Serum Bicarbonate vs. Serum CO2 on CMP
Serum bicarbonate and serum CO2 on a Comprehensive Metabolic Panel (CMP) are essentially the same measurement—the term "serum CO2" refers to total CO2 content, of which bicarbonate comprises approximately 96%, making them clinically interchangeable for practical purposes. 1, 2
What the Lab Actually Measures
The CMP reports "serum CO2" or "total CO2," which is measured using chemical methods that liberate all CO2 from the serum sample. 1, 2
This total CO2 measurement includes three components: 1, 2
- Bicarbonate (HCO3-) - approximately 96% of the total
- Dissolved CO2 gas
- Minor carbon dioxide components (carbonic acid and carbamino compounds)
Because bicarbonate represents 96% of the measured value, the terms are used interchangeably in clinical practice, though "total serum CO2" is technically more accurate. 1
Why This Matters Clinically
For screening acid-base disturbances, the serum CO2/bicarbonate on a CMP is actually more reliable than arterial blood gas (ABG) bicarbonate for certain purposes. 1
The American Thoracic Society recommends using serum bicarbonate >27 mmol/L as a screening threshold to determine when ABG confirmation is needed for conditions like hypercapnia. 1, 2
A serum bicarbonate <27 mmol/L has a 99% negative predictive value for ruling out conditions like Obesity Hypoventilation Syndrome, potentially eliminating the need for arterial sampling. 1
Key Differences from ABG Bicarbonate
ABG bicarbonate is calculated using the Henderson-Hasselbalch equation from measured pH and pCO2, while CMP bicarbonate is directly measured. 3
Discrepancies between the two can occur due to: 1
- Sequential rather than simultaneous sampling (especially in unstable patients)
- Different measurement methodologies
- Timing differences in sample collection
In critically ill patients, neither method has strong accuracy for diagnosing acidemia based on ROC curve analysis, but both show strong positive correlation. 4
Important Clinical Caveats
Different laboratory assays can produce significantly different results—enzymatic techniques may report values 4 mEq/L lower than direct electrode measurements. 5
Environmental factors matter: atmospheric CO2 concentrations in the laboratory can affect measured values by up to 2 mmol/L, particularly in samples with low bicarbonate concentrations. 6
Spuriously low bicarbonate values can occur with increased serum lipids or proteins (pseudohypobicarbonatemia), such as in multiple myeloma with elevated paraproteins. 3
In hemodialysis patients, 36-40% may be misclassified using total CO2 alone compared to pH assessment, as total CO2 <22 mmol/L corresponded to actual acidemia (pH <7.38) in only 23% of cases. 7
Practical Algorithm for Interpretation
Use the serum CO2 from CMP as your initial screening tool for metabolic acid-base disturbances. 1, 2
If serum CO2 >27 mmol/L in obese patients, obtain ABG to confirm hypercapnia and evaluate for respiratory acidosis with compensation. 1, 8
If serum CO2 <22 mmol/L in CKD patients, consider oral bicarbonate supplementation to maintain normal range. 1, 8
When serum CO2 and ABG bicarbonate values diverge significantly, verify timing of sample collection and consider laboratory methodology differences before making clinical decisions. 1, 5