How to Calculate Anion Gap
What is Anion Gap?
Anion gap measures unmeasured anions in blood, helping identify metabolic acidosis causes. It's calculated from electrolyte values.
Step-by-Step Guide
- 1Calculate: AG = [Na⁺] - ([Cl⁻] + [HCO3⁻])
- 2Normal is 8-16 mEq/L (or 6-12 using newer methods)
- 3High AG suggests organic acid accumulation (lactate, ketones)
Worked Examples
Input
Na 138, Cl 104, HCO3 14
Result
AG = 20 (elevated, high AG metabolic acidosis)
Suggests lactate or ketosis
Common Mistakes to Avoid
- ✕Interpreting AG without pH/HCO3 context
- ✕Using outdated normal ranges that don't account for albumin
Frequently Asked Questions
When is anion gap useful?
Differentiating causes of metabolic acidosis: high AG (lactic acid, DKA) vs normal AG (diarrheal losses).
What limits anion gap interpretation?
It's only useful when acidosis is present; anion gap is normal in metabolic alkalosis despite serious pathology.
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