Anion gap

The Anion Gap (AG) is a valuable tool in medical diagnostics used to identify the cause of metabolic acidosis. It represents the difference between the measured cations (positively charged ions) and the measured anions (negatively charged ions) in the blood serum. The AG can assist clinicians in identifying various metabolic disorders, including diabetic ketoacidosis, lactic acidosis, and renal failure.

Calculation

The anion gap is typically calculated using the following formula:

• AG = [Sodium] - ([Chloride] + [Bicarbonate])

Normal anion gap values may vary depending on the laboratory, but they generally range between 8 and 16 mEq/L. An elevated anion gap often suggests the presence of an unmeasured anion.

Clinical Relevance

In clinical practice, the anion gap is primarily used to identify the cause of metabolic acidosis. An increased anion gap indicates the presence of unmeasured anions, which often arise in conditions such as diabetic ketoacidosis, lactic acidosis, and ingestion of certain toxins. Conversely, a normal or low anion gap can be seen in conditions such as hyperchloremic acidosis or in the presence of multiple myeloma.

Limitations and Variations

The anion gap calculation assumes that the primary cations in the serum are sodium ions, while chloride and bicarbonate are the main anions. However, other ions can contribute to the gap, such as potassium, calcium, and magnesium. Some clinicians may include potassium in the anion gap calculation, although this is less common.

Changes in albumin levels can also significantly affect the anion gap. Since albumin is an anion, hypoalbuminemia can result in a decreased anion gap, while hyperalbuminemia can increase it. Hence, clinicians often correct the anion gap for albumin levels.

See Also

• Acid-base disorders
• Metabolic acidosis

References

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