A carbon dioxide (CO2) blood test measures the concentration of CO2 in a person’s bloodstream. This test is frequently included as part of a routine screening known as the Basic Metabolic Panel (BMP) or an electrolyte panel. The primary function of this measurement is to help evaluate the body’s electrolyte balance and screen for potential issues with acid-base balance. Understanding the result provides insight into metabolic functions.
Understanding Total CO2 and Bicarbonate
The term “Total CO2” (TCO2) on a blood test represents the sum of all forms of carbon dioxide present in the blood, including dissolved CO2, carbonic acid, and bicarbonate. Over 90% of the total CO2 measured is actually bicarbonate (HCO3-).
Bicarbonate is a negatively charged electrolyte that works alongside sodium and potassium to maintain electrical neutrality in the body’s fluids. It is a byproduct of cellular metabolism, produced when the body breaks down carbohydrates and fats. Bicarbonate is transported in the blood to the lungs for eventual exhalation as CO2. Normal serum levels typically fall within the range of 22 to 29 millimoles per liter (mmol/L).
The Role in pH Regulation
The bicarbonate system is the most significant buffer in the human body, serving as the primary defense against changes in blood acidity. A buffer absorbs excess hydrogen ions (H+) when the blood is too acidic, or releases them when the blood is too alkaline. This action is governed by the reversible chemical reaction where carbon dioxide and water form carbonic acid, which then dissociates into bicarbonate and hydrogen ions.
The lungs and the kidneys regulate this buffering system. The lungs rapidly control the carbon dioxide portion by adjusting the rate and depth of breathing. When acidity rises, breathing increases to expel more CO2, which shifts the reaction to reduce H+ and restore balance.
The kidneys provide slower, long-term control by managing bicarbonate concentration. They reabsorb nearly all filtered bicarbonate back into the blood and can generate new bicarbonate to replace what is consumed during buffering. This coordination maintains the blood’s pH within the narrow range of 7.35 to 7.45.
What Abnormal CO2 Levels Indicate
An abnormal total CO2 reading indicates an underlying acid-base imbalance. A low bicarbonate level (below 22 mmol/L) suggests metabolic acidosis. This condition occurs when the body produces too much acid or loses too much base, resulting in an excess of hydrogen ions that are buffered by the available bicarbonate.
Metabolic acidosis can lead to a compensatory response, causing rapid, deep breaths (Kussmaul breathing) to expel CO2. This state can cause symptoms such as confusion, fatigue, and reduced cellular function. Conversely, a high bicarbonate level (above 29 mmol/L) points toward metabolic alkalosis.
Metabolic alkalosis develops from an excess of bicarbonate in the blood or a loss of acid. This state makes the blood too alkaline, which can severely impact the nervous system and increase muscle excitability. Although the body attempts to compensate by slowing the breathing rate to retain CO2, severe imbalances may lead to confusion, muscle twitching, and increased risk of arrhythmias.
Common Causes of High or Low Results
A low total CO2 level, signaling metabolic acidosis, can stem from several conditions that either increase acid production or lead to bicarbonate loss.
Causes of Low Total CO2 (Metabolic Acidosis)
- Uncontrolled diabetes, which can lead to diabetic ketoacidosis, causing a buildup of acidic ketone bodies.
- Severe diarrhea, representing a significant loss of bicarbonate from the gastrointestinal tract.
- Reduced kidney function, such as in chronic kidney disease, impairs the organ’s ability to excrete acid and generate new bicarbonate.
High total CO2 levels, indicating metabolic alkalosis, are often related to a loss of stomach acid or specific medication use.
Causes of High Total CO2 (Metabolic Alkalosis)
- Severe or prolonged vomiting can cause a major loss of hydrochloric acid, leaving a relative excess of bicarbonate in the bloodstream.
- The use of certain diuretic medications can also lead to an increased loss of chloride and potassium, which indirectly forces the kidneys to retain bicarbonate.
- Conditions causing chronic CO2 retention, such as severe lung diseases, can also lead to a compensatory increase in bicarbonate by the kidneys.