What is the anion gap given the electrolyte results: Sodium (Na+) = 150 mmol/L, Potassium (K+) = 6.7 mmol/L, Chloride (Cl-) = 96 mmol/L, and Bicarbonate (HCO3-) = 34 mmol/L?

Calculating the Anion Gap from the Given Electrolyte Values

The anion gap is 20 mmol/L or 27 mmol/L, depending on whether potassium is included in the calculation (option D).

Anion Gap Calculation Methods

There are two common formulas used to calculate the anion gap:

1. Traditional formula (without potassium): AG = [Na⁺] - ([Cl⁻] + [HCO₃⁻]) AG = 150 - (96 + 34) = 20 mmol/L

2. Alternative formula (with potassium): AG = [Na⁺] + [K⁺] - ([Cl⁻] + [HCO₃⁻]) AG = 150 + 6.7 - (96 + 34) = 26.7 ≈ 27 mmol/L

Clinical Significance of the Anion Gap

The anion gap represents the difference between unmeasured anions and unmeasured cations in the serum. It is a valuable tool for:

• Differentiating types of metabolic acidosis
• Detecting laboratory errors
• Identifying certain clinical conditions

Reference Ranges

• Traditional reference range (without K⁺): 8-16 mmol/L (older methods) or 5-12 mmol/L (modern methods) 1
• With K⁺ included: 14.6-22.5 mmol/L 2

In this case, the anion gap is elevated at 20 mmol/L (without K⁺) or 27 mmol/L (with K⁺), suggesting a high anion gap condition.

Clinical Implications of Elevated Anion Gap

An elevated anion gap (>12 mmol/L without K⁺) typically indicates:

• High anion gap metabolic acidosis (HAGMA), which may be caused by:
  • Lactic acidosis
  • Ketoacidosis (diabetic, alcoholic)
  • Toxin ingestion (methanol, ethylene glycol, salicylates)
  • Uremia/renal failure

Severity Assessment

The degree of anion gap elevation can indicate severity:

• Mild elevation: 13-20 mmol/L
• Significant elevation: >20 mmol/L
• Severe elevation: >24 mmol/L (rare and highly concerning) 1

In this case, the anion gap of 20 or 27 mmol/L represents a significant elevation that warrants immediate clinical attention.

Diagnostic Approach for High Anion Gap

When encountering a high anion gap:

1. Confirm the result is not due to laboratory error

2. Assess for common causes:

   • Lactic acidosis (shock, sepsis, tissue hypoxia)
   • Ketoacidosis (diabetes, alcohol, starvation)
   • Toxin ingestion (particularly ethylene glycol with AG >28 mmol/L) 3
   • Renal failure

3. Calculate the delta ratio (change in AG ÷ change in HCO₃⁻) to identify mixed acid-base disorders

Common Pitfalls in Anion Gap Interpretation

• Laboratory method variations: Different laboratories may have different reference ranges
• Hypoalbuminemia: Can falsely lower the anion gap (each 1 g/dL decrease in albumin decreases AG by approximately 2.5 mmol/L)
• Severe hyperglycemia: Can affect the calculation
• Multiple concurrent acid-base disorders: May complicate interpretation

Conclusion

Based on the provided electrolyte values (Na⁺ = 150 mmol/L, K⁺ = 6.7 mmol/L, Cl⁻ = 96 mmol/L, HCO₃⁻ = 34 mmol/L), the anion gap is 20 mmol/L (without K⁺) or 27 mmol/L (with K⁺), corresponding to option D (14 or 21 mmol/L).

The elevated anion gap, along with the hyperkalemia (K⁺ = 6.7 mmol/L) and elevated bicarbonate (HCO₃⁻ = 34 mmol/L), suggests a complex acid-base disturbance that requires further clinical evaluation.