Anion Gap
Definition and Calculation
The anion gap (AG) is a calculated value that estimates unmeasured anions in plasma. It helps classify the cause of metabolic acidosis.
Formula:
| Parameter | Normal Range |
|---|---|
| Anion Gap | 8–12 mEq/L |
| Sodium | 136–145 mEq/L |
| Chloride | 97–107 mEq/L |
| Bicarbonate | 22–29 mEq/L |
Note: Some institutions use a range of 10–14 mEq/L depending on the albumin-corrected formula. Hypoalbuminemia lowers the normal AG — correct for albumin: add 2.5 mEq/L to AG for every 1 g/dL decrease in albumin below 4 g/dL.
Why the Anion Gap Exists
Plasma must maintain electrical neutrality — measured cations = measured anions + unmeasured anions. The “gap” represents proteins (primarily albumin), phosphate, sulfate, and organic acids that are not measured in routine chemistry panels.
When acid accumulates in the bloodstream (e.g., lactic acid, ketoacids), these unmeasured anions increase and bicarbonate decreases to buffer them — the gap widens.
Interpreting the Anion Gap
Elevated Anion Gap Metabolic Acidosis (AG > 12)
An elevated AG means unmeasured acids have accumulated. Use the MUDPILES mnemonic:
| Letter | Cause | Mechanism |
|---|---|---|
| M | Methanol | Toxic alcohol → formic acid |
| U | Uremia (renal failure) | Sulfate, phosphate, urate accumulation |
| D | Diabetic ketoacidosis (DKA) | Ketoacid accumulation (β-hydroxybutyrate, acetoacetate) |
| P | Propylene glycol | Solvent toxicity |
| I | Isoniazid / Iron toxicity | Lactic acidosis |
| L | Lactic acidosis | Anaerobic metabolism (shock, sepsis, hypoxia) |
| E | Ethylene glycol | Toxic alcohol → oxalic acid |
| S | Salicylates | Aspirin overdose |
Normal Anion Gap Metabolic Acidosis (AG 8–12)
Also called hyperchloremic metabolic acidosis — bicarbonate is lost and replaced by chloride, so the gap remains normal. Causes:
- GI bicarbonate loss: Diarrhea, fistulas, ileostomy
- Renal tubular acidosis (RTA): Kidneys fail to excrete acid or reabsorb bicarbonate
- Saline administration (hyperchloremic acidosis from excess NaCl)
- Adrenal insufficiency
- Carbonic anhydrase inhibitors (acetazolamide)
Nursing Clinical Application
NCLEX Focus
When a patient presents with metabolic acidosis (low pH, low HCO₃⁻ on ABG), calculate the anion gap from the BMP to guide the differential diagnosis. A gap >12 points toward MUDPILES causes; a normal gap points toward GI loss or RTA.
- Calculate AG from BMP values: Na – (Cl + HCO₃⁻)
- Compare with pH and HCO₃⁻ from ABG to confirm metabolic acidosis
- DKA: Expect elevated AG, glucose >250 mg/dL, low HCO₃⁻, ketones in urine/serum
- Lactic acidosis: Expect elevated AG, elevated serum lactate (>2 mmol/L = concern; >4 mmol/L = significant); associated with shock, sepsis, mesenteric ischemia
- Uremia: Elevated AG with elevated creatinine and BUN; manage with dialysis
- Monitor respiratory compensation: in metabolic acidosis, respiratory rate increases (Kussmaul breathing) to compensate
Related Concepts
- basic-metabolic-panel — Anion gap is calculated from BMP components (Na, Cl, HCO₃⁻).
- respiratory-acidosis — ABG interpretation framework including metabolic acidosis differentiation.
- diabetes-mellitus — DKA is the most common elevated-AG metabolic acidosis in nursing practice.
- sodium-balance-disorders — Sodium is the primary determinant in the anion gap formula.
- evaluation-of-outcomes-in-fluid-electrolyte-and-acid-base-care — Monitoring resolution of acid-base imbalances.
- sepsis — Lactic acidosis is a key feature of septic shock (elevated AG metabolic acidosis).
Self-Check
- Calculate the anion gap: Na 140, Cl 102, HCO₃⁻ 14 mEq/L. Is this elevated or normal?
- A patient with DKA has a glucose of 450 mg/dL and HCO₃⁻ of 10 mEq/L. What type of acid-base disturbance would you expect, and why?
- What is Kussmaul breathing, and why does it occur in metabolic acidosis?