Blood Gas
Bicarbonate
Bicarbonate (HCO3-) is the main buffer base in blood and a core component of the acid-base picture reported by blood gas analysers. On most point-of-care devices it is not measured directly but calculated from the measured pH and pCO2 using the Henderson-Hasselbalch relationship, and it is usually reported alongside total carbon dioxide (TCO2) and base excess.
Why it is measured
Bicarbonate helps distinguish the metabolic component of an acid-base disturbance from the respiratory component, supporting rapid assessment of acidosis or alkalosis. As general physiology, a low value tends towards metabolic acidosis and a high value towards metabolic alkalosis, always interpreted together with pH, pCO2 and the clinical context.
| Typical range | Indicative adult arterial range approximately 22 to 26 mmol/L. Venous samples typically read a few mmol/L higher (around 22 to 29 mmol/L). Values vary by method, sample type and whether bicarbonate is calculated or derived, so always apply the analyser's and laboratory's own reference interval. |
|---|---|
| Sample | Heparinised whole blood, arterial or venous (or capillary), analysed promptly without air exposure. Bicarbonate is then calculated from the measured pH and pCO2 of that sample. |
| Turnaround | Typically under 2 to 3 minutes from sample application, as part of a combined blood gas, electrolyte and metabolite panel at the point of care. |
Point of care devices that report it
- Abbott i-STAT (CG8+, CG4+ and EG7+ cartridges, calculated HCO3 with TCO2)
- Radiometer ABL90 FLEX and ABL90 FLEX PLUS
- Siemens Healthineers epoc Blood Analysis System
- Siemens Healthineers RAPIDPoint 500e
- Roche cobas b 123 and cobas b 221
- Nova Biomedical Stat Profile Prime CCS
Questions, answered
Is the bicarbonate from a blood gas analyser measured or calculated?
On most point-of-care blood gas analysers it is calculated, not measured directly. The device measures pH and pCO2 and derives bicarbonate using the Henderson-Hasselbalch equation. This is why a separate measured total CO2 (TCO2) on a chemistry analyser can differ slightly from the calculated HCO3 reported on a gas analyser.
What is the difference between bicarbonate (HCO3-) and total CO2 (TCO2)?
Bicarbonate is the dominant single species, while TCO2 represents all forms of carbon dioxide in the sample, including bicarbonate, dissolved CO2 and carbonic acid. TCO2 is therefore usually a little higher than HCO3-. Both are reported to help assess the metabolic component of acid-base balance and should be read together with pH and pCO2.
Why might a venous bicarbonate differ from an arterial one on the same patient?
Venous blood carries more carbon dioxide returning from the tissues, so venous bicarbonate often reads a few mmol/L higher than arterial. For consistent trending it helps to keep to the same sample type and to interpret the result against the reference interval appropriate to that sample. This is general educational guidance, not advice for any individual patient.