Bicarbonate Measurement on Basic Metabolic Panel
On a Basic Metabolic Panel (BMP), serum bicarbonate is typically measured directly as total CO2 content rather than being calculated. 1
Measurement Methods
- The bicarbonate value reported on a BMP is actually a measurement of total serum CO2, which is performed using chemical methods that measure all CO2 liberated from the serum 1
- Total serum CO2 includes bicarbonate (HCO3-), which represents approximately 96% of the total, plus dissolved CO2 and other minor carbon dioxide components 1
- Some laboratories use the more accurate term "total serum CO2" to describe what is being measured, rather than "bicarbonate" 1
Comparison with Other Methods
- In contrast to BMP measurements, bicarbonate values from arterial blood gas (ABG) analysis are typically calculated using the Henderson-Hasselbalch equation rather than directly measured 2, 3
- Studies comparing measured total CO2 (from chemistry panels) with calculated bicarbonate (from blood gases) show strong correlation but not perfect agreement 4, 5
- One study found a correlation coefficient of r = 0.977 between measured TCO2 and calculated HCO3-, with a bias of 0.87 mmol/L 5
Clinical Implications
- The slight difference between measured total CO2 and calculated bicarbonate is usually not clinically significant for most patients 4
- However, in cases of severe acid-base disturbances, these differences may become more pronounced and clinically relevant 3
- When interpreting bicarbonate levels, clinicians should be aware of which method was used (direct measurement vs. calculation) 5
Practical Applications
- Serum bicarbonate levels are useful for screening certain conditions, such as Obesity Hypoventilation Syndrome (OHS), where a level >27 mmol/L may indicate the need for arterial blood gas measurement to confirm hypercapnia 1
- For monitoring chronic kidney disease, monthly measurement of serum bicarbonate is recommended, with a target level of ≥22 mmol/L 6
- In trauma settings, serum bicarbonate measurements may be used as an alternative to arterial base deficit as they show strong correlation and similar predictive ability 4
Common Pitfalls
- Large discrepancies between measured TCO2 and calculated bicarbonate may indicate laboratory error or severe physiological disturbances 3
- At extreme values (particularly low bicarbonate levels), the agreement between measured TCO2 and calculated bicarbonate tends to be poorer 5
- Clinicians should be cautious when using these values interchangeably, especially in critical care settings or when managing complex acid-base disorders 5