Does a carbon dioxide (CO2) level of 19 indicate acidosis?

A CO2 of 19 in the Blood Does Not Indicate Acidosis by Itself

A carbon dioxide (CO2) level of 19 mmol/L alone does not indicate acidosis, as acidosis is defined by pH < 7.35, not by CO2 levels alone. 1 In fact, according to clinical guidelines, a CO2 level ≥19 mmol/L is classified as only mild metabolic acidosis, and this is only if accompanied by a low pH. 2

Understanding Acidosis and CO2 Levels

Acidosis is defined as a blood pH below 7.35, which corresponds to a hydrogen ion concentration greater than 45 nmol/L. 1 There are two main types of acidosis:

1. Respiratory acidosis: Characterized by elevated PaCO2 (hypercapnia) with a low pH
2. Metabolic acidosis: Characterized by low bicarbonate (HCO3-) with a low pH

A CO2 level of 19 mmol/L in blood tests typically refers to total CO2 content or serum bicarbonate (HCO3-), not to partial pressure of CO2 (PaCO2). While this level is at the lower end of normal or slightly below normal range, it cannot be interpreted in isolation.

Interpreting CO2 in Context

To determine if acidosis is present, you need:

• pH measurement: Primary indicator of acidosis (pH < 7.35)
• CO2 or bicarbonate level: Helps determine the type of acid-base disorder
• Anion gap calculation: Helps classify metabolic acidosis

According to clinical guidelines, severity of metabolic acidosis based on total CO2 levels can be classified as:

• ≥19 mmol/L: Mild metabolic acidosis
• <19 mmol/L: Moderate to severe metabolic acidosis 2

Clinical Implications

A CO2 of 19 mmol/L could indicate:

1. Early or mild metabolic acidosis: If pH is low (<7.35)
2. Compensated respiratory alkalosis: Where low CO2 is a compensation for primary respiratory alkalosis
3. Normal variation: In some individuals, especially with certain dietary patterns

The British Thoracic Society recognizes hypercapnia as PaCO2 ≥6 kPa (approximately 45 mmHg), which is elevated CO2, not low CO2. 1 A CO2 of 19 mmol/L would not meet this definition of hypercapnia.

Diagnostic Approach

When evaluating a CO2 of 19 mmol/L:

• Measure arterial blood gases: To determine pH and PaCO2
• Calculate anion gap: [Na+] - ([Cl-] + [HCO3-]) to help identify the cause if metabolic acidosis is present 2
• Assess clinical context: Look for causes of metabolic acidosis (renal failure, diabetic ketoacidosis, lactic acidosis, etc.)

Common Pitfalls

1. Misinterpreting isolated CO2 values: A single CO2 measurement without pH cannot diagnose acidosis
2. Confusing total CO2 with PaCO2: These are different measurements with different reference ranges
3. Overlooking compensatory mechanisms: The body compensates for primary acid-base disorders, which can normalize pH despite abnormal CO2 levels

In conclusion, while a CO2 of 19 mmol/L may suggest a mild metabolic acidosis if accompanied by a low pH, this value alone is insufficient to diagnose acidosis. A comprehensive acid-base assessment including pH and other parameters is necessary for accurate interpretation.