What is the acid‑base disorder in a patient with serum sodium 139 mmol/L, potassium 4.6 mmol/L, chloride 111 mmol/L, and bicarbonate 13 mmol/L?

Metabolic Acidosis with Elevated Anion Gap

This patient has a high anion gap metabolic acidosis. The bicarbonate of 13 mmol/L is significantly below the normal range of 22-26 mmol/L, and the calculated anion gap is elevated at approximately 15 mEq/L (139 - [13 + 111] = 15), which exceeds the normal range of 10-12 mEq/L. 1

Diagnostic Classification

The low serum bicarbonate (<22 mmol/L) almost always indicates metabolic acidosis, and the elevated anion gap (>12 mEq/L) signifies accumulation of unmeasured anions such as lactate, ketoacids, uremic toxins, or ingested toxins. 1

Key Diagnostic Features

• Serum bicarbonate 13 mmol/L indicates moderate metabolic acidosis (normal 22-26 mmol/L) 1
• Calculated anion gap ≈ 15 mEq/L (Na 139 - [Cl 111 + HCO₃ 13]) confirms high anion gap acidosis (normal 10-12 mEq/L) 1
• Chloride 111 mmol/L is at the upper limit of normal, which is appropriate for high anion gap acidosis where chloride is not elevated 1

Differential Diagnosis for High Anion Gap Metabolic Acidosis

The most common causes include:

• Lactic acidosis from tissue hypoperfusion, sepsis, or shock 2
• Ketoacidosis (diabetic, alcoholic, or starvation) 2
• Renal failure with accumulation of uremic acids 2
• Toxic ingestions including ethylene glycol, methanol, or salicylates 2

Essential Next Steps

Arterial blood gas analysis must be performed to measure pH and PaCO₂, which definitively confirms metabolic acidosis and assesses for any respiratory compensation or mixed disorder. 1

Additional Required Testing

• Serum glucose and ketones to evaluate for diabetic ketoacidosis (glucose >250 mg/dL, pH <7.3, bicarbonate <15 mEq/L, ketones positive) 1
• Serum lactate to identify lactic acidosis 2
• Serum creatinine and BUN to assess renal function 1
• Calculate effective serum osmolality: 2[Na] + glucose/18 to detect osmolar gap from toxic ingestions 1

Expected Compensatory Response

In pure metabolic acidosis, the PaCO₂ should decrease by approximately 1 mmHg for every 1 mmol/L fall in serum bicarbonate. 3 With a bicarbonate of 13 mmol/L (9 mmol/L below the normal midpoint of 22 mmol/L), the expected PaCO₂ should be approximately 31 mmHg (40 - 9 = 31). 3

• If PaCO₂ is higher than expected, a concurrent respiratory acidosis exists 4
• If PaCO₂ is lower than expected, a concurrent respiratory alkalosis is present 4
• If pH is near-normal despite low bicarbonate, suspect a mixed disorder with primary respiratory alkalosis offsetting the metabolic acidosis 1

Treatment Considerations

The primary treatment for metabolic acidosis is addressing the underlying cause and restoring adequate circulation, not routine bicarbonate administration. 5, 3

When Bicarbonate Therapy Is Indicated

• Severe acidosis with pH <7.0-7.1 after establishing adequate ventilation 1, 5
• Diabetic ketoacidosis with pH <6.9-7.0 (bicarbonate generally not needed if pH ≥7.0) 1
• Life-threatening hyperkalemia as a temporizing measure 5
• Tricyclic antidepressant or sodium channel blocker overdose with cardiotoxicity 5

When Bicarbonate Should NOT Be Used

Bicarbonate therapy is not recommended for hypoperfusion-induced lactic acidemia when pH ≥7.15, as randomized controlled trials show no benefit in hemodynamic variables or vasopressor requirements and potential harm from sodium/fluid overload, increased lactate, and decreased ionized calcium. 5

Critical Monitoring

If treatment is initiated, monitor arterial or venous blood gases and serum electrolytes every 2-4 hours to assess pH, bicarbonate response, PaCO₂, sodium, potassium, and ionized calcium. 1 Target pH should be 7.2-7.3, not complete normalization. 1