Bicarbonate vs. CO2 on Comprehensive Metabolic Panel
No, bicarbonate (bicarb) and carbon dioxide (CO2) on a Comprehensive Metabolic Panel (CMP) are not exactly the same, but they are closely related. On a CMP, what is labeled as "CO2" is actually measuring total CO2 content, which includes bicarbonate (HCO3-), dissolved CO2, and other minor carbon dioxide components, with bicarbonate representing approximately 96% of the total 1.
Understanding the Relationship Between CO2 and Bicarbonate
- The serum CO2 measurement on a CMP is more accurately described as "total serum CO2" because the chemical methods measure all CO2 liberated from the serum 1
- Bicarbonate (HCO3-) is the principal component of this total CO2 measurement, making up about 96% of the value 1
- In the body, CO2 combines with water to form carbonic acid (H2CO3), which then dissociates to bicarbonate (HCO3-) and hydrogen ions (H+) 2
- This relationship is represented by the equation: CO2 + H2O → H2CO3 → H+ + HCO3- 2
Clinical Significance and Measurement
- Normal plasma bicarbonate levels range from 24 to 31 mEq/liter 3
- Serum bicarbonate is regulated by the kidneys, which acidify urine when there is a deficit or alkalinize urine when there is an excess 3
- In a healthy adult with normal kidney function, almost all filtered bicarbonate is reabsorbed; less than 1% is excreted in urine 3
- The American Thoracic Society recommends using serum bicarbonate (measured as CO2 on CMP) with a threshold of >27 mmol/L to indicate the need for arterial blood gas (ABG) confirmation of hypercapnia in certain clinical scenarios 2, 1
Potential Discrepancies Between Measurements
- Discrepancies can occur between serum CO2 measurements and bicarbonate calculated from arterial blood gases 4
- These differences may be due to:
- Different measurement techniques (enzymatic vs. electrode-based) 5
- Environmental factors affecting the sample (atmospheric CO2 levels can influence the measurement) 6
- Timing of sample collection (sequential rather than simultaneous sampling) 1
- Presence of interfering substances like increased lipids or proteins 7
Clinical Implications
- For screening purposes, such as for Obesity Hypoventilation Syndrome, serum bicarbonate (CO2) levels are considered reliable, with values <27 mmol/L having a high negative predictive value (99.0%) for ruling out conditions like OHS 1
- When there are large discrepancies between serum CO2 and calculated bicarbonate from ABG, recalculation using the Henderson-Hasselbalch equation may help clarify the acid-base disorder 4
- Elevated bicarbonate/CO2 levels may suggest chronic respiratory acidosis with renal compensation 1
Common Pitfalls
- Assuming that CO2 on a CMP directly measures dissolved carbon dioxide gas rather than total CO2 content (which is primarily bicarbonate) 1
- Failing to consider the effect of environmental factors on CO2 measurements, which can cause circadian patterns in laboratory values 6
- Not accounting for the potential influence of organic acids or paraproteins, which can cause falsely low serum bicarbonate values 7
- Overlooking the importance of timing when comparing values from different samples 1