related conditions

Diabetic Ketoacidosis (DKA)

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