What is the anion gap for a patient with hypernatremia, hyperkalemia, hypochloremia, and severe metabolic acidosis?

Calculation of Anion Gap

The anion gap for this patient is 48 mmol/L, calculated as [Na+ + K+] - [Cl- + HCO3-] = [129 + 5] - [77 + 9] = 134 - 86 = 48 mmol/L, which indicates a severe high anion gap metabolic acidosis.

Understanding the Anion Gap Calculation

The anion gap (AG) is calculated using the following formula:

• AG = [Na+ + K+] - [Cl- + HCO3-]

For this patient:

• Na+ = 129 mmol/L
• K+ = 5.0 mmol/L
• Cl- = 77 mmol/L
• HCO3- = 9 mmol/L

Therefore:

• AG = [129 + 5] - [77 + 9] = 134 - 86 = 48 mmol/L

Clinical Interpretation

This anion gap of 48 mmol/L is significantly elevated compared to the normal reference range. According to current guidelines:

• The modern reference range for anion gap is 5-12 mmol/L when using ion-selective electrode methods 1
• An anion gap exceeding 24 mmol/L is rare and strongly suggests severe metabolic acidosis 1
• According to the EXTRIP workgroup guidelines, an anion gap >27 mmol/L when there is evidence of ethylene glycol exposure warrants extracorporeal treatment 2

Clinical Significance

This extremely high anion gap (48 mmol/L) in combination with:

• Low sodium (129 mmol/L) - hyponatremia
• Normal potassium (5.0 mmol/L)
• Low chloride (77 mmol/L) - hypochloremia
• Low bicarbonate (9 mmol/L) - severe metabolic acidosis

Strongly suggests a severe high anion gap metabolic acidosis. The hypochloremia seen in this case is consistent with findings in anion gap acidosis, where plasma chloride may decrease as part of the body's response to acidosis 3.

Differential Diagnosis Based on High Anion Gap

The differential diagnosis for a severely elevated anion gap (48 mmol/L) includes:

1. Toxic alcohol ingestion (methanol, ethylene glycol)
2. Diabetic ketoacidosis
3. Lactic acidosis
4. Salicylate poisoning
5. Uremic acidosis (severe kidney failure)

Given the history of severe vomiting for 3 days and the extremely high anion gap, this patient likely has a profound metabolic acidosis that requires immediate intervention.

Clinical Implications

According to the EXTRIP workgroup guidelines, an anion gap >27 mmol/L with evidence of ethylene glycol exposure would warrant immediate extracorporeal treatment 2. With an anion gap of 48 mmol/L, this patient's condition is critical and requires urgent management regardless of the specific etiology.

The severe metabolic acidosis (bicarbonate of 9 mmol/L) combined with the markedly elevated anion gap indicates a life-threatening condition that requires immediate medical intervention, including:

• Fluid resuscitation
• Identification and treatment of the underlying cause
• Possible bicarbonate therapy
• Consideration of hemodialysis depending on the etiology

This case demonstrates the importance of calculating and interpreting the anion gap correctly, as it provides crucial diagnostic information that guides urgent management decisions in critically ill patients.