Correct Documentation of Serum Bicarbonate from a CMP
The correct way to document serum bicarbonate from a comprehensive metabolic panel is as "total CO2" or "serum bicarbonate," reported in mmol/L, recognizing that the measured value represents total serum CO2 (of which bicarbonate comprises approximately 96%, with the remainder being dissolved CO2). 1
Understanding the Terminology
The term commonly used on a CMP—"CO2" or "total CO2"—is actually the more technically accurate descriptor for what is being measured. Some laboratories use the more correct term "total serum CO2" to describe measured serum bicarbonate 1. This distinction matters because:
- Bicarbonate represents 96% of total serum CO2
- The remainder is mostly dissolved CO2 1
- The measurement captures all CO2 liberated from the serum using chemical methods 1
Clinical Documentation Standards
When documenting in medical records, either terminology is acceptable:
- "Serum bicarbonate" - widely understood clinically
- "Total CO2" - more technically precise
- "Serum CO2" - commonly used on lab reports
Always include the unit: mmol/L 2
Key Clinical Context
Normal Reference Range
Guidelines reference normal bicarbonate levels around 22-24 mmol/L 2, though specific laboratory reference ranges may vary slightly.
Important Measurement Considerations
The CMP bicarbonate measurement differs from arterial blood gas (ABG) bicarbonate in methodology:
- CMP uses enzymatic/photometric or indirect ion-selective electrode methods 3, 4
- ABG calculates bicarbonate using the Henderson-Hasselbalch equation from measured pH and pCO2 3, 5
- Generally, there is acceptable correlation between the two methods 3, 6
Common Pitfalls to Avoid
Be aware of conditions causing spuriously low bicarbonate values on CMP:
- Severe hypertriglyceridemia (>4000 mg/dL) interferes with photometric analysis due to light-scattering effects 4
- Elevated paraproteins (as in multiple myeloma) can alter measurement assays 5
- Increased serum lipids or proteins may cause pseudohypobicarbonatemia 3, 5
When CMP bicarbonate seems inconsistent with clinical picture:
- Obtain an ABG for comparison—the direct ion-selective electrode method is free from photometric analyzer errors 4
- Consider laboratory interference if there is significant discrepancy between CMP and ABG values 3, 4
Sample Handling
For optimal concordance between methods, ensure the time between sample collection and laboratory analysis does not exceed 4 hours 6. Delays can affect accuracy of measurements.
Clinical Application Examples
In specific disease contexts, documentation follows these patterns:
Chronic Kidney Disease: "Serum total CO2 levels" should be monitored and maintained above 22 mmol/L 2
Obesity Hypoventilation Syndrome screening: A serum bicarbonate threshold of 27 mmol/L is used for clinical decision-making 1
Diabetic Ketoacidosis: Venous serum bicarbonate from the basic metabolic panel can reliably estimate arterial pH, with bicarbonate ≤20.6 mEq/L predicting arterial pH ≤7.3 with >95% sensitivity 7