Acid–Base & Anion Gap Glossary

Acid–base terminology is dense and full of acronyms. This glossary defines the 25 terms most relevant to anion gap interpretation, grouped alphabetically. For numeric reference intervals, see the companion normal ranges table; to run a value, use the anion gap calculator.

Acidemia lab

An arterial pH below the normal range (< 7.35). Acidemia is the state; acidosis is the process driving it. A patient can have a severe metabolic acidosis with a normal pH if a concurrent respiratory alkalosis is compensating.

Albumin-corrected anion gap formula

The observed anion gap adjusted for serum albumin: corrected AG = observed AG + 2.5 × (4.0 − albumin g/dL). Each 1 g/dL fall in albumin lowers the expected normal gap by ~2.5 mEq/L, so the correction unmasks HAGMA in hypoalbuminemic patients.

Alkalemia lab

An arterial pH above the normal range (> 7.45). Alkalemia may coexist with a high anion gap when a metabolic alkalosis (e.g. vomiting) layers on top of a HAGMA — a mixed disorder detectable by the delta ratio.

Anion gap (AG) core

The difference between measured cations and measured anions in serum, calculated as Na⁺ − (Cl⁻ + HCO₃⁻). It estimates the concentration of "unmeasured" anions (albumin, phosphate, lactate, ketones) and is the primary screen for metabolic acidosis. Normal range 8–12 mEq/L (conventional) or 3–11 mEq/L (ISE).

Bicarbonate (HCO₃⁻) analyte

The dominant extracellular buffer anion, measured on the metabolic panel (often reported as total CO₂). A falling bicarbonate signals metabolic acidosis or respiratory alkalosis compensation; it is a direct input to the anion gap formula.

Base excess / base deficit lab

The quantity of acid or base required to titrate blood to pH 7.40 at standard CO₂. A base deficit (negative base excess) reflects metabolic acidosis and correlates loosely with an elevated anion gap.

Buffer base concept

The sum of all buffer anions in blood (bicarbonate, phosphate, proteins). Its deviation from normal underlies the base-excess calculation.

Delta gap (ΔGap) formula

The absolute difference between the change in anion gap and the change in bicarbonate: ΔGap = (observed AG − 12) − (24 − HCO₃⁻). A non-zero delta gap flags a second, concurrent metabolic disorder.

Delta ratio (Δ/Δ) formula

The ratio of the anion-gap rise to the bicarbonate fall: ΔAG / ΔHCO₃⁻. A ratio near 1 = pure HAGMA; < 0.8 = concurrent NAGMA; > 2 = concurrent metabolic alkalosis.

Electroneutrality principle

The physical law that total positive charge equals total negative charge in any body-fluid compartment. The anion gap exists because we measure only the major ions — the "gap" is filled by unmeasured anions that balance the equation.

GOLD MARK mnemonic

Modern mnemonic for HAGMA causes: Glycols, Oxoproline, L-lactate, D-lactate, Methanol, Aspirin, Renal failure, Ketoacidosis. An update of MUDPILES reflecting contemporary toxins.

HAGMA disorder

High anion gap metabolic acidosis. The elevated gap indicates accumulation of unmeasured anions — lactate, ketones, uremic toxins, or organic acids from toxins. Worked up with lactate, ketones, renal function, and the toxic-alcohol screen.

Hyperchloremic metabolic acidosis disorder

Acidosis in which chloride rises to replace the lost bicarbonate, leaving the anion gap normal. Synonymous with NAGMA. Classic causes: diarrhea, renal tubular acidosis, aggressive saline resuscitation.

Ion-selective electrode (ISE) method

The modern autoanalyzer method for measuring Na⁺, K⁺, Cl⁻. ISE reports slightly higher chloride values than older methods, compressing the calculated anion gap into a tighter 3–11 mEq/L range.

Ketoacidosis disorder

Accumulation of β-hydroxybutyrate and acetoacetate — unmeasured anions that raise the anion gap. The driver of HAGMA in diabetic ketoacidosis (DKA) and alcoholic ketoacidosis.

Lactic acidosis disorder

Elevation of lactate (an unmeasured anion) producing HAGMA. Type A = tissue hypoxia (shock, sepsis); Type B = impaired metabolism without hypoxia (metformin, malignancy, toxins).

MUDPILES mnemonic

Classic mnemonic for HAGMA causes: Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates. Largely superseded by GOLD MARK.

NAGMA disorder

Normal anion gap metabolic acidosis, also called hyperchloremic metabolic acidosis. Bicarbonate is lost and chloride rises proportionally, leaving the gap unchanged. Differentiated from HAGMA by the urine anion gap and clinical context.

Osmolar gap formula

The difference between measured and calculated serum osmolarity: measured − calculated. A gap > 10 mOsm/kg suggests unmeasured osmoles such as methanol, ethylene glycol, or propylene glycol — the toxic-alcohol "double gap" with a high anion gap.

Renal tubular acidosis (RTA) disorder

A group of tubular disorders that impair acid excretion, producing a normal-anion-gap (hyperchloremic) metabolic acidosis. Distal (type 1), proximal (type 2), and hyperkalemic (type 4) subtypes are distinguished by urine pH and serum potassium.

Strong ion difference (SID) concept

Peter Stewart's quantitative acid–base framework. SID = (Na⁺ + K⁺ + Ca²⁺ + Mg²⁺) − (Cl⁻ + lactate + other strong anions). The Stewart model explains acid–base status from three independent variables: SID, total weak acid (Atot, mainly albumin), and PaCO₂.

Total CO₂ analyte

The metabolic-panel surrogate for bicarbonate; ~95% is dissolved HCO₃⁻. Used interchangeably with bicarbonate in the anion gap formula when direct HCO₃⁻ is not measured.

Unmeasured anions concept

The anions that fill the "gap" — albumin, phosphate, sulfate, and organic anions (lactate, ketones, urate). When they accumulate, the anion gap rises. The conceptual basis of HAGMA detection.

Urine anion gap (UAG) formula

Calculated from urine electrolytes as UAG = urine Na⁺ + urine K⁺ − urine Cl⁻. A negative UAG indicates high urinary NH₄⁺ (appropriate renal acid excretion, as in diarrhea); a positive UAG indicates low NH₄⁺ (renal tubular acidosis).

Winter's formula formula

Predicts the appropriate respiratory compensation for a metabolic acidosis: expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2. A measured PaCO₂ higher than predicted indicates a concurrent respiratory acidosis; lower indicates a concurrent respiratory alkalosis.