What does an elevated anion gap and low carbon dioxide level indicate?

Elevated Anion Gap with Low CO2: Diagnostic Implications

An anion gap of 18 with a CO2 of 17 indicates metabolic acidosis, most likely an anion gap metabolic acidosis that requires prompt evaluation for potentially life-threatening causes including diabetic ketoacidosis, lactic acidosis, toxic ingestions (particularly ethylene glycol or methanol), or renal failure.

Understanding the Laboratory Values

• Normal anion gap: 8-12 mEq/L 1
• Normal CO2 (bicarbonate): 22-29 mmol/L 1
• Anion gap calculation: [Na+] - ([Cl-] + [HCO3-]) 1

The combination of an elevated anion gap (18) and low CO2/bicarbonate (17) represents a moderate metabolic acidosis with increased unmeasured anions in the blood.

Differential Diagnosis

High-Priority Causes (Potentially Life-Threatening)

1. Diabetic Ketoacidosis (DKA)

   • Check for:
     • Blood glucose >250 mg/dL
     • Positive urine and serum ketones
     • Signs of dehydration
     • History of diabetes or new-onset diabetes 2

2. Lactic Acidosis

   • Associated with:
     • Tissue hypoperfusion/shock
     • Sepsis
     • Liver disease
     • Metformin use 1

3. Toxic Ingestions

   • Ethylene Glycol Poisoning

     • Anion gap >27 mmol/L indicates severe poisoning with high mortality (20.4%) 2
     • Look for calcium oxalate crystals in urine
     • Osmolar gap often present 2

   • Methanol Poisoning

     • Similar presentation to ethylene glycol
     • Visual disturbances may be present 2

   • Salicylate Toxicity

     • Usually presents with respiratory alkalosis plus metabolic acidosis 1

4. Uremic Acidosis

   • Check BUN and creatinine
   • Usually associated with chronic or acute kidney injury 1

Other Causes

• Alcoholic ketoacidosis
• Starvation ketosis (usually bicarbonate not below 18 mEq/L) 2
• Certain medications (e.g., biguanides) 3

Diagnostic Approach

1. Immediate Laboratory Testing:

   • Arterial blood gases
   • Complete metabolic panel
   • Serum ketones
   • Serum lactate
   • Serum osmolality (calculate osmolar gap)
   • Urinalysis (look for ketones, calcium oxalate crystals)
   • Toxicology screen if indicated 2

2. Clinical Assessment:

   • Vital signs with particular attention to respiratory pattern
   • Mental status evaluation
   • Signs of dehydration
   • Evidence of shock or tissue hypoperfusion 2

Management Considerations

Management depends on the underlying cause but generally includes:

1. For All Causes:

   • Fluid resuscitation (isotonic saline at 15-20 mL/kg/hr initially) 2
   • Electrolyte monitoring and replacement
   • Treatment of the underlying cause

2. For Specific Causes:

   • DKA: Insulin therapy, potassium replacement 2
   • Ethylene glycol/methanol poisoning: Fomepizole or ethanol therapy, consider hemodialysis for severe cases (anion gap >27) 2
   • Uremic acidosis: Consider renal replacement therapy 1
   • Lactic acidosis: Improve tissue perfusion, treat underlying cause 1

Important Caveats

• The severity of metabolic acidosis correlates with mortality, particularly in toxic ingestions 2
• An anion gap >28 mmol/L in ethylene glycol poisoning is associated with 20.4% mortality 2
• Laboratory errors can cause spurious metabolic acidosis, particularly in samples that are not processed promptly 4
• Not all elevated anion gap acidosis cases have the same clinical significance - the degree of elevation matters 5

Follow-up

• Serial monitoring of acid-base status
• Monitoring of renal function
• Resolution of the anion gap should parallel clinical improvement
• For chronic conditions, monthly monitoring of serum bicarbonate levels 1