What does a low total carbon dioxide (CO2) level of 18 indicate in a blood test?

Understanding Low Total CO2 in Blood Tests

A low total carbon dioxide (CO2) level of 18 mmol/L indicates mild metabolic acidosis, which requires further evaluation to determine the underlying cause that may affect morbidity and mortality if left untreated. 1

What Total CO2 Measures

Total CO2 in blood tests primarily reflects the bicarbonate (HCO3-) concentration in your blood, which is the main component of your body's buffer system. Carbon dioxide is carried in the blood in three forms:

• Bicarbonate (70-85%)
• Dissolved carbon dioxide (5-10%)
• Bound to hemoglobin (10-20%) 2

The normal range for total CO2 is typically 23-30 mmol/L, with values below this range indicating metabolic acidosis 3.

Causes of Low Total CO2

Low total CO2 (18 mmol/L) can result from several conditions:

1. Increased acid production:

   • Lactic acidosis from tissue hypoxia
   • Ketoacidosis (diabetic, alcoholic)
   • Excessive exercise
   • Sepsis 1, 4

2. Decreased acid excretion:

   • Kidney disease (especially when GFR falls below 60 mL/min)
   • Renal tubular acidosis 1

3. Bicarbonate loss:

   • Gastrointestinal losses (diarrhea, intestinal fistulas)
   • Drainage tubes
   • Certain medications 1, 4

4. Other causes:

   • Administration of chloride-containing solutions
   • Respiratory compensation for chronic respiratory alkalosis 2, 1

Clinical Significance

A total CO2 of 18 mmol/L indicates mild metabolic acidosis, which can have several physiological effects:

• Acute effects: Decreased cardiac output, arterial dilatation with hypotension, altered oxygen delivery, decreased ATP production, predisposition to arrhythmias, and impaired immune response 4

• Chronic effects: Increased muscle degradation, abnormal bone metabolism (osteodystrophy), and increased risk of fractures 1, 4

Diagnostic Approach

When a low total CO2 is identified, further evaluation should include:

1. Calculate the anion gap: [Na+] - ([Cl-] + [HCO3-])

   • Normal anion gap (8-12 mEq/L): Suggests bicarbonate loss or hyperchloremic acidosis
   • Elevated anion gap (>12 mEq/L): Suggests accumulation of organic acids 1, 4

2. Additional laboratory testing:

   • Arterial blood gases to confirm acidosis and assess respiratory compensation
   • Complete metabolic panel
   • Serum ketones and lactate
   • Urinalysis 1

3. Clinical assessment:

   • Vital signs
   • Mental status
   • Signs of dehydration
   • Evidence of shock or tissue hypoperfusion 1

Important Considerations

1. Pre-analytical errors: Sample handling can affect total CO2 results. Delays in processing can lead to falsely low values due to continued metabolism in the sample 5.

2. Compensation mechanisms: The body compensates for metabolic acidosis through:

   • Respiratory compensation (hyperventilation to reduce PaCO2)
   • Renal compensation (increased H+ excretion and bicarbonate regeneration) 6

3. Albumin and hemoglobin levels: The severity of acidosis and response to treatment can be influenced by blood albumin and hemoglobin concentrations 7.

Management Implications

If metabolic acidosis is confirmed:

1. Treat the underlying cause

2. Consider bicarbonate therapy for moderate to severe acidosis:

   • Oral bicarbonate: 2-4 g/day (25-50 mEq/day) divided into 2-3 doses to maintain serum bicarbonate ≥22 mmol/L
   • IV sodium bicarbonate for severe acidosis (pH <7.2) 1

3. Dietary modifications:

   • Increased fruit and vegetable consumption as alternative sources of alkali
   • Appropriate protein intake (0.8 g/kg/day for non-dialysis CKD patients)
   • Sodium restriction (<2,300 mg/day) 1

4. Regular monitoring of acid-base status and renal function 1

A low total CO2 of 18 mmol/L should never be dismissed as insignificant, as it may be the first indication of an underlying medical condition that requires treatment to prevent serious complications affecting morbidity and mortality.