The variables which we are interested in are:
[HCO3-] is not easily measured and the surrogate used is [Total CO2]. [Total CO2] includes not only the [HCO3-] but also the CO2 and other carbonates dissolved in the blood. As a result [Total CO2] is 1-2 mEq/l higher than serum [HCO3-], although this will be greater with higher pCO2 levels.
Serum [HCO3-] is not measured directly but derived from pH and pCO2 using the Henderson-Hasselbach equation. This is usually done on arterial blood and produces a lower value than the [Total CO2] of venous blood. This is because the pCO2 of venous blood is higher than arterial which will also push up the [HCO3-].
Normal values:
| Venous serum | [Total CO2] | 24 - 30 mEq/l | |
| Anion gap | 4 - 16 mEq/l | ||
| Arterial blood | pH | 7.38 - 7.42 | |
| pCO2 | 4.7 – 6.0 kPa | (35 - 45 mmHg) | |
| [HCO3-] | 23 - 29 mEq/l |
pH and pCO2 measured in venous blood will vary with local blood flow rate and therefore arterial values should be used.