How to Calculate Anion Gap
The anion gap is calculated using the formula: Anion Gap = [Na+] - ([Cl-] + [HCO3-]) 1, 2.
Understanding the Anion Gap
The anion gap (AG) is a mathematical construct that compares the blood sodium concentration with the sum of the chloride and bicarbonate concentrations. It represents the difference between measured cations and measured anions in the blood and helps categorize metabolic acidosis into:
- High anion gap metabolic acidosis (HAGMA)
- Normal anion gap (hyperchloremic) metabolic acidosis
Normal Values
The normal anion gap typically ranges from:
Clinical Applications
The anion gap calculation serves several important clinical purposes:
Categorizing metabolic acidosis: Divides acidosis into high anion gap and normal anion gap types, narrowing differential diagnosis 3
Identifying mixed acid-base disorders: When comparing the rise in anion gap (delta AG) with the fall in bicarbonate (delta HCO3-), additional acid-base disorders can be detected 5
Quality control: Serves as an additional check for electrolyte measurements 4
Important Considerations
Correcting for Albumin
Since albumin is a major contributor to the normal anion gap, hypoalbuminemia can falsely lower the calculated anion gap. For accurate clinical interpretation, the anion gap should be corrected for low albumin levels 6, 7.
Delta Gap Analysis
For more complex acid-base disorders, calculating the delta gap can be helpful:
- Delta gap = Delta AG - Delta HCO3-
- If delta gap > +6: Suggests concurrent metabolic alkalosis
- If delta gap < -6: Suggests concurrent hyperchloremic acidosis 5
Potential Pitfalls
- Negative anion gaps: May indicate laboratory error, specimen contamination, extreme hyperkalemia, or paraproteins from conditions like multiple myeloma 7
- Falsely normal anion gap: Can occur in hypoalbuminemia despite presence of high anion gap acidosis
Clinical Context
The anion gap calculation is particularly useful in:
- Evaluating unexplained metabolic acidosis
- Diagnosing conditions like diabetic ketoacidosis (DKA), where anion gap >10-12 is diagnostic 1
- Distinguishing DKA from other causes of high anion gap acidosis like lactic acidosis, salicylate toxicity, methanol or ethylene glycol ingestion 1, 2
Remember that the anion gap should always be interpreted in the context of the patient's clinical presentation and other laboratory values for optimal diagnostic utility.