Blood Gas
Base Excess
Base excess is a calculated blood gas parameter that estimates the metabolic (non-respiratory) component of an acid-base disturbance. It represents the amount of strong acid or base, in mmol/L, that would be needed to titrate one litre of blood back to a normal pH under standard conditions, helping to flag metabolic acidosis or alkalosis at the bedside.
Why it is measured
A negative value (base deficit) suggests metabolic acidosis or buffer consumption, while a positive value suggests metabolic alkalosis or base accumulation. It is widely used in emergency, critical care, anaesthetic and neonatal settings to gauge the severity of metabolic upset and to track resuscitation.
| Typical range | Approximately -2 to +2 mmol/L (adult arterial), reported as base excess of blood (BEb) or of extracellular fluid (BEecf). Some laboratories use a slightly wider interval such as -3 to +3 mmol/L. Ranges vary by analyser, algorithm and sample type (arterial versus venous), so always apply the issuing laboratory's reference interval. |
|---|---|
| Sample | Heparinised whole blood, typically arterial; venous or capillary samples are also used at the point of care. Analyse promptly and avoid air bubbles, as delays and exposure to air alter the measured gases from which base excess is derived. |
| Turnaround | Around 1 to 2 minutes once the sample is applied to the cartridge or analyser, as part of a full blood gas panel. |
Point of care devices that report it
- Abbott i-STAT (CG4+, CG8+ and EG7+ cartridges, reports base excess)
- Siemens Healthineers epoc Blood Analysis System (reports BE(ecf))
- Radiometer ABL9 / ABL90 FLEX / ABL80 FLEX blood gas analysers
- Siemens Healthineers RAPIDPoint 500e blood gas system
- Nova Biomedical Stat Profile Prime / pHOx analysers
Questions, answered
Is base excess measured or calculated?
It is a calculated (derived) parameter. The analyser measures pH and the partial pressure of carbon dioxide, then computes base excess using a standard algorithm, often alongside bicarbonate. This is why values can differ slightly between analyser makes that use different equations.
What is the difference between base excess of blood and of extracellular fluid?
Base excess of blood (BEb, sometimes called actual base excess) is calculated for the blood compartment, while base excess of extracellular fluid (BEecf, or standard base excess) models the whole extracellular space and is generally more stable across changing carbon dioxide levels. Devices may report one or both, so check which value your analyser displays.
Why might base excess differ from the bicarbonate result on the same report?
Both describe the metabolic side of acid-base balance but are derived differently, so they are complementary rather than identical. Base excess captures the contribution of all buffers, not just bicarbonate. Interpreting them together with pH and carbon dioxide gives a fuller picture; this record is educational and not a substitute for clinical assessment.