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Renal, Fluids, Electrolytes & Acid-Base · Topic 19 of 22

Anion Gap Metabolic Acidosis

Renal · acid–base

Anion Gap Metabolic Acidosis

An added acid: calculate the gap, correct it for albumin, find the acid — ketones, lactate, uremia or toxin — and check the delta ratio.

Key numbers
Anion gap: Na⁺ − (Cl⁻ + HCO₃⁻)AG > 12 = high
Correct the gap for low albumin+2.5 per 1 g/dL albumin ↓
Delta ratio: ΔAG ÷ ΔHCO₃⁻Δ ratio 1–2 = pure AGMA
Osmolar gap over 10 suggests toxic alcoholOsm gap > 10
01 · CalculateAnion gap: Na⁺ − (Cl⁻ + HCO₃⁻)

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻); the typical normal value is about 12 mEq/L (10–12), so a gap above 12 with low HCO₃⁻ is a high anion gap metabolic acidosis.

02 · CalculateCorrect the gap for low albumin

Low albumin hides a gap: corrected anion gap = measured gap + 2.5 × (4 − albumin in g/dL), ie, add 2.5 mEq/L for every 1 g/dL that albumin is below normal.

03 · CalculateDelta ratio: ΔAG ÷ ΔHCO₃⁻

Delta ratio = (anion gap − 12) ÷ (24 − HCO₃⁻): 1–2 means a pure high-gap acidosis, below 1 an added normal-gap acidosis, above 2 an added metabolic alkalosis.

01

Causes & risk factors

Why it happens
Causes: ketoacidosis, lactate, kidney failure, toxins must knowThe main causes of a high-gap acidosis are ketoacidosis (diabetic, alcoholic, starvation), lactic acidosis, kidney failure and toxins — methanol, ethylene glycol and salicylates; rhabdomyolysis is a rare cause.
Lactic acidosis: commonest acidosis in hospitalized patientsLactic acidosis is the most common metabolic acidosis in hospitalized patients: type A from shock or hypoxia, type B with normal perfusion (metformin, NRTIs, liver failure, thiamine deficiency, seizures, cancer).
02

Diagnosis & monitoring

Work-up
Osmolar gap over 10 suggests toxic alcohol must knowOsmolar gap = measured osmolality − calculated (2 × Na⁺ + glucose/18 + BUN/2.8); a gap above 10 with a high-gap acidosis suggests methanol or ethylene glycol.
Check respiratory compensation with Winters formulaCheck compensation: expected PaCO₂ = 1.5 × HCO₃⁻ + 8 (± 2); a higher measured PaCO₂ means a coexisting respiratory acidosis and a lower one a coexisting respiratory alkalosis.
03

Do right now

In this order
First moves
1
Anion gap: Na⁺ − (Cl⁻ + HCO₃⁻)AG > 12 = high
→
2
Correct the gap for low albumin+2.5 per 1 g/dL albumin ↓
→
3
Order BUN, glucose, ketones, lactate, salicylate, osmolality
→
4
Osmolar gap over 10 suggests toxic alcoholOsm gap > 10
Why each step
Anion gap: Na⁺ − (Cl⁻ + HCO₃⁻)Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻); the typical normal value is about 12 mEq/L (10–12), so a gap above 12 with low HCO₃⁻ is a high anion gap metabolic acidosis.
Correct the gap for low albuminLow albumin hides a gap: corrected anion gap = measured gap + 2.5 × (4 − albumin in g/dL), ie, add 2.5 mEq/L for every 1 g/dL that albumin is below normal.
Order BUN, glucose, ketones, lactate, salicylate, osmolalityIf the cause is not obvious, order BUN and creatinine, glucose, blood β-hydroxybutyrate, lactate, salicylate level and measured serum osmolality; urine calcium oxalate crystals point to ethylene glycol.
Osmolar gap over 10 suggests toxic alcoholOsmolar gap = measured osmolality − calculated (2 × Na⁺ + glucose/18 + BUN/2.8); a gap above 10 with a high-gap acidosis suggests methanol or ethylene glycol.
04

Management

Orders
Treat the cause; bicarbonate below pH 7.0Treat the cause — ketoacids and lactate convert back to bicarbonate. IV sodium bicarbonate is controversial in high-gap acidosis and is usually reserved for pH below 7.0, aiming for about 7.10.
Septic shock, AKI, pH ≤ 7.2: consider bicarbonateSurviving Sepsis 2026 suggests sodium bicarbonate for septic shock with pH ≤ 7.2 and AKI stage 2–3, but not to improve hemodynamics or vasopressor needs in hypoperfusion lactic acidemia.
Dialyze kidney failure, toxic alcohols, salicylatesHemodialysis is required for acidosis of kidney failure and is sometimes needed for ethylene glycol, methanol or salicylate poisoning.
05

Red flags & don'ts

Avoid harm
Bicarbonate risks: sodium, volume, hypokalemia, hypocalcemia complicationSodium bicarbonate can cause sodium and volume overload, hypokalemia, hypocalcemia, overshoot metabolic alkalosis and hypercapnia that needs higher minute ventilation; a 50-mL ampule holds 50 mEq and is hyperosmolar.
06

The rules

Know these
DKA resolution: ketones and pH, not gapIn DKA, judge resolution by ketones < 0.6 mmol/L plus venous pH ≥ 7.3 or HCO₃⁻ ≥ 18, not by the anion gap — saline often leaves a hyperchloremic normal-gap acidosis during recovery.
07

More detail

Reference
Show 8 reference facts
Lactic acidosis strictly means pH below 7.35 with lactate above 5–6 mmol/L; smaller rises are hyperlactatemia.
D-lactic acidosis follows short bowel or jejunoileal bypass: carbohydrate loads cause confusion, ataxia and slurred speech, and routine lactate assays miss D-lactate.
5-Oxoproline (pyroglutamic acid) acidosis occurs with drugs that disrupt the γ-glutamyl cycle, such as chronic acetaminophen (paracetamol) or flucloxacillin.
A low anion gap is not an acidosis: it reflects hypoalbuminemia, hypercalcemia, hypermagnesemia, lithium toxicity, paraproteins (myeloma) or bromide or iodide intoxication.
In lactic acidosis the delta ratio is classically 1.6–1.8 using population normals, but about 1.2 using each patient's own baseline gap and HCO₃⁻.
Bicarbonate dosing: target HCO₃⁻ = 0.3 × PaCO₂ (≈ pH 7.10); NaHCO₃ needed (mEq) = (target − measured HCO₃⁻) × 0.4 × weight (kg), given over hours, rechecking pH 30–60 minutes later.
Kidney failure raises the gap by retaining sulfate, phosphate, urate and hippurate; earlier kidney disease more often causes a normal-gap acidosis.
The characteristic sign of metabolic acidosis is hyperpnea (deep breaths without dyspnea); pH below about 7.10 causes nausea, vomiting, malaise, hypotension and ventricular arrhythmias.
Pearl

GOLD MARK: glycols, oxoproline, L-lactate, D-lactate, methanol, aspirin, renal failure, ketoacidosis.

Sources · Merck Manual Professional — Metabolic Acidosis (updated Dec 2025) · Merck Manual Professional — Acid-Base Disorders (anion gap, albumin, delta gap) · Merck Manual Professional — Lactic Acidosis (updated Jul 2026) · AMBOSS — Delta gap / delta ratio · Deranged Physiology — The delta gap and delta ratio: advantages and disadvantages · Acid-base disorders in ICU patients. Electrolyte Blood Press 2010 (adjusted AG = observed AG + 2.5 × [normal − measured albumin]; normal AG 7 ± 4 on modern analyzers) · Evans L et al. Surviving Sepsis Campaign: international guidelines 2021 (recommendations 71–72, bicarbonate). Intensive Care Med 2021 · Mehta AN, Emmett JB, Emmett M. GOLD MARK: an anion gap mnemonic for the 21st century. Lancet 2008 · Umpierrez GE et al. Hyperglycaemic crises in adults with diabetes: a consensus report (ADA/EASD/JBDS/AACE/DTS). Diabetologia 2024, doi 10.1007/s00125-024-06183-8 · Pyroglutamic acidosis in Staphylococcus aureus bacteremia treated with flucloxacillin — case report (2024, cites GOLDMARK) · The Δ anion gap/Δ bicarbonate ratio in lactic acidosis: time for a new baseline? (2024) · Surviving Sepsis Campaign: International Guidelines for Management of Sepsis and Septic Shock 2026 — SCCM recommendations page (sodium bicarbonate) · Jaber S et al. Sodium bicarbonate for severe metabolic acidemia and acute kidney injury: the BICARICU-2 randomized clinical trial. JAMA 2025 (Europe PMC abstract) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).