From the Guidelines
The anion gap is calculated by subtracting the major measured anions (chloride and bicarbonate) from the major measured cation (sodium) in the blood, with the formula: Anion Gap = Na⁺ - (Cl⁻ + HCO₃⁻), and some institutions also include potassium in the calculation (AG = Na⁺ + K⁺ - (Cl⁻ + HCO₃⁻)). The anion gap is a valuable diagnostic tool for differentiating types of metabolic acidosis and guiding appropriate treatment strategies based on the underlying cause 1. To calculate the anion gap, you need the values from a basic metabolic panel. A normal anion gap typically ranges from 8 to 12 mEq/L, though reference ranges may vary slightly between laboratories. An elevated anion gap indicates the presence of unmeasured anions in the blood, often seen in conditions like diabetic ketoacidosis, lactic acidosis, or certain poisonings, such as ethylene glycol poisoning 1. In the context of ethylene glycol poisoning, the anion gap can be used to guide the decision for extracorporeal treatment (ECTR), with a recommendation for ECTR if the anion gap is > 27 mmol/L 1. Some key points to consider when calculating and interpreting the anion gap include:
- The formula for calculating the anion gap: Anion Gap = Na⁺ - (Cl⁻ + HCO₃⁻) or AG = Na⁺ + K⁺ - (Cl⁻ + HCO₃⁻) if potassium is included
- The normal range for the anion gap: 8-12 mEq/L, or 10-20 mEq/L if potassium is included
- The significance of an elevated anion gap: indicates the presence of unmeasured anions in the blood, often seen in conditions like diabetic ketoacidosis, lactic acidosis, or certain poisonings
- The use of the anion gap in guiding treatment decisions, such as the decision for ECTR in ethylene glycol poisoning 1.
From the Research
Anion Gap Calculation
The anion gap is a calculated measure that helps in the diagnosis of acid-base disorders. It is calculated using the formula: Anion Gap = Sodium - (Chloride + Bicarbonate) [ 2 ].
Importance of Anion Gap
- The anion gap is useful in identifying the type of metabolic acidosis, which can be either high anion gap or non-anion gap (normal anion gap) [ 3 ].
- A high anion gap metabolic acidosis is often caused by the accumulation of unmeasured anions, such as lactate, ketones, or other organic acids [ 4 ].
- The anion gap can also be affected by the administration of sodium bicarbonate, which can increase the bicarbonate level and decrease the anion gap [ 5 ].
Clinical Application
- The anion gap is an important tool in the management of patients with lactic acidosis, as it can help guide treatment decisions [ 6 ].
- However, the use of sodium bicarbonate to correct acidemia is still a topic of debate, and its benefits and risks need to be carefully considered [ 2,3,4,5,6 ].
Calculation
- The anion gap is typically calculated using arterial blood gas values, but it can also be estimated using venous blood gas values [ 2 ].
- The normal anion gap range is typically considered to be 3-12 mmol/L, but this can vary depending on the laboratory and the specific measurement techniques used [ 3 ].