Is CO2 (carbon dioxide) in a complete metabolic panel (CMP) equivalent to bicarbonate (HCO3-)?

From the Guidelines

Yes, CO2 in a complete metabolic panel (CMP) is equivalent to bicarbonate (HCO3-). When you see "CO2" listed on a CMP lab report, it's actually measuring the bicarbonate level in your blood, not gaseous carbon dioxide. This measurement is typically reported in mEq/L or mmol/L, with normal values ranging from about 22 to 29 mEq/L. The test measures bicarbonate because it's the main form of CO2 in your bloodstream and serves as an important buffer that helps maintain proper blood pH, as explained in the BTS guideline for oxygen use in adults in healthcare and emergency settings 1.

The bicarbonate system is crucial for acid-base balance in the body. When acids build up in the blood, bicarbonate neutralizes them; conversely, when the blood becomes too alkaline, bicarbonate levels decrease. This measurement helps healthcare providers assess kidney function, respiratory status, and overall acid-base balance. Conditions like diabetic ketoacidosis, kidney disease, or respiratory disorders can affect bicarbonate levels, making this an important diagnostic indicator. According to the BTS guideline, hypercapnia and respiratory acidosis are inextricably linked and are best considered together 1.

Key points to consider:

• CO2 in a CMP measures bicarbonate levels, not gaseous carbon dioxide
• Bicarbonate serves as an important buffer for acid-base balance in the body
• The measurement helps assess kidney function, respiratory status, and overall acid-base balance
• Conditions like diabetic ketoacidosis, kidney disease, or respiratory disorders can affect bicarbonate levels. The most recent and highest quality study on this topic is from 2017, which provides guidance on oxygen use in adults in healthcare and emergency settings 1.

From the FDA Drug Label

Sodium bicarbonate in water dissociates to provide sodium (Na+) and bicarbonate (HCO3¯ ) ions Bicarbonate anion is considered “labile” since at a proper concentration of hydrogen ion (H+) it may be converted to carbonic acid (H2CO3) and thence to its volatile form, carbon dioxide (CO2) excreted by the lung

The CO2 in a complete metabolic panel (CMP) is not directly equivalent to bicarbonate (HCO3-), but rather CO2 is the volatile form that bicarbonate can be converted to in the presence of hydrogen ions. In other words, CO2 and bicarbonate are related but distinct entities, with bicarbonate being a constituent of body fluids and CO2 being its volatile form excreted by the lungs 2.

• Key point: The relationship between CO2 and bicarbonate is one of conversion, not equivalence.
• Clinical implication: When interpreting a CMP, CO2 levels should not be directly equated with bicarbonate levels.

From the Research

CO2 and Bicarbonate in CMP

• In a complete metabolic panel (CMP), CO2 is often reported as a measure of bicarbonate (HCO3-) levels in the blood 3, 4.
• Bicarbonate is a crucial component in maintaining the body's acid-base balance, and its levels can be affected by various factors, including metabolic acidosis or alkalosis 5, 6.
• The relationship between CO2 and bicarbonate is complex, and several formulas have been proposed to estimate expected pCO2 levels based on serum bicarbonate concentrations 5, 7.
• Studies have shown that there is a significant direct linear relationship between end-tidal carbon dioxide (ETCO2) and arterial blood bicarbonate levels in patients with metabolic acidosis 3.
• However, it is essential to note that CO2 and bicarbonate are not exactly the same thing, and their measurements can be influenced by different factors, such as respiratory and renal function 4, 7.

Key Findings

• A study published in 2016 found that capnography can be used to measure ETCO2 levels, which have a close correlation with PaCO2 and can be used to diagnose metabolic acidosis 3.
• Another study published in 2018 found that a simple formula, pCO2 = [HCO3-] + 15, can be used to estimate expected pCO2 levels and rule out superimposing respiratory acid-base disorders 5.
• A review article published in 2010 discussed the pathophysiology, diagnosis, and management of metabolic acidosis, highlighting the importance of measuring serum bicarbonate concentrations and calculating the anion gap to aid in diagnosis 7.

Measurement and Calculation

• The measurement of CO2 in a CMP is typically reported as a total carbon dioxide (TCO2) level, which includes both bicarbonate and dissolved CO2 4, 6.
• The calculation of expected pCO2 levels based on serum bicarbonate concentrations can be useful in identifying acid-base disorders and guiding treatment 5, 7.