Overview
This lecture explains the anion gap, its physiological basis, factors affecting it, calculation methods (including correction for hypoalbuminemia), and how it differentiates types of metabolic acidosis.
The Anion Gap: Definition and Origin
- The anion gap represents the difference between measured cations and anions in serum, reflecting unmeasured ions.
- Cations are mainly sodium (Na+) and potassium (K+), while anions are chloride (Cl-) and bicarbonate (HCO3-).
- The anion gap = [Na+] - ([Cl-] + [HCO3-]); potassium is often omitted due to its minimal impact.
- Normal anion gap range is lab-specific, usually 8–12 mEq/L.
Causes and Correction of Anion Gap Changes
- Elevated anion gap indicates accumulation of unmeasured anions, typically from pathological acids or severe renal failure.
- Normal anion gap metabolic acidosis involves loss of bicarbonate or decreased renal acid excretion, with increased chloride.
- Causes of elevated gap: lactic acidosis, ketoacidosis, methanol, ethylene glycol, renal failure.
- Non-acidosis causes of high anion gap include metabolic alkalosis, hyperphosphatemia, and rare paraproteinemias.
- Low anion gap is most often due to hypoalbuminemia, but can be seen with excess cations or lab artifacts.
- Adjust anion gap for low albumin: corrected gap = measured gap + 2.5 × (4 - albumin in g/dL).
Clinical Approach and Examples
- Stepwise acid-base analysis: assess pH, determine primary disturbance, check compensation, then calculate anion gap.
- Winters formula estimates expected pCO2 for metabolic acidosis: pCO2 ≈ 1.5 × [HCO3-] + 8.
- Examples provided: distinguishing normal vs. elevated anion gap metabolic acidosis, mixed acid-base disorders, and the impact of albumin correction.
Key Terms & Definitions
- Anion gap — difference between measured cations and anions; reflects unmeasured ions.
- Metabolic acidosis — decreased blood pH caused by increased acid or decreased bicarbonate.
- Elevated anion gap metabolic acidosis — metabolic acidosis with increased unmeasured anions.
- Normal anion gap (hyperchloremic) metabolic acidosis — acidosis with unchanged gap, chloride increase compensates for bicarbonate loss.
- Winters formula — calculates expected pCO2 in metabolic acidosis: pCO2 ≈ 1.5 × [HCO3-] + 8.
- Hypoalbuminemia — low serum albumin, affects the anion gap.
Action Items / Next Steps
- Review your lab’s normal range for anion gap.
- Practice calculating and correcting the anion gap with provided formulas.
- Prepare for the next lecture on using the anion gap to diagnose triple acid-base disorders.